US3651798A

US3651798A - Blood pressure indicator and noise

Info

Publication number: US3651798A
Application number: US37543A
Authority: US
Inventors: Paul H Egli; Warren W Brenner; Martin H Wilcox
Original assignee: Parke Davis and Co LLC
Current assignee: Parke Davis and Co LLC; Warner Lambert Co LLC
Priority date: 1970-05-15
Filing date: 1970-05-15
Publication date: 1972-03-28
Anticipated expiration: 1989-03-28

Abstract

An electronic blood pressure indicator for use with conventional inflatable arm cuff apparatus and a mercury or aneroid manometer, comprising a microphone for translation of arterial blood flow sounds into electrical signals, a narrow bandpass amplifier circuit which filters extraneous frequencies and amplifies a narrow band of low frequencies containing essential arterial blood flow information, a discriminator which senses signal strength and passes only signals exceeding a predetermined value, a pulse generator which provides a pulse output corresponding to blood flow pulses passed by the discriminator, the pulse generator cooperating with switch means to drive an indicator to provide a visual indication of the arterial blood flow sounds, and a dual lockout circuit which blocks incoming signals and spurious pulses from the pulse generator.

Description

ilnite States Patent iEglliet ai.

1151 dflifi [54] L001) PRESSURE INDICATOR AND 3,171,292 3/1965 Pantle ..12s/2.os R NOISE $13013 2:363 Vogt ..12s/2.05 A ,3 ,14 6 Young ....l28/2.05 S [72] Inventors: g g i g 3,550,582 12/1970 Wilhelmson 1 28/205 A s H. Wilcox, Castle Rock, Colo. primary E i wflli 5 Kamm [73] Assignee: Parke, Davis & Company, Detroit, Mich. Attorney-Paul Paul [22] Filed: May 15, 1970 [57] ABSTRACT PP N05 37,543 An electronic blood pressure indicator for use with conventional inflatable arm cuff apparatus and a mercury or aneroid [52] U 5 Cl 128/2 05 s 128/2 05 P 128/2 05 M manometer, comprising a microphone for translation of arteri- ST 6 al blood flow sounds into electrical signals, a narrow bandpass I 5 l 1 Int Cl sloz amplifier circuit which filters extraneous frequencies and am- 58] i 2 05 P plifies a narrow band of low frequencies containing essential 128/2 05 S arterial blood flow information, a discriminator which senses 2 d 5 signal strength and passes only signals exceeding a predetermined value, a pulse generator which provides a pulse output I 56] References Cited corresponding to blood flow pulses passed by the discriminator, the pulse generator cooperating with switch means to UNITED STATES PATENTS drive an indicator to provide a visual indication of the arterial blood flow sounds, and a dual lockout circuit which blocks ini 1 5 f coming signals and spurious pulses from the pulse generator. a1 ey 3,318,303 5/1967 Hammacher 128/205 S 9 Claims, 3 Drawing Figures -E T3 E 16 1 1 1 E323 D1SCRIMINATOR z 'ig SWITCH INDICATOR v I I I7 I DUAL LOCKOUT I i UNREGULATED POWER SUPPLY PMENTBNR28 m2 INVENTORS, PAUL H. EGLI BY WARREN vv. BRENNER MARTIN H. WILC x W #W ATTORNEYS.

BLOOD PRESSURE INDICATOR AND NOISE BACKGROUND OF THE INVENTION 1. Field of the Invention This invention lies in the field of electronic sphygmomanometer apparatus and, more particularly, electronic blood pressure detection and indicator apparatus.

