CA1322029C - Rate-responsive pacemaker - Google Patents
Rate-responsive pacemakerInfo
- Publication number
- CA1322029C CA1322029C CA000591474A CA591474A CA1322029C CA 1322029 C CA1322029 C CA 1322029C CA 000591474 A CA000591474 A CA 000591474A CA 591474 A CA591474 A CA 591474A CA 1322029 C CA1322029 C CA 1322029C
- Authority
- CA
- Canada
- Prior art keywords
- signal
- rate
- pacing rate
- pacing
- threshold
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Fee Related
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Classifications
-
- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61N—ELECTROTHERAPY; MAGNETOTHERAPY; RADIATION THERAPY; ULTRASOUND THERAPY
- A61N1/00—Electrotherapy; Circuits therefor
- A61N1/18—Applying electric currents by contact electrodes
- A61N1/32—Applying electric currents by contact electrodes alternating or intermittent currents
- A61N1/36—Applying electric currents by contact electrodes alternating or intermittent currents for stimulation
- A61N1/362—Heart stimulators
- A61N1/365—Heart stimulators controlled by a physiological parameter, e.g. heart potential
- A61N1/36585—Heart stimulators controlled by a physiological parameter, e.g. heart potential controlled by two or more physical parameters
Abstract
ABSTRACT
RATE-RESPONSIVE PACEMAKER
A pacemaker has means for producing first and second signals one of which changes rapidly in response to changes in exercise level but may inaccurately represent the appropriate pacing rate. Such signal may be derived from a vibration sensor. The second signal accurately represents the required pacing rate but changes slowly in response to exercise level and may represent any of a number of different physiological variables, such as respiratory rate.
The signals may be derived from the same or separate sensors. A control circuit provides a pacing rate appropriate to resting conditions if the first signal has a value less than a threshold regardless of the value of the second signal. The pacing rate is elevated to a predetermined level, such as 95 beats/min, if the first signal exceeds the threshold but the second signal has a value indicative of a pacing rate less than the predetermined level. The pacing rate is further elevated to substantially equal that indicated by the value of the second signal when the first signal exceeds the threshold and the second signal indicates a rate higher than the predetermined rate.
RATE-RESPONSIVE PACEMAKER
A pacemaker has means for producing first and second signals one of which changes rapidly in response to changes in exercise level but may inaccurately represent the appropriate pacing rate. Such signal may be derived from a vibration sensor. The second signal accurately represents the required pacing rate but changes slowly in response to exercise level and may represent any of a number of different physiological variables, such as respiratory rate.
The signals may be derived from the same or separate sensors. A control circuit provides a pacing rate appropriate to resting conditions if the first signal has a value less than a threshold regardless of the value of the second signal. The pacing rate is elevated to a predetermined level, such as 95 beats/min, if the first signal exceeds the threshold but the second signal has a value indicative of a pacing rate less than the predetermined level. The pacing rate is further elevated to substantially equal that indicated by the value of the second signal when the first signal exceeds the threshold and the second signal indicates a rate higher than the predetermined rate.
Description
1322a2s RATE-RESPONSIVE PACEMAKER
The present invention relates to a cardiac pacemaker.
In the normal healthy individual the output of blood by the heart is varied continuously to meet the metabolic demands of the body. The cardiac output changes with variations in the heart rate and the volume of blood pumped at each heart beat. The cardiac output is the product of these two variables.
During strenuous exercise a four or five fold increase in cardiac output may be required. Approximately 75~
of this increase is achieved by elevation of the heart rate.
~
Patients with cardiac disease resulting in very slow heart rates frequently require treatment in the form ,~ of an implanted artificial pacemaker. This consists of a small electrical pulse generator that is connected to the heart by an insulated electrode lead, usually positioned in the right ventricle.
Until reoently the ma~ority of such paaemakers were relatively simple devices that provided a constant frequency of stimulation. These suffer from the ::
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,::
~:
`
.
1322~29 disadvantage that they are unable to increase the heart rate during exercise, thereby limiting the degree of exertion that the patient is able to undertake. This drawback has led to the development of two forms of pacemaker that can increase stimulation rate during exercise. The "ideal" system is a dual chamber pacemaker which has a second lead implanted in the right atrial chamber of the heart~
This detects the rate of the heart's natural pacemaker and enables the artificial pacemaker to provide normal physiological changes in heart rate. However, such systems are not suitable for many patients and also have inherent technical and economic disadvantages.
An alternative approach that has been developed during recent years is the single chamber rate-responsive pacemaker. The fundamental principles of this device ; are as follows:
(A) A sensor is used to detect changes in a biologlcal variable that are directly or indirectly proportional to the level of metabolic activity ln the body. Evoked QT
interval, respiratory rate, mixed venous ~` 25 temperature and body vibration are the main 1322~29 variables that are sensed by currently available rate-responsive pacemakers.
(B) The output from the sensor is processed by electronic circuitry to provide a signal that can be used to vary the stimulation rate provided by the pacemaker. This circuitry is designed to perform a series of mathematical and logical functions that are known collectively as the rate-response algorithm.
The two principal functions of the rate-responsive algorithm are:
15 i ) To determine the recognition level within the pacemaker for the value of the sensor signal which must be excaeded in order for an increase in pacing rate to be initiated (threshold level).
i : :
~; 20 li) To determine the preoise mathematical relationship between changes in the sensor signal S and the change in pacing rate P to be provided by the pacemaker, (the slope dP/dS).
.
~322~2~
Although every individual requires substantially the same heart rate as other individuals for a given level or type of physical activity, the changes which take place in the sensed,biological variable as a result of a given level of physical activity differ from individual to individual. Accordingly, conventional rate response pacemakers are arranged so that the values of the threshold and slope may be adjusted to suit the individual patient. These values can be selected by telemetered instructions from an external programmer. The programmer is also used to select the minimum and maximum rates that the pacemaker will generate and to vary a number of other aspects of pacemaker function such as pulse amplitude and duration.
Conventional rate responsive pacemakers utilise a single biological sensor to modulate the pacing rate.
Studies on the clinlcal performance of such pacemakers have revealed that no single sensor system is able to accurately reproduoe the changes in heart rate that are seen in healthy lndivlduals during the per~ormance of normal daily activities. Thus pacing systems that detect movement of the body (e.g. by sensing physical ;~ 25 vibrations) are able to provide :
correctly timed changes in heart rate but do not necessarily provide a response that is proportional to the degree of exercise that the user is undertaking.
Pacing systems that indirectly detect changes in metabolic activity (e,g. by sensing the rate of respiration or the temperature of mixed venous blood) are able to generate changes in heart rate that are more physiologically appropriate in tarms of their magnitude, but these responses may take palce so slowly that they are too late to be of haemodynamic benefit to the user.
European Patent Application 249820 discloses an attempt at solving these problems. The pacemaker disclosed in that document includes several sensors each for sensing a respective different physiological variable and a selector circuit which successively selects different ones of the signals and/or combinations thereof in predetermined time steps after the start of an exeraise ayale. In the example glven, an acceleration sensor determines the start of an exercise perlod and controls the pacing rate during the initial portion of the exercise period which is predetermined and selected to be within the range 10 to 60 noconds. Purlng tho ~lddlo perlod of tho 1~2B29 exerci~e, which is predetermined and selected to be within the range 30 seconds to 20 minutes, a respiration sensor determines pacing rate. In the final period of the exercise which is also predetermined and selected to be within the range 30 seconds to 3 minutes, pacing rate continues to be determined by the respiration sensor. Although the European patent application also indicates that the selected signal and/or combination of signals may also be dependent upon other factors such as predetermined pacing rates, those output signals which fluctuate least, or the falling below a threshold of a single output signal or combination thereof, there is no specific description of what these dependencies might be in practice.
The present inventors consider that a pacemaker constructed as described in European Application 249820 would, in practice, suffer from disadvantages.
For example, during the initial predetermined time period of the exercise, namely from 10 to 60 seconds, pacing rate varies with the magnitude of the slgnal from the acaelerometer and thus, during this per~od, due to the deficiencies in such sensors, the actual pacing rate may differ substantially from the most :
.
appropriate pacing rate. For example, a person running downstairs may produce substantial vibration which would cause the accelerometer to produce a large output signal which in turn would cause the pacemaker to produce a higher pacing rate than actually required whereas a person running upstairs may produce less vibration, resulting in a lower pacing rate than actually required. Further, the predetermining of the lengths of the time periods in an exercise cycle during which different sensor signals are selected will mean that, in practice, where an individual undertakes a variety of different exercise cycles, the switching from one sensor signal to another may take place at inappropriate times.
Hertzschrittmacher, 6~ 1986 pages 64 to 67 contains a report of some studies carried out into the possibility of providing a pacemaker with two sensors, one sensing bodily vibrations and the other central venous blood temperature. Thls study proposes that the advantages of the two systems oan be obtained by utilizlng the oombination of such sensors and proposes that the pacing frequency increase should result from either a blood temperature lnoreas- or a change ln the measured .
1322~29 activity. In effect, thexefore, this system would select a pacing rate equal to the higher of the r~tes indicated by the two sensors. In view of the inaccurate relationship between the magnitude of the signal output by a vibration sensor and required pacing rate, this system would not be satsifactory.
EP 259658 (Intermedics Inc) discloses a further attempt at solving the problem that vibration sensors, whilst responding rapidly to bodily vibrations, do not provide an accurate indication of the pacing rate required whereas sensors which sense such changes in metabolic activity as rate of , respiratlon or mixed venous blood temperature respond slowly but, after response, may provide a signal which accurately indicates an appropriate pacing rate. EP 259658 suggests that filtering the output of a vibration sensor to suppress frequencies above ~; approximately 4Hz will provide a signal which notonly responds rap$dly to changes in bodily vibration but also has a "direct and substantially linear relationship" with the work belng performed by the patient. The pacing rate i9 determined by an algorithm which utilises such a signal and a signal from a second sensor sensing, for example, blood ~ 1322029 temperature. This algorithm involves establishing a number of successively higher thresholds for the amplitude of the filtered signal from the vibration sensor and a number of fixed predetermined successively higher pacing rates associated with each threshold respectively. When the amplitude of the filtered signal from the vibration sensor exceeds a given threshold, the pacing rate is increased (at a predetermined rate of increase) to the respective value and counting of a predetermined time-out period is begun. If the signal from the second sensor (eg blood temperature sensor~ increases within the time-out period to a level indicating that the required pacing rate is higher than the predetermined pacing rate associated with the relevant threshold, further increase in the pacing rate takes place in acsordance with the level of the signal from the second sensor. If, however, the signal from the second sensor fails to increase to such a level within the time-out period, it ls assumed that the lncrease in the filtered vibratlon sen~or slgnal has been in error and a fault recovery program is executed to reduce the paclng rate back to a base level.
13%2029 The algorithm disclosed in EP 25965~ is also such that if the signal from the second sensor increases without the amplitude of the filtered signal from the first sensor having,exceeded its first threshold some increase in pacing rate may take place under control of the signal from the second sensor, but this increase ls limited to a small value. The description in EP 259658 appears to envisage that, under conditions where the amplitude of the filtered signal from the first sensor has exceeded a given one of the thresholds, the further increase in pacing rate which may take place under control of the second signal (provided it has increased sufficiently within the time out period) is limited to a small value so that the higher pacing rates can only be achieved if the filtered vibration sensor signal first exceeds ` successively higher thresholds.
Regardless of whether filtering out fre~uencies above 4Hz in a signal derived from a vibration sensor would actually provide a signal substantially linearly related to the work being performed by a patient as ' suggested in EP 259658, the proposal in this patent suffers from a number of problems.
. , .
1322~29 Firstly, the setting up of even a single threshold is a relatively complicated and skilled operation requiring the patient to undergo selected exercises whilst a skilled technician or physician monitors the operation of the pacemaker in order to enable him to program in an appropriate threshold. ~he setting of a number of successively higher thresholds, therefore, would be extremely complex and time consuming and the present inventors believe that it may in fact be impossible to achieve a satisfactory set of such thresholds in the majority of patients.
