ECG MONITOR WITH IMPROVED ALARM DETECTION
The present invention relates to an automatic alarm generating system provided with cardiac rhythm sensor and using digital radio-frequency signal transmission, in which cardiac rhythm sensors positioned on a person are connected to a personal warner through a cable, one or more personal warner are connected to a central transceiver through radio-frequency signal transmission. The central transceiver is connected to a communicator through a cable, and the system is adapted to perform comparative test with the sensed cardiac rhythm signal and to generate alarm signals when the cardiac rhythm exceeds determined limit values.
It is known that as a result of physical and psychological attack the threatened person gets into a state of stress which evokes automatic, vegetative reaction in the human organism. Typically, during this state of stress the rhythm of the cardiac action speeds up which is well detectable. Also, special changes in the signal shape of the ECG are traceable.
There are known methods for measuring the rhythm of the cardiac action which methods are applied in health service, in which a sphygmometer is used and an alarm message may be sent to a central medical service for example in the form of an SMS transferred through a cellular phone or through radio-frequency transmission. The principal task of these solutions is to provide medical attention, rather than to provide absolute safe transmission or definite exclusion of a false alarm.
Patent application DE 4218042 describes a system based on the detection of cardiac rhythm which system may be used for example in banks. A small device attached individually to persons working for example in a strong-room detects a physiological signal typically the rhythm of their cardiac activity, and the system responds to signs of sudden collective stress with a warning signal which is transmitted to a police station.
Though this solution proved to be good, it still has some shortcomings. In the one hand the system does not provide for sufficiently secure transmission, since upon cutting through its cables - in so far as radio transmission section is also used - it is interceptable and it can be interfered, switched off or the staff may be forced to do so. On the other hand the collective stress does not always provide a right basis for the
reliability of the alarm. Collective stress, fright may be induced by a common transient source, e.g.: unexpected sound effects (explosion of an exhauster, bang of a dropped object or burst of a bulb, etc.). This leads to a baseless alarm which is usually termed as false alarm. Also, it is possible that only a single person is affected by stress, for example when only a few people work far from each other in a certain place where increased security measures are taken. In this case, if collectiveness is a condition of effectuating the alarm then the system does not give signals of this situation. This is called "failed detection".
Either false alarm or failed detection is undesirable, therefore the aim of the present invention is to restrain the possibility of occurrence of them. A further aim is to make smooth and undisturbable transmission possible.
In order to achieve the aim, in the system taught in the document DE 4218042, delay for making immediate switching off the system impossible, coded digital radio- frequency transmission, individually fixed adaptive base value determination are used, and in case of once exceeding the limit values in relation to the base values an authenticity test is performed based on further measurement.
According to the present invention, in the system mentioned in the preamble the personal warner is adapted to perform an authenticity test in case of exceeding given limit values, and on the basis of a positive result of the authenticity test it is adapted to activate a radio transmitter connected thereto to transmit radio-frequency signals which consist of a series of encoded signals containing encrypted identification codes; and the personal warner is provided with a switch means which switches off when a determined delay time has elapsed, and the personal warner remains active during the determined delay time.
A detailed description of the present invention will be given by means of the attached drawings in which:
Fig. 1 schematically shows the structure of the detection side of the alarm generating system;
Fig. 2 shows the structure of the communicator of Fig. 1 ; and
Fig. 3 shows the structure of the personal warner of Fig. 1.
Cardiac rhythm sensor 1 shown in Fig. 1 is attached to a person staying in a protected place. This may be a belt, an adhesive plaster or an electrode including the
electrodes of a known ECG, or a group of these, or other similar means, which is suitable for continuous monitoring of the activity of the heart and is adapted to transform it into electrical signals and to transmit these electric signals to a personal warner 2 which is connected to cardiac rhythm sensor 1 through a cable. Personal warner 2 is also attached to the given person, advantageously in such a manner that it is imperceptible for an observer. Preferably, for personal safety reasons the connection also contains a separation which eliminates direct galvanic connection. In case of cutting, breaking or other physical disconnection of the cable personal warner 2 does not detect signals. Then immediate alarm is generated. Personal warner 2 is connected to a central transceiver 3 through wireless, radio-frequency connection. Central transceiver 3 is able to handle a plurality of personal warners 2 simultaneously. Central transceiver
3 is connected to a communicator 4 by means of which data of an alarm detected in a certain case are processed, decoded, and depending on the result of this decoding further actions to be done are determined. The person for whom the alarm signal was generated can be identified during decoding on the basis of an encrypted identification code.