2. Description of the Prior Art The art of measuring blood pressure has been well developed by the medical profession, resulting in a standard and near-universal procedure for taking blood pressure measurements. The procedure involves inflating a rubber bladder in a cloth cuff encircling the patients arm, thus forming an occlusion opposing the arterial flow of blood, the blood being under pressure from the heart. When the bladder is inflated t a pressure sufficient to cut off the blood flow entirely, the pressure is slowly released until blood flow resumes in the artery. As the pressure drops, a point is reached where the pulsating blood flow first pushes through the cuff, causing audio sounds normally detectable with a stethoscope. At the point of first detection, where the decreasing bladder pressure is matched by the maximum, or systolic blood pressure, the doctor skilled in auscultation can detect the pulsatile flow in the artery, and determine the systolic pressure. As the cuff pressure is further reduced, the blood pulses generate a series of distinct sounds audible with a stethoscope to persons with average hearing in an environment of normal background noise. The further reduced cuff pressure at which the sounds fade away is the pressure which the skilled doctor detects as the diastolic pressure. Through long use of the stethoscope, this point in cuff pressure has acquired an important clinical meaning. Although cuff pressure must decrease further beyond this point until completely unimpeded flow results, it is this long used cuff pressure which is clinically important, and which any instrument must be capable of registering.

The disadvantages of the conventional stethoscope are that the operator is required to make subjective detections of the sound of blood pulses which are faint and intermixed with ambiguous external noises, and that the detennination of the end points of the blood fiow pulse train is somewhat subjective and therefore subject to inaccuracy. To overcome these disadvantages, electric sphygmomanometers have been devised, utilizing electronic to in cooperation with a microphone for amplifying and detecting the auscultatory signals. However, such electronic devices have been subject to system design deficiencies, particularly in providing the required sensitivity, avoiding inaccuracy caused by power supply variations, proper filtering of electronic noise and extraneous signals derived from muscle movements, efficient matching of the electronic circuitry with the microphone, and the problems of power consumption, size and cost.

SUMMARY OF THE INVENTION It is an object of this invention to provide an improved electronic blood pressure indicator which will provide an indication of diastolic and systolic pressure, the indication being free of background noises and extraneous body signals.

It is a further object of this invention to provide an electronic blood pressure indicator which can be used with conventional sphygmomanometer equipment and which will operate with a constant sensitivity.

It is a further object of this invention to provide a blood pressure instrument which clearly indicates blood flow signals and which prevents indication of unwanted signals derived from the patient as well as generated internally by the instrument.

It is a still further object of this invention to provide a noisediscriminating pulse amplifier adapted for blood pressure indication.

Accordingly, the invention provides a microphone sensor which is resistor-calibrated to provide a standardized output voltage; an amplifier which filters out signals outside of a narrow bandwidth of the desired auscultatory signals; a discriminator circuit which establishes the instrument sensitivity by selecting the passable signal amplitude; a pulse generator comprised of opposite polarity transistors to produce a pulse of uniform shape from the variable signal output of the discriminator; a switch and indicator light driven by the pulse generator output and powered by an unregulated power supply; a dual lockout circuit which locks out transmission of any signals to the indicator until a fixed period after a previous signal pulse; and a diode regulated power supply. In operation, the auscultatory signals are detected by the microphone and fed into the amplifier, the amplitude of the signals is sensed by the discriminator which permits passage of the signals from the time cuff pressure is at systolic pressure to the time that it drops to diastolic pressure. After each pulse, the lockout circuit prevents any further indication for a period approaching the time when the next blood pressure pulse is due to arrive.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing the system function of the different portions of the instrument.

FIG. 2 is a schematic diagram of the electronic circuitry of the preferred embodiment.

FIG. 3 is a perspective view of the preferred form of microphone assembly to be used with the instrument of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, the auscultatory sounds caused by the pulsating blood flow are sensed by a sensor 111, which transforms the sounds into electrical signals. The signals are coupled into the bandpass amplifier 12 which filters extraneous signals and amplifies the desired signals in a narrow frequency range of approximately 10 to 50 Hz. The significant sounds to be detected from the occluded blood flow in the artery have a peak intensity at about 40 Hz. Any microphone capable of responding at this low frequency would also produce signals from extraneous noise in the background, as well as from electromagnetic radiations produced by adjacent equipment, lights, etc. There is thus a requirement for a sharp frequency cut off filter to provide filtering before and during amplification of the electrical signal, as will be pointed out in more detail hereinbelow. The signals generated by the microphone and coupled into the amplifier are in the range of nanoamps or lower, and the amplifier accordingly must have a stable gain of several thousands without introducing appreciable noise which would result in ambiguous signals.