Further complexity arises in the need for setting time-out periods and the need for defining the small amounts of increase in pacing rate which may take place if the second signal increases without the amplitude of the filtered vibration sensor signal having flrst exceeded an appropriate threshold.
Secondly, the algorithm discloses in EP 259658 is uch that a number of common activities could not be adequately provided for. One example is any form of exerclse that generates significant body vlbratlons but does not require high levels of energy expendlture e.g. brushing one's teeth. Such a form of exercise will generate high amplitude vibrations ::
~322029 in the frequency range of 1-4 Hz. These will be detected by the activity sensor and cause a rapid rise in pacing rate as a number of vibration threshold levals are exceeded. The complementary sensor will not indicate the need for a pacemaker rate response and will eventually ensure that the pacemaker rate returns to near base levels.
However, this decline in rate will not occur until the termination of the time out period, and will thus not prevent a period of excessive pacing rate occurring which may last for two or three minutes, for example. Another example is any form of exercise that requires a high level of energy expenditure by the patient but does not generate high amplitude vibration signals e.g. swimming. In this situation it is likely that only a lower vibration threshold level will be exceeded, resulting in a small incresse in pacemaker rate. The rate will of course increase further during sustained exertion when the ongoing energy expenditure is detected by the complementary sensor, but it appears that the algorithm of EP 259658 wlll limlt the magnltude o~
this secondary rlse in pacemaker rate if the vibration signal remalns small.
The invention, at least in its preferred embodiment, aims to solve the above problems. The way in which this solution is achieved by the invention will be understood by considering the following description.
In a preferred aspect, the present invention provides a rate responsive pacemaker that incorporates two distinct modalities of biological sensing. These two modalities may be detected by means of two separate sensor systems or they may be derived by electronic processing of the signal from a single sensor. In order to simplify the description of the present invention it will be assumed that two separate sensors are required. Signals from the two sensors will be analysed simultaneously by a dual-channel rate response algorithm. This algorithm is designed to retain the beneficial aspects of the responses provided by each different sensory modality while at the same time minimising their adverse aspects.
The two sensors wlll be termed Sensor 1 and Sensor 2.
It must be emphasised that Sensor 1 can in practlce be any sensor that provides an immediate response to a change in the level of physical exertion being 1 3 2 2 ~ 2 9 performed by the user whereas Sensor 2 can be ~y sensor that accurately determines the magnitude of this change. However, for the purposes of the following description, it will be assumed that Sensor 1 detects body vibration and Sensor 2 measures the rate of respiration.
Sensor 1 serves to deteat whether or not the user is undertaking physical exertion of sufficient magnitude to necessitate an increase in heart rate. During a sustained period of physical inactivity, the lack of a signal from Sensor 1 will keep the entire rate response algorithm in a "deactivated" state, thereby allowing the pacemaker to function "on demand" at a preset minimum rate. In this~ state, signals from Sensor 2 will be ignored and will thus be incapable of initiating "unwanted" increases in pacing rate (e.g.
as may occur with hyperventilation in the absence of physical exertion).
With the onset of physiaal exertion, however, Sensor 1 will detect the inareased amplitude and magnitude of body vibrations and when these exceed a preset threshold value, the rate response algorithm will be activated. A first channel (Channel 1) of the ~322~29 algorithm will initiate an increase in pacing rate.
This response will be of an "all or none" character in that the pacing rate will rise progressively until a predetermined upper rate limit is reached. For the purposes of this document it will be assumed that the response will take the form of a linear increase in rate with a gradient of 1 beat/min/sec. The resting pacing rate may be 70 beats/mln and the upper rate limit should preferably be at least 20 beats/min higher than the resting rate and preferably not more than 30 beats/min higher than the resting rate. It is particularly preferred that the upper rate limit be 25 beats/min higher than the resting rate, namely 95 beats/mln when .the resting rate is 70 beats/min. It should be undertood, however, that these values are given by way of example. It should, however, be understood that a particularly preferred aspect of the invention involves the provision in channel 1 of a single threshold, which threshold is selected to represent an exercise level requiring an increase of at least 20 beast/min relatlve to the restlng rate.
The pacing rate wlll remaln elevated for as long as sensor 1 continues to detect supra-threshold body vlbrations. As described above, the highest pacing rate that can occur solely as a result of activation of Sensor 1 is say 95 beats/min. A provision of the current invention is that the signals detected by Sensor 2, in effect, serve to modulate this upper rate limit.
The output from Sensor 2 will be fed to a second channel (Channel 2) of the rate response algorithm.
This channel will incorporate circuitry that can assess the rate and/or depth of respiration and thereby determine a pacing rate that is physiologically appropriate for the level of exercise being undertaken by the user. If the signals from Sensor 2 are suc~ that a rate of less than the upper rate limit of Channel 1, e.g. 95 beats/min, is indicated, the pacemaker will continue to function at the rate (e.gO 95 beats/min) as determined by Channel 1 of the rate response algorithm. If the signal from Sensor 2 indicates a rate that is higher than the Channel 1 maximum (e.g. 95 beats/min), however, then this rate wlll be adopted by Channel 1 as a revised upper rate limit. The pacemaker rate will rise progressively until this new upper limit is attained. A programmable "maximum upper rate" value ~;~ 25 will be employed to nsure that the pacing rate cannot 1322~29 rise to levels that would adversely influence cardiovascular function and thereb~ have deleterious conse~uence for the user.
A drop in the level of exercise will be detected by Sensor 2 and this will in turn lead to a lower rate being generated by Channel 2 of the algorithm. This will reduce the upper rate limit for Channel 1 and a corresponding drop in pacing rate will follow. If the level of exercise declines to such an extent that the vibration signals detected by Sensor 1 fall below the preset threshold, the entire algorithm will return to the "deactivated" state. In this state, the pacing ! rate will fall progressively until the minimum rate of 70 beatsjmin is reached. It is envisaged that a . .
constant rate o decline of pacing rate will be provided under these circumstances, and a possible value for this rate of decline would be ~.5 beats/min/sec. It therefore follows that the higher the pacing rate ls at the moment of cessation of exercise, the longer it will take for the pacing rate to decline to the resting level.
~ 1322029 Applicants also acknowledge the disclosure of European Patent Application 191404 which discloses a pacemaker utilising an activity sensor (i.e. a vibration sensor) and a physiological sensor. In order to conserve the battery, the physiological sensor and associated circuitry is normally turned completely off. However, if the activity sensor produces a signal higher than a given threshold, then the physiological sensor and associated circuitry is turned on and pacing rate controlled, at all times, in accordance with the magnitude of the slgnal from the physiological sensor.
The sole ob~ect of the arrangement disclosed in European Application 191404 is to conserve the battery and this document does not address the problems with which thè present invention is concerned.
The invention is described further, by way of example, with reference to the accompanying drawings, in which:
Fig. 1 is a funotional block diagram illustrating a preferred embodiment of the present invention, .~
' ~ 1322029 Fig. 2 is a block diagram showing part of the embodiment of Fig. 1 in more detail:
Fig. 3 is a flow chart showing the functions performed in the preferred embodiment of the invention;
Figs. 4 and 5 are graphs illustrating operation of the preferred embodiment of the invention:
and Fig. 6 is a block diagram showing a modification to the preferred embodiment.
,, _ The illustrative implantable rate responsive pacemaker shown in Fig. 1 comprises a hermetically sealed can 1 containing a power source 2 (e.g. a battery), an activity sensor 3, an activity sensor processor 4, , ~ ~ circuitry 5 and 6 defining two channels of a rate response algorithm, and stimulating circuitry 7. A
lead 8 extends through the wall of the can to a pacing electrode (not shown) for applying an electrical stimulus to the heart of the user of the paoemaker. A
; respiratory sensor 9 is situated outside the can and .
.
is connected to a respiratory sensor processor 10 within the pacemaker.
The activity sensor 3 is sensitive to the level of physical vibration within the user's body. It generates an activity signal that is amplified and processed by processing circuitry 4 before being passed to circuitry 5 that constitutes channel l of the rate response algorithm. Under resting conditions, this channel of the algorithm defines a constant minimum pacing rate by means of a pacing control signal that is applied to the stimulation circuitry 7. The value of this minimum pacing rate, and other re~ulred values to be described below, can be selected by means of a telemetry signal from an external programming unit 15. Thus, the can l contains a telemetry receiver and transmitter device 16 having an antenna 17 so that information may be exchanged with the external programmer 15. A read switch 11 is provided in the can 1 for activating the device 9 in ~ response to the proximlty of a magnet 12 provided ln ; the external programmer 15.
:~ :
;:
` ~322~29 When the user parforms physical exercise, the amplitude and frequency of the activity signal will increase. When the amplitude and/or frequency of the signal exceeds a preset threshold value, channel 1 of the rate response algorithm will respond by increasing the rate of the pacing control signal. The activity signal threshold value may be selected by means of a telemetry signal from the external programming unit.
The resulting lncrease in the rate of the pacing control signal will be in the form of an "all or none"
response. The rate will rise progressively from the preset minimum value until a preset upper rate limit ~ is reached (for example, the rate will rise linearly ; 15 by 1 beat/min/sec from 70 beats/min up to 95 beats/min). Having reached the upper rate limit, the paclng control signal of Channel 1 will remain at this level for as long as the activity signal remains above its threshold value.
If the level of physiaal exertion being undertaken by the user declines to ~uch an extent that the actlvity slgnal falls below lts threshold value, the entlre rate response algorithm will return to a "deactivated"
state and the rate of the pacing control signal will fall progressively to the preset minimum value (for example, it will decline linearly by 0.5 beats/min/sec until a rate of 70 beats/min is reached).
The respiratory sensor 9 detects changes in the elctrical impedance of the user's chest wall. Chest wall impedance varies with respiratory movement, and the user's respiratory rate at any given moment is represented by the number of cyclical variations in impedance that are occurring per minute. The signal representing these impedance changes is amplified and processed by sensor processor 4 before being analysed in channel 2 of the rate response algorithm. If the amplitude of this impedance signal falls below a predeter~ined threshold level, it will be rejected by the rate response algorithm. Under these circumstances, the upper rate limit of channel 1 of the algorithm will remain unchanged.
The present invention relates to a cardiac pacemaker.
In the normal healthy individual the output of blood by the heart is varied continuously to meet the metabolic demands of the body. The cardiac output changes with variations in the heart rate and the volume of blood pumped at each heart beat. The cardiac output is the product of these two variables.
During strenuous exercise a four or five fold increase in cardiac output may be required. Approximately 75~
of this increase is achieved by elevation of the heart rate.
~
Patients with cardiac disease resulting in very slow heart rates frequently require treatment in the form ,~ of an implanted artificial pacemaker. This consists of a small electrical pulse generator that is connected to the heart by an insulated electrode lead, usually positioned in the right ventricle.
Until reoently the ma~ority of such paaemakers were relatively simple devices that provided a constant frequency of stimulation. These suffer from the ::
', , .
,::
~:
`
.
1322~29 disadvantage that they are unable to increase the heart rate during exercise, thereby limiting the degree of exertion that the patient is able to undertake. This drawback has led to the development of two forms of pacemaker that can increase stimulation rate during exercise. The "ideal" system is a dual chamber pacemaker which has a second lead implanted in the right atrial chamber of the heart~
This detects the rate of the heart's natural pacemaker and enables the artificial pacemaker to provide normal physiological changes in heart rate. However, such systems are not suitable for many patients and also have inherent technical and economic disadvantages.
