Photoelectric physiological measuring apparatus

I claim:

1. An apparatus for the non-invasive measurement of physiological data obtainable from monitoring blood flow in the circulatory ststem comprising:

a. a pulsating light source adapted to be placed in non-invasive adjacent relationship to at least a portion of said circulatory system;

b. light receiving means for receiving light signals from said light source after reflection from said circulatory system, and for generating electrical signals representative of said received light signals;

c. a demodulating circuit means, connected to said light receiving means, for demodulating said electrical signals to amplitude modulate said pulsating light representations, and having an output terminal for transmitting said demodulating signals;

d. circuit means, connected to said output terminal, for providing at least one of the electrical functions:

i. averaging over a predetermined time interval,

ii. detecting peak signal magnitudes, and

iii. passing signals over a predetermined frequency band; and

e. display means, connected to said circuit means, for providing a visual display of said electrical functions.

2. The apparatus of claim 1, further comprising a common housing wherein said pulsating light source and said light receiving means are mounted.

3. The apparatus of claim 2 wherein said pulsating light source further comprises a plurality of light emitting elements mounted in a symmetrical relationship about said light receiving means in said housing.

4. The apparatus of claim 3 wherein said pulsating light source further comprises three light-emitting diodes and said light receiving means further comprises a photocell.

5. The apparatus of claim 2 wherein said circuit means providing the electrical function of averaging over a predetermined time further comprises an integrating circuit means having the capability of averaging over at least 50 seconds.

6. The apparatus of claim 2 wherein said circuit means for providing the electrical function of passing signals over a predetermined frequency band further comprises a band pass filter circuit means for passing signals in the frequency range 3.75 - 6.75 Hz.

7. The apparatus of claim 2 wherein said circuit means for providing the electrical function of detecting peak signal magnitude further comprises a trigger circuit adjustable for detecting signal magnitudes representative of heart beats.

8. The apparatus of claim 2 further comprising a pulse amplitude circuit, connected to said output terminal, said pulse amplitude circuit having an integrating means for storing a voltage representative of said demodulated signals peak amplitudes; and display means for providing a visual display of the value of said stored voltage.

9. The apparatus of claim 8, further comprising a means for selectively discharging said integrating means, said discharging means comprising a timing circuit activated by said circuit means for detecting peak signal magnitude.

10. The apparatus of claim 9 wherein said timing circuit activation interval is predetermined and fixed.

11. The apparatus of claim 10 wherein said integrating means is discharged by said discharging means at a rate corresponding to the heartbeat rate.

12. An apparatus for the non-invasive measurement of physiological data obtainable from monitoring blood flow in the circulatory system, comprising:

a. a housing including a pulsating light source adapted to be placed in non-invasive relationship and adjacent to at least a portion of the circulatory system;

b. a light receiving means, located in said housing in fixed relationship to said pulsating light source, for receiving light signals reflected from said circulatory system portion, and for generating electrical signals representative of said received light signals;

c. demodulating circuit means, connected to said light receiving means, for developing an envelope signal tracking said electrical signals' peak amplitudes;

d. an averaging circuit connected to said demodulating circuit means, said averaging circuit having means for developing a voltage representative of said envelope signal average value over a time period of at least about 50 seconds; and

e. an indicator connected to said averaging circuit, said indicator having a visual display means for displaying said average voltage signal.

13. The apparatus of claim 12 wherein said pulsating light source further comprises a plurality of light emitting elements mounted in a symmetrical relationship about said light receiving means in said housing.

14. The apparatus of claim 13 wherein said pulsating light source further comprises three light-emitting diodes and said light receiving means further comprises a photocell.

15. The apparatus of claim 14 wherein said pulsating light source frequency of pulsation is about 1000 Hz.

16. The apparatus of claim 15 wherein said pulsating light source has a duty cycle of about 10%, and a light wavelength of about 9000 A.

17. The apparatus of claim 16 wherein said averaging circuit further comprises an integrating capacitor circuit having a discharge time constant of about 50 seconds and a charge time constant of about 1 second.