The output of the amplifier 12 is coupled to the discriminator circuit 13 which serves as the amplitude selecting component of the system, rejecting smaller level signals generated by extraneous noise. The signal level required to pass the discriminator is the design criterion which sets the sensitivity of the overall system.

The output from the discriminator 13 is coupled into a pulse generator 14 which produces a pulse of uniform shape and amplitude to activate switch 15. The signals passed from the discriminator are highly variable in both frequency and amplitude because of the irregularity of the arterial sounds during the course of the cuff pressure drop. Further, there is a considerable difference in signal strength from patient to patient depending upon the patients arm build and blood pressure. Thus, the pulse generator transforms these variable signals into standardized signals which contain the required information. The switch 15, driven by the output of the pulse generator M, operates the indicator 16, preferably an incandescent light bulb, which provides the instrument output. As will be noted hereinbelow, the light bulb constitutes the largest drain on the battery power source, and it is accordingly important to provide that it be turned on for the shortest possible time consistent with a clearly visible light pulse. Accordingly, the shape of the pulse from the pulse generator and the switching characteristics of the switch are important design features.

In practice, the light indicator 16, which provides signals representing the blood flow sounds, will be used in cooperation with a conventional pressure gauge marking, so that the operator can quickly and efficiently read the systolic pressure when the light first starts to blink, and read the diastolic pressure when the light ceases to blink. For an aneroid pressure gauge the best position for the indicator is in the center of the radial dial, and for a manometer, it is best placed immediately alongside the calibrations on the mercury column. The indicator light may be of any conventional form, although the preferred embodiment incorporates an incandescent bulb having the advantages of operating on low voltage and being long lived.

Still referring to FIG. 1, a dual lockout circuit 17 is indicated, which is activated by the output from the switch 15, and which controls the operation of the pulse generator 14. The regulated power supply 18 provides power to all of the circuitry except the indicator light 16, which is supplied by the battery or unregulated supply 19. Indicator 16 is therefore subject to fading as the battery becomes exhausted. However, the user is assured of accurate readings so long as he can see the flashing light, since the supply 18 will maintain sensitivity and accuracy of the measurements with battery deterioration up to the point where the indicator light is no longer visible.

Referring now to FIG. 3, the active element is a conventional piezoelectric crystal 21 such as a Rochelle salt or a ceramic bimorph. The crystal 21 is mounted from the bottom of the housing on cork or rubber pads 22 on opposite corners of the crystal 21. The pads are suitably 0.03 inch thick and one-sixteenth inch wide, providing a stable platform without unduly restricting the crystal from bending. On top of the crystal a light metal bridge 23 is rigidly attached to the two corners opposite from the support pads, and extends up to a point at the center of the diaphragm 24. The front of the microphone housing is round so that the diaphragm will have the symmetrical vibration pattern of a circular plate. The back of the microphone is made rectangular so that the rubber bladder within the cuff, which folds over the microphone, distributes the pressure evenly and holds the microphone into close contact with the skin.

The microphone is designed to produce an output of about an order of magnitude larger than necessary to drive the amplifier. The desired output is trimmed by inserting, in parallel with the microphone output, a selected resistor 25. The resistor 25 is chosen of a value to trim the effective output voltage of the microphone to a standardized value.

Referring now to FIG. 2, the narrow bandpass amplifier component of the circuitry is comprised of transistors 51, 52, 53, 54 and 55, with associated circuitry. The filtering function is distributed throughout the amplifier. The intrinsic capacitor of the microphone in combination with resistor 42 provides a first low frequency rejection circuit. Additional low frequency filtering is accomplished by capacitor-resistor combinations 41, 43 and 44 connected to the base terminals of transistors 51, 53 and 54 respectively, and combination 45 connected between ground and the emitter of transistor 55. High frequency rejection is obtained by capacitor-resistor combinations 56, 57 and 58 connected between ground and the collectors of transistors 51, 53 and 54 respectively. These networks thus provide an effective narrow bandwidth of frequencies having a peak amplification occurring at approximately 25 Hz, and providing adequate amplification at 40 Hz to detect the desired arterial signals. The bandwidth is peaked at 25 Hz rather than 40 Hz to improve the rejection of 60 Hz noise signals.