An alternative approach that has been developed during recent years is the single chamber rate-responsive pacemaker. The fundamental principles of this device ; are as follows:
(A) A sensor is used to detect changes in a biologlcal variable that are directly or indirectly proportional to the level of metabolic activity ln the body. Evoked QT
interval, respiratory rate, mixed venous ~` 25 temperature and body vibration are the main 1322~29 variables that are sensed by currently available rate-responsive pacemakers.
(B) The output from the sensor is processed by electronic circuitry to provide a signal that can be used to vary the stimulation rate provided by the pacemaker. This circuitry is designed to perform a series of mathematical and logical functions that are known collectively as the rate-response algorithm.
The two principal functions of the rate-responsive algorithm are:
15 i ) To determine the recognition level within the pacemaker for the value of the sensor signal which must be excaeded in order for an increase in pacing rate to be initiated (threshold level).
i : :
~; 20 li) To determine the preoise mathematical relationship between changes in the sensor signal S and the change in pacing rate P to be provided by the pacemaker, (the slope dP/dS).
.
~322~2~
Although every individual requires substantially the same heart rate as other individuals for a given level or type of physical activity, the changes which take place in the sensed,biological variable as a result of a given level of physical activity differ from individual to individual. Accordingly, conventional rate response pacemakers are arranged so that the values of the threshold and slope may be adjusted to suit the individual patient. These values can be selected by telemetered instructions from an external programmer. The programmer is also used to select the minimum and maximum rates that the pacemaker will generate and to vary a number of other aspects of pacemaker function such as pulse amplitude and duration.
Conventional rate responsive pacemakers utilise a single biological sensor to modulate the pacing rate.
Studies on the clinlcal performance of such pacemakers have revealed that no single sensor system is able to accurately reproduoe the changes in heart rate that are seen in healthy lndivlduals during the per~ormance of normal daily activities. Thus pacing systems that detect movement of the body (e.g. by sensing physical ;~ 25 vibrations) are able to provide :
correctly timed changes in heart rate but do not necessarily provide a response that is proportional to the degree of exercise that the user is undertaking.
Pacing systems that indirectly detect changes in metabolic activity (e,g. by sensing the rate of respiration or the temperature of mixed venous blood) are able to generate changes in heart rate that are more physiologically appropriate in tarms of their magnitude, but these responses may take palce so slowly that they are too late to be of haemodynamic benefit to the user.
European Patent Application 249820 discloses an attempt at solving these problems. The pacemaker disclosed in that document includes several sensors each for sensing a respective different physiological variable and a selector circuit which successively selects different ones of the signals and/or combinations thereof in predetermined time steps after the start of an exeraise ayale. In the example glven, an acceleration sensor determines the start of an exercise perlod and controls the pacing rate during the initial portion of the exercise period which is predetermined and selected to be within the range 10 to 60 noconds. Purlng tho ~lddlo perlod of tho 1~2B29 exerci~e, which is predetermined and selected to be within the range 30 seconds to 20 minutes, a respiration sensor determines pacing rate. In the final period of the exercise which is also predetermined and selected to be within the range 30 seconds to 3 minutes, pacing rate continues to be determined by the respiration sensor. Although the European patent application also indicates that the selected signal and/or combination of signals may also be dependent upon other factors such as predetermined pacing rates, those output signals which fluctuate least, or the falling below a threshold of a single output signal or combination thereof, there is no specific description of what these dependencies might be in practice.
The present inventors consider that a pacemaker constructed as described in European Application 249820 would, in practice, suffer from disadvantages.
For example, during the initial predetermined time period of the exercise, namely from 10 to 60 seconds, pacing rate varies with the magnitude of the slgnal from the acaelerometer and thus, during this per~od, due to the deficiencies in such sensors, the actual pacing rate may differ substantially from the most :
.
appropriate pacing rate. For example, a person running downstairs may produce substantial vibration which would cause the accelerometer to produce a large output signal which in turn would cause the pacemaker to produce a higher pacing rate than actually required whereas a person running upstairs may produce less vibration, resulting in a lower pacing rate than actually required. Further, the predetermining of the lengths of the time periods in an exercise cycle during which different sensor signals are selected will mean that, in practice, where an individual undertakes a variety of different exercise cycles, the switching from one sensor signal to another may take place at inappropriate times.
Hertzschrittmacher, 6~ 1986 pages 64 to 67 contains a report of some studies carried out into the possibility of providing a pacemaker with two sensors, one sensing bodily vibrations and the other central venous blood temperature. Thls study proposes that the advantages of the two systems oan be obtained by utilizlng the oombination of such sensors and proposes that the pacing frequency increase should result from either a blood temperature lnoreas- or a change ln the measured .
1322~29 activity. In effect, thexefore, this system would select a pacing rate equal to the higher of the r~tes indicated by the two sensors. In view of the inaccurate relationship between the magnitude of the signal output by a vibration sensor and required pacing rate, this system would not be satsifactory.
EP 259658 (Intermedics Inc) discloses a further attempt at solving the problem that vibration sensors, whilst responding rapidly to bodily vibrations, do not provide an accurate indication of the pacing rate required whereas sensors which sense such changes in metabolic activity as rate of , respiratlon or mixed venous blood temperature respond slowly but, after response, may provide a signal which accurately indicates an appropriate pacing rate. EP 259658 suggests that filtering the output of a vibration sensor to suppress frequencies above ~; approximately 4Hz will provide a signal which notonly responds rap$dly to changes in bodily vibration but also has a "direct and substantially linear relationship" with the work belng performed by the patient. The pacing rate i9 determined by an algorithm which utilises such a signal and a signal from a second sensor sensing, for example, blood ~ 1322029 temperature. This algorithm involves establishing a number of successively higher thresholds for the amplitude of the filtered signal from the vibration sensor and a number of fixed predetermined successively higher pacing rates associated with each threshold respectively. When the amplitude of the filtered signal from the vibration sensor exceeds a given threshold, the pacing rate is increased (at a predetermined rate of increase) to the respective value and counting of a predetermined time-out period is begun. If the signal from the second sensor (eg blood temperature sensor~ increases within the time-out period to a level indicating that the required pacing rate is higher than the predetermined pacing rate associated with the relevant threshold, further increase in the pacing rate takes place in acsordance with the level of the signal from the second sensor. If, however, the signal from the second sensor fails to increase to such a level within the time-out period, it ls assumed that the lncrease in the filtered vibratlon sen~or slgnal has been in error and a fault recovery program is executed to reduce the paclng rate back to a base level.
13%2029 The algorithm disclosed in EP 25965~ is also such that if the signal from the second sensor increases without the amplitude of the filtered signal from the first sensor having,exceeded its first threshold some increase in pacing rate may take place under control of the signal from the second sensor, but this increase ls limited to a small value. The description in EP 259658 appears to envisage that, under conditions where the amplitude of the filtered signal from the first sensor has exceeded a given one of the thresholds, the further increase in pacing rate which may take place under control of the second signal (provided it has increased sufficiently within the time out period) is limited to a small value so that the higher pacing rates can only be achieved if the filtered vibration sensor signal first exceeds ` successively higher thresholds.
Regardless of whether filtering out fre~uencies above 4Hz in a signal derived from a vibration sensor would actually provide a signal substantially linearly related to the work being performed by a patient as ' suggested in EP 259658, the proposal in this patent suffers from a number of problems.
. , .
1322~29 Firstly, the setting up of even a single threshold is a relatively complicated and skilled operation requiring the patient to undergo selected exercises whilst a skilled technician or physician monitors the operation of the pacemaker in order to enable him to program in an appropriate threshold. ~he setting of a number of successively higher thresholds, therefore, would be extremely complex and time consuming and the present inventors believe that it may in fact be impossible to achieve a satisfactory set of such thresholds in the majority of patients.
Further complexity arises in the need for setting time-out periods and the need for defining the small amounts of increase in pacing rate which may take place if the second signal increases without the amplitude of the filtered vibration sensor signal having flrst exceeded an appropriate threshold.
Secondly, the algorithm discloses in EP 259658 is uch that a number of common activities could not be adequately provided for. One example is any form of exerclse that generates significant body vlbratlons but does not require high levels of energy expendlture e.g. brushing one's teeth. Such a form of exercise will generate high amplitude vibrations ::
~322029 in the frequency range of 1-4 Hz. These will be detected by the activity sensor and cause a rapid rise in pacing rate as a number of vibration threshold levals are exceeded. The complementary sensor will not indicate the need for a pacemaker rate response and will eventually ensure that the pacemaker rate returns to near base levels.
However, this decline in rate will not occur until the termination of the time out period, and will thus not prevent a period of excessive pacing rate occurring which may last for two or three minutes, for example. Another example is any form of exercise that requires a high level of energy expenditure by the patient but does not generate high amplitude vibration signals e.g. swimming. In this situation it is likely that only a lower vibration threshold level will be exceeded, resulting in a small incresse in pacemaker rate. The rate will of course increase further during sustained exertion when the ongoing energy expenditure is detected by the complementary sensor, but it appears that the algorithm of EP 259658 wlll limlt the magnltude o~
this secondary rlse in pacemaker rate if the vibration signal remalns small.
The invention, at least in its preferred embodiment, aims to solve the above problems. The way in which this solution is achieved by the invention will be understood by considering the following description.
In a preferred aspect, the present invention provides a rate responsive pacemaker that incorporates two distinct modalities of biological sensing. These two modalities may be detected by means of two separate sensor systems or they may be derived by electronic processing of the signal from a single sensor. In order to simplify the description of the present invention it will be assumed that two separate sensors are required. Signals from the two sensors will be analysed simultaneously by a dual-channel rate response algorithm. This algorithm is designed to retain the beneficial aspects of the responses provided by each different sensory modality while at the same time minimising their adverse aspects.
The two sensors wlll be termed Sensor 1 and Sensor 2.
It must be emphasised that Sensor 1 can in practlce be any sensor that provides an immediate response to a change in the level of physical exertion being 1 3 2 2 ~ 2 9 performed by the user whereas Sensor 2 can be ~y sensor that accurately determines the magnitude of this change. However, for the purposes of the following description, it will be assumed that Sensor 1 detects body vibration and Sensor 2 measures the rate of respiration.
Sensor 1 serves to deteat whether or not the user is undertaking physical exertion of sufficient magnitude to necessitate an increase in heart rate. During a sustained period of physical inactivity, the lack of a signal from Sensor 1 will keep the entire rate response algorithm in a "deactivated" state, thereby allowing the pacemaker to function "on demand" at a preset minimum rate. In this~ state, signals from Sensor 2 will be ignored and will thus be incapable of initiating "unwanted" increases in pacing rate (e.g.
as may occur with hyperventilation in the absence of physical exertion).
With the onset of physiaal exertion, however, Sensor 1 will detect the inareased amplitude and magnitude of body vibrations and when these exceed a preset threshold value, the rate response algorithm will be activated. A first channel (Channel 1) of the ~322~29 algorithm will initiate an increase in pacing rate.
This response will be of an "all or none" character in that the pacing rate will rise progressively until a predetermined upper rate limit is reached. For the purposes of this document it will be assumed that the response will take the form of a linear increase in rate with a gradient of 1 beat/min/sec. The resting pacing rate may be 70 beats/mln and the upper rate limit should preferably be at least 20 beats/min higher than the resting rate and preferably not more than 30 beats/min higher than the resting rate. It is particularly preferred that the upper rate limit be 25 beats/min higher than the resting rate, namely 95 beats/mln when .the resting rate is 70 beats/min. It should be undertood, however, that these values are given by way of example. It should, however, be understood that a particularly preferred aspect of the invention involves the provision in channel 1 of a single threshold, which threshold is selected to represent an exercise level requiring an increase of at least 20 beast/min relatlve to the restlng rate.
The pacing rate wlll remaln elevated for as long as sensor 1 continues to detect supra-threshold body vlbrations. As described above, the highest pacing rate that can occur solely as a result of activation of Sensor 1 is say 95 beats/min. A provision of the current invention is that the signals detected by Sensor 2, in effect, serve to modulate this upper rate limit.