18. The apparatus of claim 17 wherein said indicator further comprises a voltage meter of the D'Arsonval type.

19. An apparatus for the non-invasive measurement of physiological data obtainable from monitoring blood flow in the circulatory system, comprising:

a. a housing including a pulsating light source adapted to be placed in non-invasive relationship and adjacent to at least a portion of the circulatory system;

b. a light receiving means, located in said housing in fixed relationship to said pulsating light source, for receiving light signals reflected from said circulatory system portion, and for generating electrical signals representative of said received light signals;

c. demodulating circuit means, connected to said light receiving means, for developing an envelope signal tracking said electrical signals' peak amplitudes;

d. a band pass circuit connected to said demodulating circuit means, said band pass circuit having means for passing signals in the frequency range 3.75 - 6.75 Hz; and

e. an indicator connected to said band pass circuit, said indicator having a visual display means for displaying signals passed.

20. The apparatus of claim 19 wherein said pulsating light source further comprises a plurality of light emitting elements mounted in a symmetrical relationship about said light receiving means in said housing.

21. The apparatus of claim 20 wherein said pulsating light source further comprises three light-emitting diodes and said light receiving means further comprises a photocell.

22. The apparatus of claim 21 wherein said pulsating light source frequency of pulsation is about 1000 Hz.

23. The apparatus of claim 22 wherein said pulsating light source has a duty cycle of about 10%, and a light wavelength of about 9000 A.

24. An apparatus for the non-invasive measurement of physiological data obtainable from monitoring blood flow in the circulatory system, comprising:

a. a housing including a pulsating light source adapted to be placed in non-invasive relationship and adjacent to at least a portion of the circulatory system;

b. a light receiving means, located in fixed relationship to said pulsating light source, for receiving light signals reflected from said circulatory system portion, and for generating electrical signals representative of said received light signals;

c. demodulating circuit means, connected to said light receiving means for developing an envelope signal tracking said electrical signals' peak amplitudes;

d. an amplitude indicating circuit connected to said demodulating circuit means, said amplitude indicating circuit having means for developing a voltage representative of the average value of said envelope signals' peak amplitudes;

e. an indicator connected to said amplitude indicating circuit, said indicator having a visual display means for displaying said developed voltage signal.

25. The apparatus of claim 24 further comprising a peak detector circuit connected to said demodulating circuit means and to said amplitude indicating circuit, said peak detector circuit having means for detecting peak envelope signal voltages and for disabling said amplitude indicating circuit for a predetermined time interval after each peak voltage detection.

26. The apparatus of claim 25 wherein said pulsating light source further comprises a plurality of light emitting elements mounted in a symmetrical relationship about said light receiving means in said housing.

27. The apparatus of claim 26 wherein said pulsating light source further comprises three light-emitting diodes and said light receiving means further comprises a photocell.

28. The apparatus of claim 27 wherein said pulsating light source frequency of pulsation is about 1000 Hz.

29. The apparatus of claim 28 wherein said pulsating light source has a duty cycle of about 10%, and a light wavelength of about 9000 A.

Description Submit all comments and votes

BACKGROUND OF THE INVENTION

This invention relates to measurement of physiological data in general, in particular to measurement of data representative of the circulatory system, and specifically to photoelectric measurement of the blood volume waveform of the circulatory system. The circulatory system is contained in a continuous endothelial sac from the heart to the terminal microcirculation ending in the capillaries and venules. The covering of the endothelial sac varies from the thick muscular covering of the heart to no covering at all in the capillaries.

The control of the microcirculation, i.e., small arteries and veins, arterioles and meterarterioles, is essentially muscular and primarily neurogenic. The capillaries respond to local cellular needs, i.e., pH, O.sub.2 levels and nutritional needs. The microcirculation functions to sustain life itself, the larger components of the system are merely subservient to local needs.