The signal is amplified by transistors 51, 53 and 54 and associated circuits acting in cascade. Transistor 52, in emitterfollower configuration, acts to isolate transistor 51 from transistor 53. Transistor 55 provides additional power amplification. Amplifier voltage gain is established and maintained at a known value by holding the base bias voltage of transistors 51, 53 and 56 independent of battery voltage, and by the negative feedback provided by the high value emitter resistors of those transistors.

The discriminator circuit 13 is coupled to the output of the amplifier through capacitor 61, and is comprised of diode 62 and the base-emitter junction of transistor 63. Both diode 62 and the base-emitter junction of transistor 63 have forward bias, or tum-on voltages. The voltage coupled through capacitor 61 must attain the magnitude of the two combined tum-on voltages before signal current will flow into the base of transistor 63. If the signal amplitude supplied through capacitor 61 is less than the sum of the tum-on voltages of diode 62 and transistor 63, no signal current flows into the base of transistor 63, and consequently pulse generator 14 remains in its stable state. By this circuitry, amplitude selection is obtained, with the smaller level noise signals and extraneous body signals being rejected. It is noted that the turn-on voltages of diode 61 and transistor 63 are uninfluenced by battery voltage changes and vary negligibly within component production tolerances.

The pulse generator 14 is comprised of

transistors

63 and 64 and associated elements, and operates in the following manner. Between pulses,

transistors

63 and 64 are non-conducting, and the capacitor 73 is discharged, having no voltage across it. When the signal exceeds the threshold level and turns on transistor 63, a large current flows through

resistors

74 and 75 which are series connected between the collector of transistor 63 and the power supply, developing a forward bias voltage at the base of transistor 64, thus turning it on. With transistor 64 switched on, current is supplied to resistor 71, generating a positive voltage which is impressed across capacitor 73 and resistors 77 and 78 in series with diode 79. Capacitor 73 charges exponentially through diode 79 to a voltage which is approximately the voltage across resistor 71. During the rise in voltage across capacitor 73 diode 80 is non-conducting, and the charging current flows through resistor 77, adding to the current initially flowing through diode 62, thus providing a greater input to transistor 63 and driving it further into an on condition. Since the output of transistor 63 is coupled to the input of transistor 64, transistor 64 is also driven into a greater on condition, in turn increasing the input to transistor 63, and so on. Thus,

transistors

63 and 64, through this regenerative action, are quickly driven into saturation and appear as essentially short circuits, with each supplying the driving current required by the other. This condition is maintained so long as the current passing through resistor 77 is sufficient to maintain saturation of transistor 63, and is a function of the time constant by which capacitor 73 is charged. It is noted that this time constant is determined solely by capacitor 73 and resistors 77 and 78, and not by the amplitude of the incoming signal. When capacitor 73 approaches its fully charged condition, the charging current tapers off and a reverse or degenerative process sets in, quickly taking both

transistor

63 and 64 out of conduction and turning them off, thus terminating the voltage pulse appearing across resistor 71. Capacitor 73 at this time discharges rapidly through diode 80 and resistor 81, diode 79 being non-conducting during this discharge.

From the above, it is seen that pulse duration can be determined by design of capacitor 73 and resistors 77 and 78. Further,

transistors

63 and 64 are chosen as opposite polarity transistors, one being an NPN and the other a PNP, so that both transistors can be in the off state during the relatively long period between pulses, thereby avoiding energy drain from the battery.

The indicator 72 is driven by an indicator switch 15, comprised of transistors 91 and 92 in cascade. When the pulse generator delivers a voltage across resistor 71, the signal is transmitted through resistor 93, turning on transistors 91 and 92. Transistor 92 is driven heavily into conduction, thereby effectively closing the circuit through the power supply 94 and the indicator 72, for the duration of the pulse generator output.