The output from Sensor 2 will be fed to a second channel (Channel 2) of the rate response algorithm.
This channel will incorporate circuitry that can assess the rate and/or depth of respiration and thereby determine a pacing rate that is physiologically appropriate for the level of exercise being undertaken by the user. If the signals from Sensor 2 are suc~ that a rate of less than the upper rate limit of Channel 1, e.g. 95 beats/min, is indicated, the pacemaker will continue to function at the rate (e.gO 95 beats/min) as determined by Channel 1 of the rate response algorithm. If the signal from Sensor 2 indicates a rate that is higher than the Channel 1 maximum (e.g. 95 beats/min), however, then this rate wlll be adopted by Channel 1 as a revised upper rate limit. The pacemaker rate will rise progressively until this new upper limit is attained. A programmable "maximum upper rate" value ~;~ 25 will be employed to nsure that the pacing rate cannot 1322~29 rise to levels that would adversely influence cardiovascular function and thereb~ have deleterious conse~uence for the user.
A drop in the level of exercise will be detected by Sensor 2 and this will in turn lead to a lower rate being generated by Channel 2 of the algorithm. This will reduce the upper rate limit for Channel 1 and a corresponding drop in pacing rate will follow. If the level of exercise declines to such an extent that the vibration signals detected by Sensor 1 fall below the preset threshold, the entire algorithm will return to the "deactivated" state. In this state, the pacing ! rate will fall progressively until the minimum rate of 70 beatsjmin is reached. It is envisaged that a . .
constant rate o decline of pacing rate will be provided under these circumstances, and a possible value for this rate of decline would be ~.5 beats/min/sec. It therefore follows that the higher the pacing rate ls at the moment of cessation of exercise, the longer it will take for the pacing rate to decline to the resting level.
~ 1322029 Applicants also acknowledge the disclosure of European Patent Application 191404 which discloses a pacemaker utilising an activity sensor (i.e. a vibration sensor) and a physiological sensor. In order to conserve the battery, the physiological sensor and associated circuitry is normally turned completely off. However, if the activity sensor produces a signal higher than a given threshold, then the physiological sensor and associated circuitry is turned on and pacing rate controlled, at all times, in accordance with the magnitude of the slgnal from the physiological sensor.
The sole ob~ect of the arrangement disclosed in European Application 191404 is to conserve the battery and this document does not address the problems with which thè present invention is concerned.
The invention is described further, by way of example, with reference to the accompanying drawings, in which:
Fig. 1 is a funotional block diagram illustrating a preferred embodiment of the present invention, .~
' ~ 1322029 Fig. 2 is a block diagram showing part of the embodiment of Fig. 1 in more detail:
Fig. 3 is a flow chart showing the functions performed in the preferred embodiment of the invention;
Figs. 4 and 5 are graphs illustrating operation of the preferred embodiment of the invention:
and Fig. 6 is a block diagram showing a modification to the preferred embodiment.
,, _ The illustrative implantable rate responsive pacemaker shown in Fig. 1 comprises a hermetically sealed can 1 containing a power source 2 (e.g. a battery), an activity sensor 3, an activity sensor processor 4, , ~ ~ circuitry 5 and 6 defining two channels of a rate response algorithm, and stimulating circuitry 7. A
lead 8 extends through the wall of the can to a pacing electrode (not shown) for applying an electrical stimulus to the heart of the user of the paoemaker. A
; respiratory sensor 9 is situated outside the can and .
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is connected to a respiratory sensor processor 10 within the pacemaker.
The activity sensor 3 is sensitive to the level of physical vibration within the user's body. It generates an activity signal that is amplified and processed by processing circuitry 4 before being passed to circuitry 5 that constitutes channel l of the rate response algorithm. Under resting conditions, this channel of the algorithm defines a constant minimum pacing rate by means of a pacing control signal that is applied to the stimulation circuitry 7. The value of this minimum pacing rate, and other re~ulred values to be described below, can be selected by means of a telemetry signal from an external programming unit 15. Thus, the can l contains a telemetry receiver and transmitter device 16 having an antenna 17 so that information may be exchanged with the external programmer 15. A read switch 11 is provided in the can 1 for activating the device 9 in ~ response to the proximlty of a magnet 12 provided ln ; the external programmer 15.
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` ~322~29 When the user parforms physical exercise, the amplitude and frequency of the activity signal will increase. When the amplitude and/or frequency of the signal exceeds a preset threshold value, channel 1 of the rate response algorithm will respond by increasing the rate of the pacing control signal. The activity signal threshold value may be selected by means of a telemetry signal from the external programming unit.
The resulting lncrease in the rate of the pacing control signal will be in the form of an "all or none"
response. The rate will rise progressively from the preset minimum value until a preset upper rate limit ~ is reached (for example, the rate will rise linearly ; 15 by 1 beat/min/sec from 70 beats/min up to 95 beats/min). Having reached the upper rate limit, the paclng control signal of Channel 1 will remain at this level for as long as the activity signal remains above its threshold value.
If the level of physiaal exertion being undertaken by the user declines to ~uch an extent that the actlvity slgnal falls below lts threshold value, the entlre rate response algorithm will return to a "deactivated"
state and the rate of the pacing control signal will fall progressively to the preset minimum value (for example, it will decline linearly by 0.5 beats/min/sec until a rate of 70 beats/min is reached).
The respiratory sensor 9 detects changes in the elctrical impedance of the user's chest wall. Chest wall impedance varies with respiratory movement, and the user's respiratory rate at any given moment is represented by the number of cyclical variations in impedance that are occurring per minute. The signal representing these impedance changes is amplified and processed by sensor processor 4 before being analysed in channel 2 of the rate response algorithm. If the amplitude of this impedance signal falls below a predeter~ined threshold level, it will be rejected by the rate response algorithm. Under these circumstances, the upper rate limit of channel 1 of the algorithm will remain unchanged.
2~ However, if the amplitude of the impedance signal exceeds the threshold value, channel 2 of the algorithm will respond and inorease its output rate.
The mathematiaal relationshlp between the change in the frequenoy of the respiratory sensor signal (and thus a change in the rate of respiration) and the `:~
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subsequent change in algorithm output rate is set by a predetermined slope value. Values for both the respiratory threshold and the respiratory slope can be selected using telemetry signals from the external programming unit. In clinical use, values will be selected by the physician so that channel 2 of the algorithm will produce an output rate that is physiologically appropriate for the user's rate of respiration (and hence appropriate for the level of physical exercise he is undertaking).
If the channel 2 output rate is less than the initial upper rate limit of channel 1 of the algorithm (for , example, less than 95 beats/min), then the upper rate limit of channel 1 will remain unchanged. If the channel 2 output rate exceeds this value, however, then the upper rate limit of channel 1 will be raised accordingly. Thus, in effect, the upper rate limit of channel 1 of the rate response algorithm is modulated by the output of channel 2 of the algorithm. There will be a ceillng rate that the upper rate limit cannot exceed, and this value will be selected by a telemetry signal from the external programmer.
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The pacing control signal produced by channel 1 of the rate response algorithm is fed to conventional pacemaker stimulation circuitry 7. Sensing circuitry 7a is employed to monitor the user's intrinsic cardiac behaviour. If the user's heart beats spontaneously at a rate higher than that determined by the pacing control signal, the output of the pacemaker stimulation circuitry will be inhibited. The pacemaker will thus function "on demand" in that it will only stimulate the user's heart to beat if the inherent natural heart rate falls below the physiologically appropriate rate (as determined by the rate response algorithm).
Stimulation circuitry 7 is employed to deliver impulses of predetermined amplitude and duration to the user's heart via a conventional unipolar or bipolar transvenous pacing lead 8. The rate at which these impulses are delivered is determined by the pacing control signal and the timing of impulse delivery may be influenced by the pacemaker sensing oircuitry.
i322029 The rate response algorithm circuitry 5, 6 may be switched into a permanently "deactivated" state by means of a telemetered instruction from the external programming unit 15. In this state the incoming signals from both biological sensors will be ignored and the pacemaker will continue to function "on demand" at the praset minimum pacing rate.
The telemetry reception/transmission circuitry 9 may be conventional and will be used to control the following aspects of pacemaker function:
(a) Mode, namely "Demand,Fixed Rate" or "Demand,Rate Responsive".
(b3 Minimum pacing rate - for both modes.
(c) Maximum pacing rate - for "Rate Response" mode only.
(d) Activity sensor threshold value.
(e) Respiratory sensor threshold value.
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(g) Pacemaker stimulation and sensing parameters -including s*imulus amplitude and duration, refractory period, and sensitivity.
(h) Telemetry function - to enable the pacemaker to transmit the values of all programmable parameters out to the external programmer.
Fig. 2 illustrates in more detail the circuitry enclosed within broken line 20 of Pig. 1, in accordance with a preferred embodiment of the invention. As ~hown in Fig. 2, the signal from the activity sensor 3, after processing in circuitry 4, is applied to circuitry 21 whiah may be in accordance with US Patent 4,428,378 (Anderson et al), and thus may comprise a band pass amplifier 22, level detector 24 and zero crossing detector 26.
If the amplitude of the signal supplied by the amplifier 22 to the level detector 24 exceeds a threshold defined by a 8~ gnal supplied on line 27 to cirauitry 21 from the telemetry recelver and tr~n~mittor circuitry 16, zero cro~ing detoctor 26 ' :
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- 27 i 1322029 outputs a series of pulses at a frequency indicative of the level of activity sensed by the activity sensor 3. These output pulses are supplied to a rate-response logic and control circuit 28.
~he signal from respiratory sensor 9 and processing circuitry 10 is applied to circuitry 30 definlng the respiration rate response algorithm. This circuitry may be in accordance with European Patent Application 89014, and has stored therein respiratory threshold and slope values received from the telemetry device 9. The output of circuitry 30 is also supplied to the rate-resppnsive logic and control circuit 28.
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Circuit 28 has stored therein values for minimum and maximum pacing rate received from the telemetry device 9 and i8 operative to perform the above described functionQ. These functions may be provided by hardware or software or a combination of both but, in the present embodiment, it will be assumed that circuit 28 includes a microprocessor which is programmed to carry out the required functions in accordanae wlth the flow ohart shown in Fig 3.
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In Fig. 3, the frequency of the signals from circuit 21 is represented by A and T represents the threshold frequency thereof which defines the level of activity above which pacing rate will be increased. The actual current pacing rate is represented by P and the signal output by circuit 30, representing the pacing rate indicated by the respiratory sensor, is represented by R.
At step 1 in Fig. 3, the microprocessor determines whether the frequency of activity signal A is greater than the threshold T. In this embodiment, this determination is preferably made by counting the number of pulses.received by circuitry 28 from zero crossing detector 26 in a preselected period of time.
In this way, circuitry 28 determines whether the fre~uency of the pulses output by sensor 4 having an amplitude above the level set by level detector 24 is greater than the selected threshold frequency T. If A
ls less than T, step 2 determines whether the pacing rate is greater than the minimum paoing rate stored in oirauitry 28. If it ls not, the program returnq to step 1 without changing the pacing rate. If it is, the program moves to step 3 which initlates a decrease in the pacing rate, at a fixed rate such as 0.5 beats/min/sec. Following this initiation, the program returns to step 1 and the decrease in P
continues, at the fixed rate, with the program moving through steps 1, 2 and 3 until such time as step 2 in~icates that P is not greater than the minimum. At this time, the decrease is stopped and the program loops to step 1.
If, at any time, step 1 indicates that A is greater than T, the program branches to step 4 which determines whether the actual pacing rate is less than a predetermined level stored in circuitry 28 and indicated as being 95 beats/min. If step 4 indicates that P is less t~an 95 beats/min, the program moves to step 5 ~which initiates an increase in P at a fixed rate, such as 1 beat/min/sec. Following this initiation, the program loops back to step 1 and continues to move through steps 1, 4 and 5 until such time as step 4 indicates that P is not less than 9S.