The function of blood pressure, i.e., heart action and compliance of larger vessels, are secondary to local needs of perfusion. These needs vary from second to second, from organ to organ; but the end result of all body functions is to maintain homeostasis of the total organism. As the organism ages or changes in response to any condition of stress or environment, drugs, disease, etc., the microcirculation makes compensatory corrections to meet these conditions of cellular demand. The microcirculation makes this change long before central reactions are noted in a compensatory way; indeed the central reactions are ultimately fixed in response to the continuing cellular demand of an aging or disease process. These same demands are evidenced in an acute way, i.e., shock, surgery or sudden environmental changes, usually well in advance of central changes.

By measuring changes in the microcirculation as they occur, it is possible to anticipate and correlate diagnosis and treatment of a great number of body conditions. Many ways have been devised to measure circulatory change; commonly used blood pressure cuffs, indwelling venous and arterial catheters, fiber optic catheters, angiography, dye dilution techniques, glass electrodes on muscle tissue, retinal microscopy, E.C.G., Doppler principle transducers, Wheatstone bridge plethysmography, microsurgery in animals, and many more. With few exceptions, most highly accurate techniques are invasive and are concerned mainly with the "large" circulation and its changes, changes brought about for the most part by demands of the microcirculation in response to life itself.

The present invention provides an apparatus for practicing a non-invasive, rapid technique for measuring microcirculatory homeostasis or change. It operates by the simple topical application of a probe on the skin, and includes circuitry which enables the user to "read" the microcirculation in any suitable area of the body.

DESCRIPTION OF THE PRIOR ART

The circulatory system characteristic of blood volume has long been recognized as an important element of diagnostic data. For example, the U.S. Pat. No. 161,821, which issued on Apr. 6, 1875, to E. A. Pond, is for a "Sphygmoscope." The Sphygmoscope is a forerunner of the apparatus which is now referred to as an oscillometer. Basically the sphygmoscope and oscillometer utilize the principle of hydraulics to measure blood volume. The earlier devices, such as the above referred to sphygmoscope, were essentially quantitative data measurement mechanisms in that they were mostly concerned with providing an indication of maximum and minimum blood volume. Later devices provided increasingly more sophisticated data as illustrated by U.S. Pat. No. 3,083,705 which issued Apr. 2, 1963, to Carl A. Johnson for Vascular Recording Apparatus. This patent discloses an oscillometric apparatus having a response time comparable to the electrocardiograph in that it records circulatory events occurring at intervals on the order of 0.04 seconds. In short, these later devices provide qualitative data, specifically data representative of the blood volume waveform, including such waveform characteristics as the crest time and time of the diastolic slope.

Improvements during the past 20 years, in part at least to overcome certain limitations of the hydraulic principle devices, have led to development of photoelectric measurement apparatus. While these photoelectric apparatus are free of certain shortcomings of the hydraulic devices, they are inferior in other respects. Specifically, the photoelectric devices of the prior art characteristically provide data of the end-point variety (systolic and diastolic pressures) together with other quantitative data such as pulse rate. These photoelectric devices provide measurements of physiological data by introducing light into a measurement area of the body. In the body, the light is modulated such as by absorption and reflection. The modulated light, after either transmission through or reemission from the body, is collected and demodulated. Generally, the circulatory system of the measurement area is the variable characteristic and the demodulated physiological data measured is thus representative of the circulatory system. Briefly, photoelectric devices provide a composite signal which includes both meaningful and non-meaningful data. Non-meaningful data includes data which is characteristic of the light source or electrical circuitry associated therewith, or data which is representative of circulatory system functions or structure not of direct and immediate interest in the test being performed. In separating (demodulating) the meaningful from the non-meaningful data, the devices distort the data such that, while still retaining quantitative data such as minimum and maximum pressures and pulse rate information the more sophisticated qualitative data of the blood volume waveform is "distorted," most often to the extent that no meaningful waveform qualitative data is provided.