Still referring to FIG. 2, the dual lockout circuitry 17 is comprised of the feedback loop from the collector of transistor 92 to the base of transistor 63, comprising resistor 101, capacitor 102, and resistor 104 in series, resistor 104 being tied to the base of transistor 106, the collector of which is connected to the base of transistor 03. Diode 103 is connected between ground and the node joining resistor 104 and capacitor 102. Resistor 105 is connected between the base of transistor 106 and ground. In operation, between output pulses capacitor 102 is charged to appreciably the battery voltage. When an output pulse drives transistor 92 into conduction, capacitor 102 discharges through resistor 101, diode 103 and transistor 02 until the voltage across its terminals approaches zero. At the end of the output voltage pulse, transistor 92 turns off again, impressing the battery voltage across capacitor 102. The capacitor 102 charges through resistors 101, 104 and 105, with the voltage across resistor 105 appearing on the base of transistor 106, driving it into conduc tion. As transistor 100 is driven into conduction, it clamps the base of transistor 63 appreciably to ground, disenabling it from passing any signals through to the output. Thus, the pulse generator will remain inactive as long as transistor 106 remains in its conductive condition, which condition will be maintained for a duration set by the time constant of the charging path for capacitor 102, i.e., capacitor 102, resistor 101, 104, and 105. When the charging current becomes insufficient to maintain transistor 106 in conduction, it switches to an off state, thereby enabling transistor 63 to pass signals which appear at its input.

The above described lockout circuit serves a dual function. The first function is that of eliminating all extraneous signals other than the anticipated arterial sounds. As the approximate time period between such auscultatory sounds is well known, the time constant of the lockout circuit can be designed to eliminate most extraneous signals which might occur between the desired arterial pulses. The second function of the lockout circuit relates to the power drain of the indicator light. When the indicator light 72 is flashed on, it initially draws an appreciable amount of current as compared to the current load of the entire circuit. Consequently, the voltage source 94 will suffer a drop in voltage, which drop will cause a transient signal to be amplified through the circuit, thereby driving one or more of the transistors into saturation. Due to the relatively large amount of capacitance in the circuit for frequency selectivity purposes, the saturated transistors would not return to their linear state until appreciably after the pulse, at which time additional undesired transients would be generated and result in multiple lamp output flashes, with consequent user confusion and additional battery drain.

Still referring to FIG. 2, the regulated power supply is comprised of battery 941,

capacitors

111, 112 and 113,

diodes

115, 116, 117 and 120, and resistors 118 and 119. Capacitor 113 provides the initial current necessary to drive indicator 72, which requires an initial current about eight times the steady state current of the circuit. Capacitor 112, connected to battery means 941 through a diode 120 maintains an adequate power supply for the amplifier stages during the indicator on period, when light 72 is loading down the battery. Capacitor 111 and resistor 110, connected between capacitor 112 and ground, provide additional filtering of the power supply to transistors 51 and 53, the two most sensitive amplifier stages. The series combination of diodes 115, 116 and 117 in combination with resistor 119 provide an amplifier bias voltage supply substantially independent of the battery voltage. This provision of a stable bias voltage is combined with large value emitter resistors to render the instrument sensitivity relatively immune from battery voltage variations and transistor production tolerances.

What is claimed is:

1. Blood pressure indicator apparatus for indicating the blood pressure of an individual by detecting auscultatory signals originating in the individual, said blood pressure indicator apparatus comprising:

a. microphone transducer means, adapted to be placed against an arm of said individual, for generating electrical signals representative of blood flow sounds within said arm of said individual;

b. narrow bandpass amplifier means having input terminals connected to the output terminals of said microphone transducer means for amplifying said representative signals, having filter sections providing for the amplification only of a narrow bandwidth of low frequency signals corresponding to the appreciable bandwidth of said blood flow signals;

c. discriminator means connected to and driven by said amplifier means for providing output responsive only to signal amplitudes greater than a predetermined amplitude;