At this time, the program branahes to step 6 whioh determines whether the actual pacing rate is greater than the rate indiaated by the respiratory sensor and circuitry 30. Normally, at the beginning of a period of exercise, there will be some delay be~ore signal R
i~crea~es beyond the level indicative of 95 beats/min ` 1~22029 so that, initially, it will be expected that step 6 will result in a "yes" and the program will move to step 6A which determines whether P is greater than 95 beats/min. As P will not have risen above 95 beats/min, this step will give a "no" and the program will continue to loop through steps 1,4,6 and 6A
without further change of P. With sustained heavy exertion, R will increase until it exceeds 95 beats/min. When this occurs, step 6 will result in a "no", and the program will move to step 7. Step 7 is included to ensure that when P = R, the program will loop through steps 1, 4, 6 and 7 without changing P.
If R is greater than P, however, step 7 will result in a "yes". The program will move to step 8, where it is determinëd whether P is less than its permitted maximum value as stored in circuitry 28. If P is less than the maximum permitted value (as would be expected during the early stages of exercise), the program moves to step 5 whereby P is increased at the above mentioned fixed rate.
The program will contlnue to move through steps 1, 4, 6, 7, 8 and 5 in sequence until such time as steps 6 and 7 indicate that P ls equal to R. When this condition occurs, the program will loop through steps 1, 4, 6 and 7 without further increase and without decrease in the actual pacing rate. If the level of physical activity is further increased so that R
increases, step 7 will result in a "yes" and, so long as step 8 does not indicate that P has reached its maximum permitted value, a further increase in P, at the fixed rate above mentioned, will be initiated in step 5. If at any time, step 8 indicates that P has reached its maximum permitted value, further increases in R would not result ln any further increase in P. If at any time, the level of physical activity decreasas such that R becomes less than P, but does not decrease to an extent that A becomes less than T, the program will branch from step 6 through step 6A to step 3 so as to aecrease the pacing rate P at the above mentioned fixed rate. This will continue until the rate P falls to 95 beats/min. At this time, step 6A
wlll result in a "no" and step 3 will thereby be omitted from the loop, preventing a further decline in .1 P.
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The above sequence ls continued until suoh tlme as the fre~uency A falls below the threshold T and/or the circult 21 ceases to output pulses, due to the amplitude of the signal from amplifier 22 falling :
~.322029 below the threshold stored in level detector circuity 24. As a result, even though R may remain at a high level, the program moves from step 1 to step 2 at which it will be determined that P is greater than its minimum value and the program will then move to step 3, and loop through steps 1, 2 and 3, so as to reduce the pacing rate at the above mentioned fixed rate of reduction until such time as step 2 indicates that P
is not greater than the minimum, whereafter the program will sequence through steps 1 and 2 until A
again exceeds T.
It will be understood from the above description that pacing rate is only increased if both the amplitude of the signals from the activity sensor 3 exceeds a threshold defined by the level detector circuit 24 and the frequency of those signals exceeds the threshold frequency T. Thus, an advantage of the preferred embodiment of the invention is that undesired pacing rate increases do not arise either from isolated or low frequency signals of relatively large amplitude from the aatlvity sensor 3 or from low amplitude relatively high frequency signals from the sensor 3.
i322029 It will be understood from consideration of the flow chart of Fig 3 and the foregoing description that, after the pacing rate has been increased to the present level (95 beats/min in the example given) as a result of A exceeding T, all further increase in pacing rate is dependent solely upon the level of signal R derived from the second sensor. By this means, activities such as swimming may be adequately provided for since swimming may produce sufficient vibration for A to exceed T but would not produce sufficient vibration for A to exceed a significantly higher threshold. Further, activities such as brushing teeth producing high amplitude signals from the activity sensor would not result in an excessive increase in heart rate, which would be undesirable, since there would be no appreciable increase in the signal R derived from the second sensor. However, in the proposal in EP 259658, the high amplitude signals from the activity sensor would be expected to exceed several thresholds and therefore cause an excessive and undesired increase in heart rate. Further, with the present invention, upon the rapid onset of heavy exercise, the selected increase in paclng rate (25 beats/min giving a pacing rate of 95 beats/min in the example) Day be such as to enable the patient to cope ':
with such exercise for a period long enough to permit the signal from sensor 2 to catch up and then further increase the pacing rate as required. Since, in the embodiment under discussion, there is no time-out period within which the signal from sensor 2 must increase to any particular level, there is no risk of deactivating the algorithm with resultant reduction in heart rate as may arise in the arrangement disclosed in EP 259658 upon rapid onset of heavy exercise due to the inclusion of the timeout period.
Fig. 4 illustrates graphically the operation of the pacemaker according to the preferred embodiment as so far described and, by way of an example, illustrates a 10 minut~ period involving both periods of activity - and of rest.
As shown in curve I of Fig. 4, the patient undergoes a relatively low level of exerclse during the first minute, rests durlng the next two minutes, undergoes a much higher level of exercise during the following - three minutes and thereafter returns to a condltlon of rest.
` 1322029 As shown by curve II, the level of exercise in each period is sufficient for the signal from the activity sensor and associated circuitry to exceed its threshold level.
There is a delay following the beginning of an exercise period before the patient's rate of respiration increases. Curve III shows that the first period of exercise, during which the exercise level is relatively low, is not long enough to cause the respiration rate to increase at all but the second period of exercise causes the respiration rate to increase, the increase beginning after a delay of about three-quarters of a minute from the beginning of the period of exerclse.
., Curve IV illustrates the pacing rate which the output of circuitry 30 indicates as appropriate. In view of the above mentioned delay, circuitry 30 does not indlcate any increase in pacing rate during the first period of exercise and thus, if this alone were relied upon to control the pacemaker, no increase in heart rate at thi~ time would result. This would be disadvantageous. Simllarly, curve IV indicates that there is delay of about three-quarters of a minute i322029 after the beginning of ths second period of exercise before circuitry 30 begins to indicate that the heart rate should be increased. Again, if the heart rate were not to be increased during this delay period, that would be disadvantageous.
Curve V shows that, despite the absence of a signal from circuitry 30 indicating that the heart rate should be increased during the first period of exercise, the pacing rate is nevertheless increased at a fixed rate as indicated at reference number 40.
This increase arises from the program sequencing through steps 1, 4 and 5 illustrated in Fig. 3. As indicated by reference number 42 in curve 5, the pacing rate increases to a maximum level of 95 beats/min during the first period of exercise, this being achieved by the program sequencing through steps 1, 4, 6 and 6A.
Reference number 44 in curve V indicates that after the activity signal has fallen below the threshold, the paaing rate i8 reduced at the above mentloned fixed rate. This is achleved by the program seguencing through steps 1, 2 and 3 until the minimum rate of 70 beats/min is reached.
1322~29 Reference number 46 in curve V shows that immediately the second period of activity begins, the pacing rate increases at the above mentioned fixed rate to 95 beats/min, as shown by reference number 48 and remains at that rate until there has been sufficient time for signal R to increase to a level indicating a rate greater than 95 beats/min, this taking about 1 1/2 minutes in the example shown from the beginning of the second period of exercise.
Reference number 50 in curve V indicates that, after this point, the pacing rate P is increased, again at the same fixed rate as previously described, in response to the continuing increase in signal R.
During the period corresponding to reference number 50, the program of Fig. 3 se~uences through steps 1, 4, 6, 7, 8 and 5.
: At some point, the signal R reaches the maximum value appropriate to the level of exercise being undertaken ~ :~ and, as indicated by reference number 52 in curve V, ; the paclng rate P is maintained oonstant at this maximum value. Thl9 is achleved by the program shown in Fig. 3 sequencing through steps 1, 4, 6 and 7.
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As shown by reference number 54 in curve V, immediately the period of exercise terminates, the pacing rate begins to decrease at the above mentioned fixed rate since the activity signal drops below the threshold and the program sequences through steps 1, 2 and 3. This decreass takes place despite the fact that there is a delay before the respiration rate, and therefore signal R, decreases. Thus, the preferred embodiment of the invention provides a substantial advantage that the paclng rate begins to decrease immed~ately the exercise terminates rather than remaining at a high rate until such time as the respiration rate has decreased.
Fig. 5 is included to illustrate, with curves I to V
which ha~e the same meaning as curves I to V of Fig.
4, that from time to time, although the patient may not undergo any physical activity of significance so that there is no increase in the activity si~nal, respiration rate may nevertheless increase to a level such that the signal from clrcuitry 30 lndioates that the paalng rate should be lncreased. However, ln these oircumstances, paclng rate lncrease ls inapproprlate and, as shown in Fig. 5, it does not in fact take place in the preferred embodiment of the invention since the program will remain sequencing through steps 1 and 2 until such time as the actlvity signal exceeds its threshold and regardless of the level of signal R.
Thus, in the invention the pacing rate is always substantially at the required level, any changes required begin to take place without any undesirable delay and the rate of change may be preselected and therefore selected to be optimum for increasing and optimum for decreasing rate. This combination of advantages is not achievable in the prior art.
In the modification shown in Fig. 6, the respiratory sensor 9 is omitted and instead a single activity or vibration sensor 3 is employed. The output from this sensor is applied to a band pass amplifler 60 centred at 10 Hz to provide an activity signal which is then supplied to sensor processor 10 and is also applied to a low pass amplifier 62 having a pass band of zero to 0.5 Hz to provide a signal representing respiration rate whlah is applied to proaessing alrauit 10. The sensor 3 may be as described in US Patent 4,428,378 and the airouitry shown in Fig. 6 enables that slngle sensor to provide both the activity signal and the .
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~322029 respiratory signal, any unwanted cardiac signal being rejected. The modified embodiment illustrated in Fig.
6 ls otherwise the same as the embodiment described with reference to Figs. 1 to 5.
As will be appreciated, various modifications may be made within the scope of the invention. The various figures for increase or decrease of rate and maximum and minimum rate are merely given by way of illustration. For example, although the minimum rate has been given as 70 beats/min, it would be possible within the scope of the invention to provide for an extra reduction in rate to say 60 beats/min, where the 3 patient is sleeping, this condition, for example, being i~dicated by the absence of any activity signal for a predetermined period of time. Thus, the reference to a minimum rate above should be understood as meaning the normal minimum rate, such as 70 beats/min. It is also within the scope of the invention for other values between the limits of say 30 to 100 beats/min to be chosen and telemetred into the pacemaker using the external programmer 15, the sntenna 17 and receiving clrcuitry 16.
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Further, although in the embodiment described with reference of the accompanying drawings, it has been assumed that detection of the frequency of signal A
has been carried out by counting the number of pulses generated by zero crossing detector 26 in a predetermined period, other methods are possible. For example, the pulses output by the band pass amplifier could be integrated over a predetermined period and compared to an appropriate threshold. -~
In the above description, it has been indicated that the band pass amplifier 22 may pass signals within a band centred at lOHz. Typically, the bandwidth of this amplifier might be say 6Hz whereby signals in the range 7Hz to 13Hz would be passed although alternative arrangements are possible, for example a band width of 4Hz whereby signals in the range 8 to 12Hz would be passed. It is not essential that the centre frequency of the band pass amplifier should be lOHz although this is a particularly preferred value. It might, for example, be lower or hlgher. The processing performed on the signal from acitivity sensar 4 should be such that the processed signal may be aompared with a threshold which corresponds to a level of activity for whlch the approprlate pacing rate is preferably at 1322~2~
least 20 beats per minute higher than the resting rate, for example 20 to 30 beats per minute higher than the resting rate and particularly preferably about 25 beats per minute higher than the resting rate. Thus a particularly preferred combination of features in accordance with the invention involves the detection of signals from the vibration sensor 4 having an amplitude above a predetermined level and a frequency of at least 7 or 8Hz, and preferably lO~z, the circuitry causing an increase in heart rate of between 20 and 30 beats per minute, preferably 25 beats per minute above resting rate in response to the detection of such signals, further increases beyond this rate being under the sole control of the level of the signal from the second sensor.