One known photoelectric apparatus which provides a blood volume waveform of some utility, however, is disclosed in U.S. Pat. No. 3,412,729, which issued Nov. 26, 1968, to J. R. Smith, Jr. The apparatus of this Smith patent provides two representations of the blood volume waveform. A first representation is provided as the output of a DC amplifier (FIG. 1, element 50) which extracts the meaningful data by application of a "bucking voltage" which tends to cancel so-called direct current (DC) non-meaningful data. The bucking voltage is adjusted to a predetermined constant level. The DC non-meaningful data, however, is not a constant but varies according to such things as the physical position of the probe. Probe movement during a measurement has been found to be a problem, even when a patient has been anesthetized. The DC non-meaningful data, and hence the amount of bucking voltage which would be required, can also shift gradually in accordance with such factors as physiological changes in the patient which alter the optical properties of the tissues, including changes in fluid balance. These latter changes in particular can sometimes occur very rapidly and thus, from an electronic circuit standpoint, at the time of their occurrence they are effectively a high frequency signal which of course are not cancelled by the bucking voltage. As for the second waveform representation, it is provided as the output of a so-called AC amplifier (FIG. 1, element 60) and is useful for one of the objects of the patent, namely for determining the difference between systolic and diastolic pressures. This amplifier 60 provides a signal which is measured with reference to, and alternates between positive and negative excursions above and below, a potential of zero volts. For such an alternating signal, the peak positive excursion corresponds to systolic pressure and the peak negative excursion corresponds to diastolic pressure, the excursions are approximately equal, and an approximation of the pressure difference is simply either their difference or twice the positive peak excursion value. It is thus seen that according to Smith, a first waveform is provided which is subject to error as a result of changes in the non-meaningful data signal and a second waveform is provided which provides quantitative data.

SUMMARY OF THE INVENTION

The present invention is an electronic apparatus for continuously monitoring tissue perfusion. The apparatus utilizes a pulsed light source and sensor mounted in a probe, which is attached to the outer surface of the patient's skin by non-invasive techniques. The apparatus provides an indication of one or more body functions of interest to the physician treating the patient. In the preferred embodiment, the following four indications are provided by the electronic circuits connected to the sensor:

Pulse rate

The sensor signal is amplified, demodulated, filtered and processed through a suitable indicator circuit to provide a direct count indication of the patient's heart beat.

Pulse wave

The sensor signal is amplified, demodulated, and passed through an electronic gain and filter circuit to provide an indication of cardiac output, particularly ventricular ejection, as seen by the tissues.

Pulse amplitude

The pulse wave signal is integrated over a predetermined time interval to provide an indication of the magnitude of the excursion between minimum and maximum values of the pulse wave signal. The resultant signal provides an indication of the left ventricular capacity.

Vaso bed

The pulse amplitude signal is integrated over a predetermined time interval to provide an indication of the patient's average, or baseline, tissue perfusion. The continuing monitoring of tissue perfusion changes provides a predictive and diagnostic tool for impending shock, blood loss, cardiac failure, dilution effects, etc. This indication provides an early warning which can be used to evaluate the total body reaction to a given set of circumstances, both vasodilation and vasoconstriction.

Photoelectric measurements of the circulatory system provide a composite signal representative of all activity and structures in the measurement area. The blood volume waveform, including qualitative data free of distortion of high frequency non-meaningful data, is provided by extracting from said composite signal those variations having a frequency greater than about 10 Hz. Such a waveform permits measurement of qualitative data such as data of the left ventricular ejection phase of a heart beat cycle and also permits of accurate measurement of quantitative data such as tissue perfusion. According to a further feature of the invention non-meaningful low frequency data components are also removed from the composite signal.