. pulse generator means connected to and driven by said discriminator means for providing output pulses of fined amplitude and time duration responsive to the output signals of said discriminator means;

e. indicator means for providing a visual output corresponding to the output pulses of said pulse generator means;

f. switch means connected to and driven by the output of said pulse generator means and connected to and driving said indicator means for driving said indicator means in response to the output of said pulse generator means;

. lockout means connected to said indicator means and to the input of said pulse generator means, for locking out the operation of said pulse generator means for a fixed predetermined time period subsequent to each pulse operation of said indicator means; and,

. power supply means connected to said indicator means, said switch means, said pulse generator means, said discriminator means, and said narrow bandpass amplifier means for energizing same.

2. The apparatus as described in claim 1 wherein said lockout means comprises a capacitor, a low time constant path through which said capacitor discharges when said switch means switches on said indicator means, a second and higher time constant path through which said capacitor charges when said switch means switches said indicator means off, and a normally open transistor switch connected in shunting relationship to the input of said pulse generator means and connected to said second path such that said pulse generator means is held inactive for appreciably the time constant of said second path.

3. The apparatus as described in claim 1 wherein said pulse generator means comprises two transistors of opposite polarity, each having an output terminal, an input terminal, and a common terminal, a direct coupling resistance network connecting the output terminal of the first of said transistors to the input terminal of said second transistor, a second normally non-signal coupling network connecting the output terminal of said second transistor to the input terminal of said first transistor, said second network containing a capacitor in series with a second resistance network to provide a fixed time period of signal coupling from the output terminal of said second transistor to the input terminal of said first transistor.

4. The apparatus as described in claim 3 wherein each of said common terminals is connected to a respective voltage reference, and each of said input terminals is resistively con nected to its respective common terminal, thereby maintaining each of said transistors non-conducting except when generating a pulse in response to a signal from said discriminator means.

5. The apparatus as described in claim 41 wherein a diode is in series with the base-to-emitter junction of said first transistor, said series combination constituting said discriminator means.

6. The apparatus as described in claim 1 wherein said microphone transducer means has a circular front housing and a rectangular back housing, and has a trimming resistor in parallel with its output terminals for standardizing the microphone output voltage.

7. The apparatus as described in claim 1 wherein an unregulated power supply is connected to said indicator means, and a regulated power supply is connected to said switch means, said pulse generator means, said discriminator means, and said narrow bandpass amplifier means.

and to turn off said first and second transistors after a predetermined time period, thereby providing a pulse output;

f. said discharge path resetting said combined circuit to a 8. In an electronic instrument for amplifying a pulse train and for providing an indication of such pulses which exceed a predetermined lower limit in amplitude, a combined discriminator and pulse generator circuit comprising:

a. a first diode, a first transistor with its base-to-emitter junction in series with said first diode, both said diode and said junction having forward bias voltages cooperating to provide a discriminator turn-on voltage level;

b. a second transistor having an opposite polarity, and a resistive network for connecting the collector of said first transistor to the base of said second transistor;

c. a second diode, and a capacitor and first resistor in series connected between the collector of said second transistor and the base of said first transistor through said second diode, thereby providing a feedback path from the output of said second transistor to the input of said first transistor;

. a third diode and a second resistor in series, connected to and providing a discharge path for said capacitor;

. said capacitor having a charging path through said second diode and said first resistor, said charging path cooperating with said first and second transistors to turn on both said transistors when an input voltage in excess of said turn-on voltage is applied to the input of said first diode condition to provide a next pulse output;

g. switch means connected to and driven by the output of said second transistor for providing a switchable output; and

h. lockout means connected to the output of said switch means and to the input of said first transistor, for locking out the operation of said combined discriminator and pulse generator circuit for a fixed pre-determined time period subsequent to each of said pulse outputs.

9. The apparatus as described in claim 8 wherein said lockout means comprises a capacitor, a low time constant path through which said capacitor discharges when said switch means switches on, a second and higher time constant path through which said capacitor charges when said switch means switches off, and a normally open transistor switch connected in shunting relationship to the input of said first transistor and connected to said second path such that said first transistor is held inactive for a second predetermined time period of about the time constant of said second path.