To perform these tasks in the exemplary arrangement of Fig. 2 communicator 4 includes a built-in microprocessor 14 and an LCD (Liquid Crystal Display) 13 for displaying data and information. Also, microprocessor 14 actuates an output controller 15 which is connected to an actual alarm device, which in this case is a conventional alarm device, through cable 18. Also, via cable 18 connection to an external controlling station may be established, for example a security centre or the police. Communicator
4 is supplied with energy necessary for operation via power-supply unit 17. Further, in an advantageous embodiment communicator 4 is provided with push buttons 16 which may be necessary for performing functions executable by an operator on the spot. These functions may be for example changing the mode of alarm, storing the received and decoded data or transferring these data to a computer. Communicator 4 is further connected to a transceiver circuit 11 through another cable 12. Transceiver circuit 11 is provided with an antenna 10. These are collectively adapted to serve simultaneously a plurality of personal warners 2 and to identify the individual personal warners by means of safety codes so that interfere, interception or manipulation of the transmission is substantially impossible. To this end methods known in the technology may be
used, for example data transfer in which skip-codes are used or other known data encrypting digital transmission techniques. Security of data transmission is enhanced by the fact that in basic situation, when there is no alarm or when the system is not in a test mode, then radio-frequency transmission between individual personal warners 2 and central transceiver 3 by no means exists, therefore there is no physical opportunity for interception or for breaking the code.
Fig. 3 is a block diagram showing the detailed structure of an exemplary personal warner 2. In Fig. 3 signaller 22 is shown as an integral unit, which transmits electric signals representing the cardiac activity to a signal transformer 25 through cable 23 and connector 24. The signal is then transmitted to signal processor 26 which is a microprocessor. Signal processor 26 determines if significant deviation from the normal state is detected in the electric pulse series of signaller 22. It should be noted that this deviation in the simplest form may be the deviation noticeable in the frequency of the rhythm of the cardiac activity, however, in a more complex embodiment certain characteristics of the signal shape may be suitable for drawing conclusions. Methods for signal shape analysis are known in the field of cardiography therefore they are not specified in the present description, they may include evaluating algorithms of various aspects. However, it can be stated that analysis of the signal shape makes signal processing substantially more complicated, an additional advantage of this method is that it also implies the applicability in the field of health care, moreover, by separation of the two functions (alarm system and health care) both may be effectuated at the same time.
Further, personal warner 2 shown in Fig. 3 contains an accumulator 27 and a LED 20 for displaying the current mode of operation of the activated device. Push buttons 21 may be used for switching on/off the device or test function may be activated with them. It should be noted that in the system according to the present invention when the device is switched off by means of pushing the respective button, the personal warner remain active for a determined delay time, so that in case of an attempted robbery the offender is unable to switch off the sensing element(s) of the system even if the staff is forced to do so. This delay time may be for example 10 minutes which is enough for generating an alarm signal and also for control of the same. However, the delay time may be different, but advantageously it is between 5-15 minutes.
Personal warner 2 performs a comparative test in which the detected cardiac rhythm signal is compared with one or more given cardiac rhythm limit values. In case of exceeding one of the limit values automatic alarm is generated which is preceded by an authenticity test which will be described later. The detected cardiac rhythm signal itself as an averaged signal in a time-window of a given length is taken as the basis of this comparative test. With this simple averaging interference of short duration may be eliminated, while it does not result in significant delay of alarm. Preferably, the length of the time-window may be 3-15 sec. Basically, cardiac rhythm limit values are of frequency nature, but also, they may be ECG shape featuring values produced by performing certain methods.
Personal warner 2 according to the invention may be made suitable for determination of an individually fixed adaptive base value. Then in an adjusting mode of operation the detected cardiac rhythm signal is averaged in a base-determination time window, and the limit values for the given cardiac rhythm are determined relative to the averaged value. The length of the base-determination time window may be of a few seconds, the limit values may differ from this measured average values by a certain percent in either way. The adaptive base value may be automatically determined upon switching on the personal warner 2 and possibly during operation at fixed intervals, for example a few times a day. The person carrying the device does not need to do anything. In this way reliability is increased, and possibility of false alarm as well as failed detection is decreased.
In addition to delayed switching off an authenticity test is included in the system according to the invention in order to eliminate the possibility of a false alarm when as a result of an unjustified detection signal - due to an accidental event or a contact fault - one of the given limit values is exceeded. The system performs a repeated test and a second sampling to verify the validity of the previously measured value. Thus, the authenticity test is a repeated comparative test with the sensed cardiac rhythm signal which is a signal averaged in a time window of a given length. Advantageously, a certain period of waiting time may be set before the repeated comparative test is performed. During this time shock effects resulting from a sudden fright diminish. This waiting time may be 10-30 seconds. If the second measuring result equals to or greater than the normal value, that is, the result of the authenticity test is positive, then
radio transmitter 28 is activated to transmit coded radio-frequency signal series containing encrypted identification code, by which alarm is actually generated.
Personal warner 2 contains a radio transmitter 28 provided with an antenna 29, for transmission of an appropriately coded encrypted digital radio-frequency signal series to communicator 4. Therefore radio transmitter 28 and antenna 29 must be designed so that they are able to communicate in a safe manner with the transceiver circuit 11 of communicator 4 being at a certain distance. Practically, this distance is not more than a few times ten or a few times hundred metres. For transmission well known radio-frequency transceiver subassemblies may be used.
An exemplary arrangement of the alarm generating system has been described but other arrangements satisfactory for the required functions and mode of operation are also feasible without departing from the scope of the present invention as defined in the appended claims.