In a preferred embodiment, extraction of meaningful data signals from high and low frequency non-meaningful data is accomplished by means of a combination shunt and series capacitor. The embodiment measures a reflected signal (as opposed to a signal transmitted entirely through a body member) and the embodiment also operates in a pulsed mode; the pulse duty cycle is about ten percent (approximately a 100 micro-second pulse interval and a 900 microsecond inter-pulse interval). The shunt capacitor is chosen to have a capacitive reactance at high frequencies (frequencies above about 10 Hz.) which is low to provide a shunt path to ground thereby extracting such high frequency signals from the composite signal. For this pulsed mode embodiment, the capacitor also performs an integrating function to further demodulate the composite signal. The non-relevant data of the composite signal includes a base component which is by far the largest of the components making up the composite signal. The signal base component includes light reflected from the various skin layers, tissue cells and other matter not a part of the circulatory system. The base component has been found to represent up to 99 percent of the composite signal, and includes the aforementioned DC component discussed with reference to the Smith patent. The capacitive reactance characteristic of the series capacitor effectively blocks the complete range of base component signal low frequency non-relevant data signals. In selecting the values of the components which make up the charge and discharge time constants of the circuit, for a pulsed mode operated embodiment, values are chosen which provide a charge time constant fast enough to preserve the waveform. For the preferred embodiment inter-pulse interval of 900 micro-seconds, relevant data signal peak amplitude of about 0.01 volts amplified and applied to the integration capacitor, and a ratio of relevant to non-relevant data of about 1 to 99, an integration circuit having a charge time constant of about 0.001 second and a discharge time constant of about 0.1 second provides an accurate waveform. (The terms "low" and "high" frequency are used in a relative sense and not in a literal technical sense of precise frequency ranges as set forth in manuals of electronic standards). Also, definition of the discharge time constant in terms of the amplitude of the relevant data signal is in terms of an assumed mean-value signal since the actual signal does of course vary in amplitude.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustration of a photoelectric physiological data measuring system according to the present invention;

FIG. 2, Parts A and B, is an electrical schematic diagram of an essentially discrete component implementation of the system of FIG. 1;

FIG. 3 is an illustration of waveforms representative of a blood volume waveform at various points in the circuit of FIG. 2;

FIG. 4 is a mechanical schematic diagram of the probe 12 or FIG. 2;

FIG. 5, Parts A, B and C, is a schematic diagram of a basically integrated circuit implementation of the system of FIG. 1; and,

FIG. 6 is a block diagram of digital readout circuitry for use in combination with the measurement devices of FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, a photoelectric physiological data measuring system is shown generally as 10. The system includes a photoelectric probe 12 and a demodulator circuit 14. Photoelectric probes are well known in the art, and include both those which measure relected light and those which measure light transmitted through the body. The demodulator 14 includes a circuit for operating on a composite signal from probe 12 to extract signal variations less than about 10 Hz. to provide a demodulated signal the waveform of which includes circulatory system qualitative data free of distortion by non-meaningful high frequency data. The exact method of extraction is immaterial; an amplifier or filter for preferentially passing the desired signal or a shunt or blocking circuit for selectively removing undesired signal may be used.

Also shown in FIG. 1 by means of phanton lines are a group of blood volume waveform measurement devices shown generally as 16. Such devices may be any of a variety of well known devices, such as a graphic recorder, also known as a strip or pen recorder, or a pulse rate meter. The devices may also be measurement apparatus such as tissue perfusion and left ventricular ejection meters.