The pacemaker according to the invention may be arranged for operation in any of a variety of different modes. For example, it may be used in the so-called AAI mode in which both pacing and sensing take place in the atrium and pacing pulses are inhibited if spontaneous aotlvity ls ~ensed, ln the so-aalled WI mode ln which both paalng and sensing take place in the ventricle and pacing pulses are inhibited when spontaneous activity is sensed or the ~ 1322029 so-called DDD mode in which pacing and sensing take place in both the atrium and the ventricle with inhibition of the pacing pulses or triggering taking place in accordance with sensed activity, in a well-known manner.
Further, although in the preferred embodiment as described, the change in pacing rate takes place linearly w1th time, it i8 possible within the scope of the invention for the changes in pacing rate to take place non-linearly and it is possible for the changes to be made continuously or step-wise.
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The mathematiaal relationshlp between the change in the frequenoy of the respiratory sensor signal (and thus a change in the rate of respiration) and the `:~
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subsequent change in algorithm output rate is set by a predetermined slope value. Values for both the respiratory threshold and the respiratory slope can be selected using telemetry signals from the external programming unit. In clinical use, values will be selected by the physician so that channel 2 of the algorithm will produce an output rate that is physiologically appropriate for the user's rate of respiration (and hence appropriate for the level of physical exercise he is undertaking).
If the channel 2 output rate is less than the initial upper rate limit of channel 1 of the algorithm (for , example, less than 95 beats/min), then the upper rate limit of channel 1 will remain unchanged. If the channel 2 output rate exceeds this value, however, then the upper rate limit of channel 1 will be raised accordingly. Thus, in effect, the upper rate limit of channel 1 of the rate response algorithm is modulated by the output of channel 2 of the algorithm. There will be a ceillng rate that the upper rate limit cannot exceed, and this value will be selected by a telemetry signal from the external programmer.
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The pacing control signal produced by channel 1 of the rate response algorithm is fed to conventional pacemaker stimulation circuitry 7. Sensing circuitry 7a is employed to monitor the user's intrinsic cardiac behaviour. If the user's heart beats spontaneously at a rate higher than that determined by the pacing control signal, the output of the pacemaker stimulation circuitry will be inhibited. The pacemaker will thus function "on demand" in that it will only stimulate the user's heart to beat if the inherent natural heart rate falls below the physiologically appropriate rate (as determined by the rate response algorithm).
Stimulation circuitry 7 is employed to deliver impulses of predetermined amplitude and duration to the user's heart via a conventional unipolar or bipolar transvenous pacing lead 8. The rate at which these impulses are delivered is determined by the pacing control signal and the timing of impulse delivery may be influenced by the pacemaker sensing oircuitry.
i322029 The rate response algorithm circuitry 5, 6 may be switched into a permanently "deactivated" state by means of a telemetered instruction from the external programming unit 15. In this state the incoming signals from both biological sensors will be ignored and the pacemaker will continue to function "on demand" at the praset minimum pacing rate.
The telemetry reception/transmission circuitry 9 may be conventional and will be used to control the following aspects of pacemaker function:
(a) Mode, namely "Demand,Fixed Rate" or "Demand,Rate Responsive".
(b3 Minimum pacing rate - for both modes.
(c) Maximum pacing rate - for "Rate Response" mode only.
(d) Activity sensor threshold value.
(e) Respiratory sensor threshold value.
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(g) Pacemaker stimulation and sensing parameters -including s*imulus amplitude and duration, refractory period, and sensitivity.
(h) Telemetry function - to enable the pacemaker to transmit the values of all programmable parameters out to the external programmer.
Fig. 2 illustrates in more detail the circuitry enclosed within broken line 20 of Pig. 1, in accordance with a preferred embodiment of the invention. As ~hown in Fig. 2, the signal from the activity sensor 3, after processing in circuitry 4, is applied to circuitry 21 whiah may be in accordance with US Patent 4,428,378 (Anderson et al), and thus may comprise a band pass amplifier 22, level detector 24 and zero crossing detector 26.
If the amplitude of the signal supplied by the amplifier 22 to the level detector 24 exceeds a threshold defined by a 8~ gnal supplied on line 27 to cirauitry 21 from the telemetry recelver and tr~n~mittor circuitry 16, zero cro~ing detoctor 26 ' :
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- 27 i 1322029 outputs a series of pulses at a frequency indicative of the level of activity sensed by the activity sensor 3. These output pulses are supplied to a rate-response logic and control circuit 28.
~he signal from respiratory sensor 9 and processing circuitry 10 is applied to circuitry 30 definlng the respiration rate response algorithm. This circuitry may be in accordance with European Patent Application 89014, and has stored therein respiratory threshold and slope values received from the telemetry device 9. The output of circuitry 30 is also supplied to the rate-resppnsive logic and control circuit 28.
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Circuit 28 has stored therein values for minimum and maximum pacing rate received from the telemetry device 9 and i8 operative to perform the above described functionQ. These functions may be provided by hardware or software or a combination of both but, in the present embodiment, it will be assumed that circuit 28 includes a microprocessor which is programmed to carry out the required functions in accordanae wlth the flow ohart shown in Fig 3.
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In Fig. 3, the frequency of the signals from circuit 21 is represented by A and T represents the threshold frequency thereof which defines the level of activity above which pacing rate will be increased. The actual current pacing rate is represented by P and the signal output by circuit 30, representing the pacing rate indicated by the respiratory sensor, is represented by R.
At step 1 in Fig. 3, the microprocessor determines whether the frequency of activity signal A is greater than the threshold T. In this embodiment, this determination is preferably made by counting the number of pulses.received by circuitry 28 from zero crossing detector 26 in a preselected period of time.
In this way, circuitry 28 determines whether the fre~uency of the pulses output by sensor 4 having an amplitude above the level set by level detector 24 is greater than the selected threshold frequency T. If A
ls less than T, step 2 determines whether the pacing rate is greater than the minimum paoing rate stored in oirauitry 28. If it ls not, the program returnq to step 1 without changing the pacing rate. If it is, the program moves to step 3 which initlates a decrease in the pacing rate, at a fixed rate such as 0.5 beats/min/sec. Following this initiation, the program returns to step 1 and the decrease in P
continues, at the fixed rate, with the program moving through steps 1, 2 and 3 until such time as step 2 in~icates that P is not greater than the minimum. At this time, the decrease is stopped and the program loops to step 1.
If, at any time, step 1 indicates that A is greater than T, the program branches to step 4 which determines whether the actual pacing rate is less than a predetermined level stored in circuitry 28 and indicated as being 95 beats/min. If step 4 indicates that P is less t~an 95 beats/min, the program moves to step 5 ~which initiates an increase in P at a fixed rate, such as 1 beat/min/sec. Following this initiation, the program loops back to step 1 and continues to move through steps 1, 4 and 5 until such time as step 4 indicates that P is not less than 9S.
At this time, the program branahes to step 6 whioh determines whether the actual pacing rate is greater than the rate indiaated by the respiratory sensor and circuitry 30. Normally, at the beginning of a period of exercise, there will be some delay be~ore signal R
i~crea~es beyond the level indicative of 95 beats/min ` 1~22029 so that, initially, it will be expected that step 6 will result in a "yes" and the program will move to step 6A which determines whether P is greater than 95 beats/min. As P will not have risen above 95 beats/min, this step will give a "no" and the program will continue to loop through steps 1,4,6 and 6A
without further change of P. With sustained heavy exertion, R will increase until it exceeds 95 beats/min. When this occurs, step 6 will result in a "no", and the program will move to step 7. Step 7 is included to ensure that when P = R, the program will loop through steps 1, 4, 6 and 7 without changing P.
If R is greater than P, however, step 7 will result in a "yes". The program will move to step 8, where it is determinëd whether P is less than its permitted maximum value as stored in circuitry 28. If P is less than the maximum permitted value (as would be expected during the early stages of exercise), the program moves to step 5 whereby P is increased at the above mentioned fixed rate.
The program will contlnue to move through steps 1, 4, 6, 7, 8 and 5 in sequence until such time as steps 6 and 7 indicate that P ls equal to R. When this condition occurs, the program will loop through steps 1, 4, 6 and 7 without further increase and without decrease in the actual pacing rate. If the level of physical activity is further increased so that R
increases, step 7 will result in a "yes" and, so long as step 8 does not indicate that P has reached its maximum permitted value, a further increase in P, at the fixed rate above mentioned, will be initiated in step 5. If at any time, step 8 indicates that P has reached its maximum permitted value, further increases in R would not result ln any further increase in P. If at any time, the level of physical activity decreasas such that R becomes less than P, but does not decrease to an extent that A becomes less than T, the program will branch from step 6 through step 6A to step 3 so as to aecrease the pacing rate P at the above mentioned fixed rate. This will continue until the rate P falls to 95 beats/min. At this time, step 6A
wlll result in a "no" and step 3 will thereby be omitted from the loop, preventing a further decline in .1 P.
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The above sequence ls continued until suoh tlme as the fre~uency A falls below the threshold T and/or the circult 21 ceases to output pulses, due to the amplitude of the signal from amplifier 22 falling :
~.322029 below the threshold stored in level detector circuity 24. As a result, even though R may remain at a high level, the program moves from step 1 to step 2 at which it will be determined that P is greater than its minimum value and the program will then move to step 3, and loop through steps 1, 2 and 3, so as to reduce the pacing rate at the above mentioned fixed rate of reduction until such time as step 2 indicates that P
is not greater than the minimum, whereafter the program will sequence through steps 1 and 2 until A
again exceeds T.
It will be understood from the above description that pacing rate is only increased if both the amplitude of the signals from the activity sensor 3 exceeds a threshold defined by the level detector circuit 24 and the frequency of those signals exceeds the threshold frequency T. Thus, an advantage of the preferred embodiment of the invention is that undesired pacing rate increases do not arise either from isolated or low frequency signals of relatively large amplitude from the aatlvity sensor 3 or from low amplitude relatively high frequency signals from the sensor 3.
i322029 It will be understood from consideration of the flow chart of Fig 3 and the foregoing description that, after the pacing rate has been increased to the present level (95 beats/min in the example given) as a result of A exceeding T, all further increase in pacing rate is dependent solely upon the level of signal R derived from the second sensor. By this means, activities such as swimming may be adequately provided for since swimming may produce sufficient vibration for A to exceed T but would not produce sufficient vibration for A to exceed a significantly higher threshold. Further, activities such as brushing teeth producing high amplitude signals from the activity sensor would not result in an excessive increase in heart rate, which would be undesirable, since there would be no appreciable increase in the signal R derived from the second sensor. However, in the proposal in EP 259658, the high amplitude signals from the activity sensor would be expected to exceed several thresholds and therefore cause an excessive and undesired increase in heart rate. Further, with the present invention, upon the rapid onset of heavy exercise, the selected increase in paclng rate (25 beats/min giving a pacing rate of 95 beats/min in the example) Day be such as to enable the patient to cope ':
with such exercise for a period long enough to permit the signal from sensor 2 to catch up and then further increase the pacing rate as required. Since, in the embodiment under discussion, there is no time-out period within which the signal from sensor 2 must increase to any particular level, there is no risk of deactivating the algorithm with resultant reduction in heart rate as may arise in the arrangement disclosed in EP 259658 upon rapid onset of heavy exercise due to the inclusion of the timeout period.
Fig. 4 illustrates graphically the operation of the pacemaker according to the preferred embodiment as so far described and, by way of an example, illustrates a 10 minut~ period involving both periods of activity - and of rest.
As shown in curve I of Fig. 4, the patient undergoes a relatively low level of exerclse during the first minute, rests durlng the next two minutes, undergoes a much higher level of exercise during the following - three minutes and thereafter returns to a condltlon of rest.