UNITED STATES PATENT OFFICE tE TIFIcA E or CORRECTION Patent No. 3 ,65l,798 Dated September 28 1972 Inventor(s) Paul H. Egli, et al I It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the Title, after "NOISE", add -DI' SCRIMINATING PULSE AMPLIFIER.

Column 1, line 15, after "inflated", tshould be -to-.

Column 1, line 44, after "electronic";v delete -toand insert therefor -circuits-.

Signed and sealed this 3nd day of October 1972.

(SEAL) Attest:

EDWARD MQFLETCHERJRQ ROBERT GOTTSCHALK Attesting Officer v Commissioner of Patents USCOMM-DC 6O376-P69 ".5. GOVERNIIINT PRINTING OFNCE I969 036-)34 "OHM PC4050 (10-69)

Claims

1. Blood pressure indicator apparatus for indicating the blood pressure of an individual by detecting auscultatory signals originating in the individual, said blood pressure indicator apparatus comprising: a. microphone transducer means, adapted to be placed against an arm of said individual, for generating electrical signals representative of blood flow sounds within said arm of said individual; b. narrow bandpass amplifier means having input terminals connected to the output terminals of said microphone transducer means for amplifying said representative signals, having filter sections providing for the amplification only of a narrow bandwidth of low frequency signals corresponding to the appreciable bandwidth of said blood flow signals; c. discriminator means connected to and driven by said amplifier means for providing output responsive only to signal amplitudes greater than a predetermined amplitude; d. pulse generator means connected to and driven by said discriminator means for providing output pulses of fixed amplitude and time duration responsive to the output signals of said discriminator means; e. indicator means for providing a visual output corresponding to the output pulses of said pulse generator means; f. switch means connected to and driven by the output of said pulse generator means and connected to and driving said indicator means for driving said indicator means in response to the output of said pulse generator means; g. lockout means connected to said indicator means and to the input of said pulse generator means, for locking out the operation of said pulse generator means for a fixed predetermined time period subsequent to each pulse operation of said indicator means; and, h. power supply means connected to said indicator means, said switch means, said pulse generator means, said discriminator means, and said narrow bandpass amplifier means for energizing same.

4. The apparatus as described in claim 3 wherein each of said common terminals is connected to a respective voltage reference, and each of said input terminals is resistively connected to its respective common terminal, thereby maintaining each of said transistors non-conducting except when generating a pulse in response to a signal from said discriminator means.

5. The apparatus as described in claim 4 wherein a diode is in series with the base-to-emitter junction of said first transistor, said series combination constituting said discriminator means.

8. In an electronic instrument for amplifying a pulse train and for providing an indication of such pulses which exceed a predetermined lower limit in amplitude, a combined discriminator and pulse generator circuit comprising: a. a first diode, a first transistor with its base-to-emitter junction in series with said first diode, both said diode and said junction having forward bias voltages cooperating to provide a discriminator turn-on voltage level; b. a second transistor having an opposite polarity, and a resistive network for connecting the collector of said first transistor to the base of said second transistor; c. a second diode, and a capacitor and first resistor in series connected between the collector of said second transistor and the base of said first transistor through said second diode, thereby providing a feedback path from the output of said second transistor to the input of said first transistor; d. a third diode and a second resistor in series, connected to and providing a discharge path for said capacitor; e. said capacitor having a charging path through said second diode and said first resistor, said charging path cooperating with said first and second transistors to turn on both said transistors when an input voltage in excess of said turn-on voltage is applied to the input of said first diode and to turN off said first and second transistors after a predetermined time period, thereby providing a pulse output; f. said discharge path resetting said combined circuit to a condition to provide a next pulse output; g. switch means connected to and driven by the output of said second transistor for providing a switchable output; and h. lockout means connected to the output of said switch means and to the input of said first transistor, for locking out the operation of said combined discriminator and pulse generator circuit for a fixed pre-determined time period subsequent to each of said pulse outputs.