FIG. 2 is a schematic diagram of a basically discrete component implementation of a preferred embodiment of a measurement system as shown in FIG. 1. As shown, probe 12 includes a light source, shown generally as 18, which consists of three light emitting diodes 20 adapted by a terminal 22 for interconnection to an energy source, not shown. Also included in probe 12 is a photo-transistor 24. The mechanical structure for mounting the diodes 20 and photo-transistor 24 is not shown as it may take any of a variety of configurations. U.S. Pat. Nos. 3,040,737 issued June 26, 1962 to A. D. Kompelien et al. and 3,602,213 issued Aug. 31, 1971 to William L. Howell and William B. Leaf disclose two such configurations. For the actual embodiment, the phototransistor 24 is mounted in the center of a flat circular disc. The three diodes are positioned on radials forming angles of 120.degree. about midway between the center and outer periphery of the disc. The diodes 20 are optically isolated from phototransistor 24. More important than the mechanical structure of the probe 12, is the optical characteristics of the diodes 20. The exact diodes employed in the preferred embodiment are set forth in Table 1 hereinafter. The important optical characteristics of the diodes 20 are that they provide light at a wavelength other than that of ambient light (i.e., light from ordinary incandescent and fluorescent lamps) and at a wavelength insensitive to blood oxygen saturation of the blood. The diodes 20 of the preferred embodiment have a wavelength peak emission of 9,000 A. Such a wavelength also provides good penetration of the epidermis, dermis, and subcutaneous layers of the skin, and is minimally absorbed by the skin pigments B-carotene and melanin to permit use of low wattage diodes to avoid excessive tissue heating. When the diodes are operated in a pulsed mode, the diodes 20 can even be safely over-driven. For the preferred embodiment duty cycle of ten percent and pulse interval of one-hundred microseconds, each diode is driven by a current of 83.5 milli-amps which amounts to over-driving the diode steady state maximum rating by about 67%. This 83.5 milli-amp driving current is provided by a pair of monostable multivibrators (not shown), one for pulse duration and the other for repetition rate. Each multivibrator is a separate integrated circuit; both are externally adjustable, regeneratively connected by four gates and drive a high speed silicon transistor. In order to respond fast enough to a pulse and be sufficiently responsive at wavelengths about 9,000 A, a silicon phototransistor is employed as photodetector 24.

Referring again to FIG. 2, the demodulator circuit 14 is shown as comprising an input interface section 26, combination shunnt and integration section 28, and an output interface section 30. The input section 26 includes an isolation capacitor 32, an impedance matching emitter follower shown generally as 34 and a common emitter amplifier shown generally as 36. Demodulator 28 includes a resistor 38 and capacitor 40. Also included in demodulator 28 is a diode 42 which is necessary, in a pulsed mode operation embodiment such as that of FIG. 2, in order to prevent excessive discharge of capacitor 40 through common emitter amplifier 36 during inter-pulse intervals. The output stage 30 comprises another impedance matching emitter follower shown generally as 44. It is not essential that the demodulator 28 be intermediate the input and output sections 26 and 30. The demodulator could, for example, precede the input section 26 in which case a collector resistor would need to be provided for amplifier 36 because resistor 38 serves as both such a collector resistor and as the resistive component of an integrator formed principally by the resistor 38 and capacitor 40. In addition to being the capacitive component of the integrator, capacitor 40 is a shunt path for high frequency non-meaningful data signals.

The balance of FIG. 2 comprises a group of utilization devices shown generally as 16. This group of devices includes an input interface section shown generally as 46 and four meter sections. One meter section is a pulse amplitude meter, shown generally as 48, another a pulse wave meter, shown generally as 50, a third is a pulse rate meter, shown generally as 52, and the other meter section is a "vaso bed" or tissue perfusion meter, shown generally as 54. Briefly, the input interface section 46 provides impedance matching to the probe circuitry which derives a signal representative of the blood volume waveform from the composite measurement signal which corresponds to the light pulses as modulated in the measurement area. Section 46 also includes those stages of amplification common to two or more meter sections, in order to avoid redundant circuitry. A further consolidation of circuitry is possible and was actually made in the actual embodiment which corresponds to FIG. 2; specifically, it is possible to entirely eliminate the circuitry between nodes 56 and 58 in pulse wave meter 50 by reconnecting capacitor 60 at nodes 62 and 64 and interconnecting node 58 to node 66. The reason for including the redundant circuitry in FIG. 2 will be made apparent shortly.

Considering now the input interface section 46 of the utilization devices 16, it comprises merely a pair of common emitter amplifiers and their associated biasing components. The common emitter amplifier 70 in addition to providing signal amplification, provides impedance matching with and is capacitively coupled to the output interface section 30 of demodulator 14. Further amplification is provided by the second common emitter amplifier, shown generally as 72. Also included in interface section 46 are variable resistances 74 and 76.