` 1322029 As shown by curve II, the level of exercise in each period is sufficient for the signal from the activity sensor and associated circuitry to exceed its threshold level.
There is a delay following the beginning of an exercise period before the patient's rate of respiration increases. Curve III shows that the first period of exercise, during which the exercise level is relatively low, is not long enough to cause the respiration rate to increase at all but the second period of exercise causes the respiration rate to increase, the increase beginning after a delay of about three-quarters of a minute from the beginning of the period of exerclse.
., Curve IV illustrates the pacing rate which the output of circuitry 30 indicates as appropriate. In view of the above mentioned delay, circuitry 30 does not indlcate any increase in pacing rate during the first period of exercise and thus, if this alone were relied upon to control the pacemaker, no increase in heart rate at thi~ time would result. This would be disadvantageous. Simllarly, curve IV indicates that there is delay of about three-quarters of a minute i322029 after the beginning of ths second period of exercise before circuitry 30 begins to indicate that the heart rate should be increased. Again, if the heart rate were not to be increased during this delay period, that would be disadvantageous.
Curve V shows that, despite the absence of a signal from circuitry 30 indicating that the heart rate should be increased during the first period of exercise, the pacing rate is nevertheless increased at a fixed rate as indicated at reference number 40.
This increase arises from the program sequencing through steps 1, 4 and 5 illustrated in Fig. 3. As indicated by reference number 42 in curve 5, the pacing rate increases to a maximum level of 95 beats/min during the first period of exercise, this being achieved by the program sequencing through steps 1, 4, 6 and 6A.
Reference number 44 in curve V indicates that after the activity signal has fallen below the threshold, the paaing rate i8 reduced at the above mentloned fixed rate. This is achleved by the program seguencing through steps 1, 2 and 3 until the minimum rate of 70 beats/min is reached.
1322~29 Reference number 46 in curve V shows that immediately the second period of activity begins, the pacing rate increases at the above mentioned fixed rate to 95 beats/min, as shown by reference number 48 and remains at that rate until there has been sufficient time for signal R to increase to a level indicating a rate greater than 95 beats/min, this taking about 1 1/2 minutes in the example shown from the beginning of the second period of exercise.
Reference number 50 in curve V indicates that, after this point, the pacing rate P is increased, again at the same fixed rate as previously described, in response to the continuing increase in signal R.
During the period corresponding to reference number 50, the program of Fig. 3 se~uences through steps 1, 4, 6, 7, 8 and 5.
: At some point, the signal R reaches the maximum value appropriate to the level of exercise being undertaken ~ :~ and, as indicated by reference number 52 in curve V, ; the paclng rate P is maintained oonstant at this maximum value. Thl9 is achleved by the program shown in Fig. 3 sequencing through steps 1, 4, 6 and 7.
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As shown by reference number 54 in curve V, immediately the period of exercise terminates, the pacing rate begins to decrease at the above mentioned fixed rate since the activity signal drops below the threshold and the program sequences through steps 1, 2 and 3. This decreass takes place despite the fact that there is a delay before the respiration rate, and therefore signal R, decreases. Thus, the preferred embodiment of the invention provides a substantial advantage that the paclng rate begins to decrease immed~ately the exercise terminates rather than remaining at a high rate until such time as the respiration rate has decreased.
Fig. 5 is included to illustrate, with curves I to V
which ha~e the same meaning as curves I to V of Fig.
4, that from time to time, although the patient may not undergo any physical activity of significance so that there is no increase in the activity si~nal, respiration rate may nevertheless increase to a level such that the signal from clrcuitry 30 lndioates that the paalng rate should be lncreased. However, ln these oircumstances, paclng rate lncrease ls inapproprlate and, as shown in Fig. 5, it does not in fact take place in the preferred embodiment of the invention since the program will remain sequencing through steps 1 and 2 until such time as the actlvity signal exceeds its threshold and regardless of the level of signal R.
Thus, in the invention the pacing rate is always substantially at the required level, any changes required begin to take place without any undesirable delay and the rate of change may be preselected and therefore selected to be optimum for increasing and optimum for decreasing rate. This combination of advantages is not achievable in the prior art.
In the modification shown in Fig. 6, the respiratory sensor 9 is omitted and instead a single activity or vibration sensor 3 is employed. The output from this sensor is applied to a band pass amplifler 60 centred at 10 Hz to provide an activity signal which is then supplied to sensor processor 10 and is also applied to a low pass amplifier 62 having a pass band of zero to 0.5 Hz to provide a signal representing respiration rate whlah is applied to proaessing alrauit 10. The sensor 3 may be as described in US Patent 4,428,378 and the airouitry shown in Fig. 6 enables that slngle sensor to provide both the activity signal and the .
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~322029 respiratory signal, any unwanted cardiac signal being rejected. The modified embodiment illustrated in Fig.
6 ls otherwise the same as the embodiment described with reference to Figs. 1 to 5.
As will be appreciated, various modifications may be made within the scope of the invention. The various figures for increase or decrease of rate and maximum and minimum rate are merely given by way of illustration. For example, although the minimum rate has been given as 70 beats/min, it would be possible within the scope of the invention to provide for an extra reduction in rate to say 60 beats/min, where the 3 patient is sleeping, this condition, for example, being i~dicated by the absence of any activity signal for a predetermined period of time. Thus, the reference to a minimum rate above should be understood as meaning the normal minimum rate, such as 70 beats/min. It is also within the scope of the invention for other values between the limits of say 30 to 100 beats/min to be chosen and telemetred into the pacemaker using the external programmer 15, the sntenna 17 and receiving clrcuitry 16.
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Further, although in the embodiment described with reference of the accompanying drawings, it has been assumed that detection of the frequency of signal A
has been carried out by counting the number of pulses generated by zero crossing detector 26 in a predetermined period, other methods are possible. For example, the pulses output by the band pass amplifier could be integrated over a predetermined period and compared to an appropriate threshold. -~
In the above description, it has been indicated that the band pass amplifier 22 may pass signals within a band centred at lOHz. Typically, the bandwidth of this amplifier might be say 6Hz whereby signals in the range 7Hz to 13Hz would be passed although alternative arrangements are possible, for example a band width of 4Hz whereby signals in the range 8 to 12Hz would be passed. It is not essential that the centre frequency of the band pass amplifier should be lOHz although this is a particularly preferred value. It might, for example, be lower or hlgher. The processing performed on the signal from acitivity sensar 4 should be such that the processed signal may be aompared with a threshold which corresponds to a level of activity for whlch the approprlate pacing rate is preferably at 1322~2~
least 20 beats per minute higher than the resting rate, for example 20 to 30 beats per minute higher than the resting rate and particularly preferably about 25 beats per minute higher than the resting rate. Thus a particularly preferred combination of features in accordance with the invention involves the detection of signals from the vibration sensor 4 having an amplitude above a predetermined level and a frequency of at least 7 or 8Hz, and preferably lO~z, the circuitry causing an increase in heart rate of between 20 and 30 beats per minute, preferably 25 beats per minute above resting rate in response to the detection of such signals, further increases beyond this rate being under the sole control of the level of the signal from the second sensor.
The pacemaker according to the invention may be arranged for operation in any of a variety of different modes. For example, it may be used in the so-called AAI mode in which both pacing and sensing take place in the atrium and pacing pulses are inhibited if spontaneous aotlvity ls ~ensed, ln the so-aalled WI mode ln which both paalng and sensing take place in the ventricle and pacing pulses are inhibited when spontaneous activity is sensed or the ~ 1322029 so-called DDD mode in which pacing and sensing take place in both the atrium and the ventricle with inhibition of the pacing pulses or triggering taking place in accordance with sensed activity, in a well-known manner.
Further, although in the preferred embodiment as described, the change in pacing rate takes place linearly w1th time, it i8 possible within the scope of the invention for the changes in pacing rate to take place non-linearly and it is possible for the changes to be made continuously or step-wise.
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Claims (29)
1. A pacemaker comprising:
sensing means for providing a first signal which changes relatively rapidly in response to changes in a patient's exercise level but may inaccurately represent the pacing rate appropriate to said exercise level, and a second signal that changes from a normal condition relatively slowly in response to changes in exercise level but which, after having changed from said normal condition, relatively accurately represents a required pacing rate; and control means for generating a pacing signal to control pacing rate in response to said sensing means, said pacing signal being operative to provide:
(A) a relatively low pacing rate when said first signal has a value less than a threshold;
(B) a predetermined pacing rate higher than said relatively low pacing rate when the first signal exceeds said threshold and the second signal has a value indicative of a pacing rate less than said predetermined pacing rate; and (C) a pacing rate substantially equal to that indicated by the value of said second signal when the first signal exceeds said threshold and the second signal indicates a rate higher than said predetermined pacing rate, regardless of the amount by which said first signal may exceed said threshold.
sensing means for providing a first signal which changes relatively rapidly in response to changes in a patient's exercise level but may inaccurately represent the pacing rate appropriate to said exercise level, and a second signal that changes from a normal condition relatively slowly in response to changes in exercise level but which, after having changed from said normal condition, relatively accurately represents a required pacing rate; and control means for generating a pacing signal to control pacing rate in response to said sensing means, said pacing signal being operative to provide:
(A) a relatively low pacing rate when said first signal has a value less than a threshold;
(B) a predetermined pacing rate higher than said relatively low pacing rate when the first signal exceeds said threshold and the second signal has a value indicative of a pacing rate less than said predetermined pacing rate; and (C) a pacing rate substantially equal to that indicated by the value of said second signal when the first signal exceeds said threshold and the second signal indicates a rate higher than said predetermined pacing rate, regardless of the amount by which said first signal may exceed said threshold.
2. A pacemaker according to claim 1, wherein the pacing signal generated by said control means is operable to change the pacing rate linearly.
3. A pacemaker according to claim 1, wherein the pacing signal generated by said control means is operable to change the pacing rate nonlinearly.
4. A pacemaker according to claim 1, wherein the pacing signal generated by said control means is operable to change the pacing rate stepwise.
5. A pacemaker according to claim 1, wherein said sensing means comprises a first sensor means for providing said first signal and a second sensor means for providing said second signal.
6. A pacemaker according to claim 5, wherein said first sensor means is operable to sense bodily vibrations and said second sensor means is operable to sense a physiological variable.
7. A pacemaker according to claim 6, wherein said second sensor means senses a physiological variable selected from evoked QT interval, respiratory rate, minute ventilation, mixed venous temperature, venous oxygen saturation, or rate of change of right ventricular pressure.
8. A pacemaker according to claim 6, wherein said threshold is defined as both a magnitude and a frequency of said first signal.
9. A pacemaker according to claim 1, wherein said sensing means comprises a single sensor and processing means for processing signals from said single sensor to produce said first and second signals respectively.
10. A pacemaker according to claim 9, wherein said single sensor is a vibration sensor and said processing means comprises filtering means.
11. A pacemaker according to claim 10, wherein said filtering means is operable to provide with first signal by passing signals having frequencies within a frequency band in the region of 10 Hz and to provide said second signal by passing frequencies less than about 0.5 Hz.
12. A pacemaker according to claim 11, wherein said second signal represents respiratory rate.
13. A pacemaker according to claim 1, wherein said predetermined pacing rate is approximately 95 beats/-min.
14. A pacemaker according to claim 1, wherein said control means is operable to define predetermined minimum and maximum values for said pacing rate and to maintain the pacing rate within said values regardless of the values of said frst and second signals.
15. A pacemaker comprising:
sensing means for providing a first signal which changes relatively rapidly in response to changes in a patient's exercise level but many inaccurately represent the pacing rate appropriate to said exercise level, and a second signal that changes from a normal condition relatively slowly in response to changes in exercise level but which, after having changed from said normal condition, relatively accurately represents a required pacing rate; and control means for generating a pacing signal to control pacing rate in response to said sensing means, said pacing signal being operative to provide:
(A) a relatively low pacing rate unless said first signal includes frequency components having an amplitude above a predetermined amplitude and a frequency above a threshold frequency of at least 7 Hz;
(B) a predetermined pacing rate higher than said relatively low pacing rate when said first signal includes frequency components having an amplitude above said predetermined amplitude and a frequency above said threshold frequency and the second signal has a value indicative of a pacing rate less than said predetermined rate; and (C) a pacing rate substantially equal to that indicated by the value of said second signal when the first signal exceeds said threshold and the second signal indicates a rate higher than said predetermined pacing rate.
sensing means for providing a first signal which changes relatively rapidly in response to changes in a patient's exercise level but many inaccurately represent the pacing rate appropriate to said exercise level, and a second signal that changes from a normal condition relatively slowly in response to changes in exercise level but which, after having changed from said normal condition, relatively accurately represents a required pacing rate; and control means for generating a pacing signal to control pacing rate in response to said sensing means, said pacing signal being operative to provide:
(A) a relatively low pacing rate unless said first signal includes frequency components having an amplitude above a predetermined amplitude and a frequency above a threshold frequency of at least 7 Hz;
(B) a predetermined pacing rate higher than said relatively low pacing rate when said first signal includes frequency components having an amplitude above said predetermined amplitude and a frequency above said threshold frequency and the second signal has a value indicative of a pacing rate less than said predetermined rate; and (C) a pacing rate substantially equal to that indicated by the value of said second signal when the first signal exceeds said threshold and the second signal indicates a rate higher than said predetermined pacing rate.
16. A pacemaker according to claim 15, wherein said threshold frequency is 10 cycles per second.
17. A pacemaker comprising:
sensing means for providing a first signal which changes relatively rapidly in response to changes in a patient's exercise level but may inaccurately represent the pacing rate appropriate to said exercise level, and a second signal that changes from a normal condition relatively slowly in response to changes in exercise level but which, after having changed from said normal condition, relatively accurately represents a required pacing rate and control means for generating a pacing signal to control pacing rate in response to said sensing means, said pacing signal being operative to define, for said first signal, a single substantially constant threshold value, and to provide:
(A) a relatively low pacing rate when said first signal has a value less than said threshold value;
(B) a predetermined pacing rate higher than said relatively low rate when the first signal exceeds said threshold value and the second signal has a value indicative of a pacing rate less than said predetermined pacing rate; and (C) a pacing rate substantially equal to that indicated by the value of said second signal when the first signal exceeds said threshold and the second signal indicates a rate higher than said predetermined pacing rate.
sensing means for providing a first signal which changes relatively rapidly in response to changes in a patient's exercise level but may inaccurately represent the pacing rate appropriate to said exercise level, and a second signal that changes from a normal condition relatively slowly in response to changes in exercise level but which, after having changed from said normal condition, relatively accurately represents a required pacing rate and control means for generating a pacing signal to control pacing rate in response to said sensing means, said pacing signal being operative to define, for said first signal, a single substantially constant threshold value, and to provide:
(A) a relatively low pacing rate when said first signal has a value less than said threshold value;
(B) a predetermined pacing rate higher than said relatively low rate when the first signal exceeds said threshold value and the second signal has a value indicative of a pacing rate less than said predetermined pacing rate; and (C) a pacing rate substantially equal to that indicated by the value of said second signal when the first signal exceeds said threshold and the second signal indicates a rate higher than said predetermined pacing rate.
18. A pacemaker comprising:
sensing means for providing a first signal which changes relatively rapidly in response to changes in a patient's exercise level but may inaccurately represent the pacing rate appropriate to said exercise level, and a second signal that changes from a normal condition relatively slowly in response to changes in exercise level but which, after having changed from said normal conditions relatively accurately represents a required pacing rate; and control means for generating a pacing signal to control pacing rate in response to said sensing means, said pacing signal being operative to provide:
(A) a relatively low pacing rate when said first signal has a value less than a threshold;
(B) a predetermined pacing rate higher than said relatively low rate when the first signal exceeds said threshold and the second signal has a value indicative of a pacing rate less than said predetermined pacing rate;
(C) a pacing rate substantially equal to that indicated by the value of said second signal when the first signal exceeds said threshold and the second signal indicates a rate higher than said predetermined pacing rate; and said pacing signal being operative to increase said pacing rate above said predetermined pacing rate solely in response to increase in said second signal.
sensing means for providing a first signal which changes relatively rapidly in response to changes in a patient's exercise level but may inaccurately represent the pacing rate appropriate to said exercise level, and a second signal that changes from a normal condition relatively slowly in response to changes in exercise level but which, after having changed from said normal conditions relatively accurately represents a required pacing rate; and control means for generating a pacing signal to control pacing rate in response to said sensing means, said pacing signal being operative to provide:
(A) a relatively low pacing rate when said first signal has a value less than a threshold;
(B) a predetermined pacing rate higher than said relatively low rate when the first signal exceeds said threshold and the second signal has a value indicative of a pacing rate less than said predetermined pacing rate;
(C) a pacing rate substantially equal to that indicated by the value of said second signal when the first signal exceeds said threshold and the second signal indicates a rate higher than said predetermined pacing rate; and said pacing signal being operative to increase said pacing rate above said predetermined pacing rate solely in response to increase in said second signal.
19. A pacemaker according to claim 18, wherein said predetermined rate is at least 20 beats per minute above said relatively low pacing rate.
20. A pacemaker according to claim 19, wherein said predetermined rate is between 20 and 30 beats per minute above said relatively low pacing rate.
21. A pacemaker according to claim 20, wherein said predetermined rate is approximately 25 beats per minute above said relatively low pacing rate.
22. A pacemaker comprising:
first sensing means for providing a first signal in response to bodily vibrations arising from a patient's activity;
second sensing means for providing a second signal which changes in response to a physiological parameter which varies in response to a patient's activity; and control means for generating a pacing signal to control pacing rate in response to said first and second sensing means, said pacing signal being operative to define for said first signal a threshold for components of said first signal having a frequency of at least 7 Hz, and to provide:
(A) a relatively low pacing rate when said first signal is such that said threshold is not exceeded;
(B) a predetermined pacing rate which is from 20 to 30 beats/min higher than said relatively low pacing rate when the first signal is such that said threshold is exceeded and the second signal has a value indicative of a pacing rate less than said predetermined pacing rate; and (C) a pacing rate substantially equal to that indicated by the value of said second signal when the first signal is such that said threshold is exceeded and the second signal indicates a rate higher than said predetermined rate; and said pacing signal being operative to increase said pacing rate above said predetermined pacing rate solely in response to increase in said second signal.
first sensing means for providing a first signal in response to bodily vibrations arising from a patient's activity;
second sensing means for providing a second signal which changes in response to a physiological parameter which varies in response to a patient's activity; and control means for generating a pacing signal to control pacing rate in response to said first and second sensing means, said pacing signal being operative to define for said first signal a threshold for components of said first signal having a frequency of at least 7 Hz, and to provide:
(A) a relatively low pacing rate when said first signal is such that said threshold is not exceeded;
(B) a predetermined pacing rate which is from 20 to 30 beats/min higher than said relatively low pacing rate when the first signal is such that said threshold is exceeded and the second signal has a value indicative of a pacing rate less than said predetermined pacing rate; and (C) a pacing rate substantially equal to that indicated by the value of said second signal when the first signal is such that said threshold is exceeded and the second signal indicates a rate higher than said predetermined rate; and said pacing signal being operative to increase said pacing rate above said predetermined pacing rate solely in response to increase in said second signal.
23. A pacemaker comprising:
means to control pacing rate by (a) rapidly responding to a change in a patieint's exercise level as indicated by the value of a first signal representative of a first biological variable, and (b) following such rapid response, accurately setting the pacing rate relative to the patient's exercise level as indicated by the value of a second signal representative of a second biological variable;
a single sensor including means for measuring activity by the patient to produce an activity signal;
and signal processing means for generating said first and second signals from said activity signal.
means to control pacing rate by (a) rapidly responding to a change in a patieint's exercise level as indicated by the value of a first signal representative of a first biological variable, and (b) following such rapid response, accurately setting the pacing rate relative to the patient's exercise level as indicated by the value of a second signal representative of a second biological variable;
a single sensor including means for measuring activity by the patient to produce an activity signal;
and signal processing means for generating said first and second signals from said activity signal.
24. An arrangement according to claim 23, wherein said first biological variable is bodily vibration and said second biological variable is a physiological variable.
25. An arrangement according to claim 24, wherein said sensor is a vibration sensor.
26. An arrangement according to claim 25, wherein said processing means comprises filtering means.
27. An arrangement according to claim 26, wherein said filtering means is operable to provide said first signal by passing a frequency band in the region of 10 Hz and to provide said second signal by passing frequencies less than about 0.5 Hz.
28. An arrangement according to claim 25, wherein said physiological variable is respiratory rate.
29. A cardiac pacer comprising:
(a) single means for sensing a plurality of signal variables indicative of a patient's biological parameters;
(b) said sensing means including output means for identifying and separating said signal variables;
(c) means coupled to said output means for selectively combining said separated signal variables in order to define a pacing rate of the cardiac pacer; and (d) means coupled to said combining means for generating signals at said pacing rate.
(a) single means for sensing a plurality of signal variables indicative of a patient's biological parameters;
(b) said sensing means including output means for identifying and separating said signal variables;
(c) means coupled to said output means for selectively combining said separated signal variables in order to define a pacing rate of the cardiac pacer; and (d) means coupled to said combining means for generating signals at said pacing rate.
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB8803613 | 1988-02-17 | ||
| GB888803613A GB8803613D0 (en) | 1988-02-17 | 1988-02-17 | Rate-responsive pacemaker |
| GB8819268.7 | 1988-08-12 | ||
| GB888819268A GB8819268D0 (en) | 1988-02-17 | 1988-08-12 | Rate-responsive pacemaker |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1322029C true CA1322029C (en) | 1993-09-07 |
Family
ID=26293501
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000591474A Expired - Fee Related CA1322029C (en) | 1988-02-17 | 1989-02-17 | Rate-responsive pacemaker |
Country Status (5)
| Country | Link |
|---|---|
| US (2) | US5063927A (en) |
| EP (1) | EP0331309B1 (en) |
| CA (1) | CA1322029C (en) |
| DE (2) | DE68927447T2 (en) |
| GB (1) | GB2216011B (en) |
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-
1989
- 1989-02-16 EP EP89301474A patent/EP0331309B1/en not_active Revoked
- 1989-02-16 DE DE68927447T patent/DE68927447T2/en not_active Revoked
- 1989-02-16 US US07/311,638 patent/US5063927A/en not_active Expired - Lifetime
- 1989-02-16 GB GB8903597A patent/GB2216011B/en not_active Expired - Fee Related
- 1989-02-16 DE DE68929280T patent/DE68929280T2/en not_active Expired - Lifetime
- 1989-02-17 CA CA000591474A patent/CA1322029C/en not_active Expired - Fee Related
-
1990
- 1990-04-26 US US07/515,085 patent/US5044365A/en not_active Expired - Lifetime
Also Published As
| Publication number | Publication date |
|---|---|
| EP0331309B1 (en) | 1996-11-13 |
| GB2216011B (en) | 1990-02-28 |
| GB8903597D0 (en) | 1989-04-05 |
| DE68927447T2 (en) | 1997-04-03 |
| DE68927447D1 (en) | 1996-12-19 |
| DE68929280T2 (en) | 2001-05-31 |
| EP0331309A3 (en) | 1989-10-04 |
| US5044365A (en) | 1991-09-03 |
| GB2216011A (en) | 1989-10-04 |
| EP0331309A2 (en) | 1989-09-06 |
| US5063927A (en) | 1991-11-12 |
| DE68929280D1 (en) | 2001-03-08 |
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| MKLA | Lapsed |