The pulse amplitude meter 48 provides an indication of the waveform characteristic of pressure; specifically, meter 48 indicates the difference between the waveform maximum and minimum pressures (the systolic and diastolic pressures, respectively). Meter 48 amplifies the waveform signal provided by the input interface section 46 on lead 90 by means of amplifier 92. The waveform signal after amplification by amplifier 92 is integrated by the combination of resistor 94, capacitor 96, and capacitor 98 and applied to the base of amplifier 100 which drives a conventional D'arsonval pointer-indicator meter 102. Also included in pulse amplitude meter 48 is a compensation circuit, shown generally as 104, which compensates for variations in heart beat rate. Without such a compensation circuit, an increase in heart rate would appear on indicator 102 as an increase in the pressure differential between systolic and diastolic pressures and, conversely, a decrease would appear as a pressure differential decrease. Compensation circuit 104 provides, during a portion of each waveform pulsatile representation of a heart beat, essentially a short circuit to ground for both the pulse waveform representation out of amplifier 92 and the discharge path of the integrating components of resistor 94 and capacitors 96 and 98. The compensation circuit 104 provides this short circuit to ground for a fixed interval. As the heart beat rate increases, the short circuit exists a greater percentage of the time and, conversely, as the heart beat rate decreases, the short circuit exists a lesser percentage of the time. Briefly, compensation circuit 104 includes a transistor gate 106 conduction of which is controlled by impulses received via lead 108 from a monostable multivibrator (single-shot) of the pulse rate circuit 52.

The pulse wave meter 50 shown in FIG. 2 is specifically designed to preferentially measure signals of a frequency in the range of about 3.75 Hz. to 6.75 Hz. in the signal it receives on lead 110 from input interface section 46. For the actual embodiment of the preferred embodiment illustrated in FIG. 2, capacitors 60 and 68 were selected to have a value providing a frequency response curve the 3 db points of which are 0.63 Hz. and 40 Hz.; the curve is essentially flat within a range from 3.75 Hz. to 6.75 Hz. to preferentially amplify components of a waveform within this range. Series coupled capacitor 60 attenuates signals of a frequency below about 3.75 Hz. and shunt connected capacitor 68 shunts to ground signals above about 6.75 Hz. Briefly, the waveform components within this range are those representative of the heart beat phase known as "left ventricular ejection." It is sufficient for purposes of this discussion, that it be understood that the capacitors 60 and 68 emphasize the left ventricular ejection phase of the heart beat cycle. When the probe 12 is positioned over an arterial measurement area, the effect of capacitors 60 and 68 is to preferentially pass the left ventricular ejection phase of a heart beat waveform and attenuate other phases of the waveform, particularly those immediately preceding and succeeding the left ventricular phase. When the probe 12 is positioned over a microcirculatory area, the waveform represents blood flow at a significant distance from the heart and in vessels small compared to the arteries. The blood waveform loses some of the waveform arterial characteristics, including the steepness of the slope of the waveform leading edge of each heart beat pulse in traveling such a distance and as a result of the damping effect of the small vessels of the microcirculation. This leading edge corresponds to the left ventricular ejection phase of the heart beat cycle. When the probe 12 is positioned over a microcirculatory area, the effect of capacitors 60 and 68 is to restore the left ventricular ejection characteristic to the waveform thereby permitting a measurement of the microcirculation, typically used for indicating tissue perfusion, to also provide qualitative waveform data such as left ventricular ejection. Also included in pulse wave meter 50 is a common emitter amplifier 72A and a D'arsonval movement pointer indicator 116. As previously stated, the amplifier 72A is redundant and can be eliminated simply by re-positioning capacitor 60 and re-connecting node 58. The amplifier 72A was included in FIG. 2 in order to permit a logical grouping of the various circuit components into input interface section 46 and four meter sections 48, 50, 52 and 54.

The pulse rate meter 52 derives its input from the second amplifier 72 of the input interface section 46 via lead 120. Basically, the meter 52 counts the pulsatile waveform representations of heart beats, each pulsatile representation ("pulse") corresponding to a single heart beat. According to the illustrated embodiment, each pulse is converted to a unit of voltage and applied to a voltage integrator. Accordingly, the voltage stored by the voltage integrator at any time is representative of pulse rate. Meter 52 includes a mon