Blood pressure measuring system

Abstract

The blood pressure measuring system includes the generation of output signals by correlation of pressure pulses detected in a fluid system used to close off the blood flow, with acoustic pulses detected downstream near the blood flow close-off point in a patient's arm or other portion of his body. Correlating the pressure pulses with the acoustic pulses and generating output signals only when there is a coincidence between the pressure and acoustic pulses minimizes the effect of spurious signals and artifacts to the end that a more accurate systolic and diastolic blood pressure reading may be provided. In this respect, the systolic pressure is indicated at the point in time that the output signals start and the diastolic pressure is indicated at the point in time that the output signals terminate.

Description

This invention relates to blood pressure measuring systems and more particularly to an improved system for automatically indicating and/or recording a patient's systolic and diastolic blood pressures by the use of pneumatic and electronic components so that it is no longer necessary for a doctor or nurse to conduct the various steps involved in determining such pressures.

BACKGROUND OF THE INVENTION

The systolic blood pressure of a patient or person is a measure of the peak or maximum pressure in the patient's circulatory system whereas the diastolic blood pressure is a measure of the average pressure of blood flowing through the circulatory system. The conventional and normal procedure for determining these pressures involves a doctor or nurse closing off a portion of the patient's circulatory system, such as at the patient's arm to block flow of blood to the lower portion of the arm. A fluid pressure chamber normally operated by air is wrapped about the upper portion of the patient's arm and pressure is applied until the blood flow is cut off. By the use of a stethoscope positioned downstream of the close-off point, the doctor can determine when sufficient pressure has been applied to close off the blood flow, there being an absence of acoustic pulses resulting from blood pressure and flow pulses detected in the stethoscope.

After the close-off has been achieved, the doctor or nurse will gradually release the pressure and carefully listen with the stethoscope for the start of blood flow to the lower portion of the arm. At the instant a first acoustic pulse is detected in the stethoscope, a reading of a suitable manometer or other pressure indicating device connected to the fluid chamber is taken, thereby indicating the systolic blood pressure. The pressure is gradually decreased further until such time as there is an absence of pronounced acoustic pulses detected by the doctor listening in the stethoscope and at this point, another pressure reading is taken, which serves to indicate the diastolic blood pressure.

The acoustic pulses, called Korotkoff sounds in the medical profession, are divided into five phases based on changes in the characteristics of the sounds as cuff pressure is decreased. Phase IV is a muffled sound relative to the first three phases. Phase V is silent. There is some disagreement on whether the end of Phase III or Phase IV more accurately represents the true diastolic pressure.

While it is a widespread practice for technicians and nurses to take a patient's blood pressure in accord with the foregoing procedure, very few persons even among professionals are really skilled at determining accurately the systolic and diastolic pressure without taking internal taps. One of the major problems is the introduction of extraneous signals (artifacts) by movement or activity of the patient and incidental interactions between the instrument components, such as the stethoscope and the patient, his clothing, and so forth. Extraneous room noises also interfere with a proper detection of changes in blood flow. A properly trained physician or nurse can, after much experience, conduct fairly accurate blood pressure measurements. In this respect, the doctor's sensitivity to proper acoustic pulses and improper or erroneous signals becomes developed to the point that he can exclude most spurious signals in making his measurement.

Since there are relatively few persons really capable of taking accurate blood pressure measurements, several systems have been proposed for mechanizing the process to the end that the subjective factors introduced when an untrained person attempts to take the pressure can be eliminated. However, such mechanized systems for automatically indicating or recording the systolic and diastolic pressures are also subject to false triggering by spurious signals and it has been found that a trained doctor or nurse can still best provide the most accurate measurements. Nevertheless, it would still be desirable if there were some means for accurately determining a patient's blood pressure in a thoroughly automatic manner. Such an automatic system would enable large numbers of patients to have their blood pressures taken with consistent results. Moreover, the necessity of trained specialists for carrying out the procedure could be avoided.

BRIEF DESCRIPTION OF THE PRESENT INVENTION

With the foregoing considerations in mind, the present invention contemplates a system for automatically measuring a patient's systolic and diastolic blood pressure wherein most extraneous noises and spurious signals are eliminated all to the end that great accuracy is achieved in a fully automatic manner.

Briefly, the system involves applying pressure to close off blood flow to a portion of the patient. The applied pressure is then decreased gradually and then pressure pulses are detected corresponding to the patient's heartbeat and blood flow pulses in the closed off portion. Also acoustic pulses are detected in a portion of the patient downstream of the blood flow as the applied pressure is being decreased. Electrical signals in turn are generated corresponding to the detected pressure and acoustic pulses. These signals are then quantized and correlated to provide an output signal only when a pressure signal occurs simultaneously with an acoustic signal. Spurious signals or extraneous pulses are thus substantially eliminated, since most disturbances causing pressure fluctuations do not produce significant acoustic output while acoustic interference does not produce significant pressure transients. On the other hand, heartbeats within the appropriate cuff pressure range produce pressure and sound in essential time synchronism.

The output signals resulting from coincidence of the pressure and acoustic signals are then utilized to display or record automatically a pressure reading at points in time corresponding to the start and termination of the output signals.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the system of this invention will be had by now referring to the accompanying drawings in which:

FIG. 1 is a block diagram of the automatic blood pressure measuring system of this invention wherein the blood pressure is being taken from a patient's arm; and,

FIG. 2 illustrates a pressure analog signal plot and pulse diagrams indicating conditions at correspondingly lettered points in the block diagram of FIG. 1, useful in explaining the operation of the system.

DETAILED DESCRIPTION OF THE INVENTION

Referring first to FIG. 1, there is shown a patient 10 together with means for applying pressure to a portion of his body, such as his left arm 11, as by means of a flexible fluid chamber 12. The chamber 12 may constitute a wrap-around flat tubular arrangement which can be secured so that increasing the fluid pressure within the chamber applies pressure to the arm 11.

Towards the foregoing end, there is provided an inlet tube 13 including an inlet control valve 14 in turn receiving fluid under pressure from a source 15. The valve 14 is of the proportional type so that the rate of pressure build up or application to the fluid chamber 12 can be controlled by the degree of opening of the valve 14.

Means for gradually decreasing the pressure in the fluid chamber 12 includes an outlet tube 16 having an outlet valve 17. The outlet valve 17 is also of the proportional type so that the rate of bleeding off of fluid pressure from the chamber can be controlled. A pressure transducer 18 in the tube 13 serves to convert the actual pressure changes occurring in the fluid chamber 12 to an electrical analog signal.

In the particular embodiment illustrated, a means for generating pressure pulses corresponding to the patient's heartbeat takes the form of a differentiating circuit 19 connected to receive the analog signal from the pressure transducer 18. As will become clearer as the description proceeds, the differentiating circuit 19 detects pressure fluctuations in the analog signal to provide essentially a series of electrical pressure pulses which are then passed through a filter and threshold detector network 20 to provide quantized electrical pressure signals. As shown, these electrical pressure signals are passed into one side of a coincidence circuit 21.

Means responsive to pulses in the downstream side of the close-off pressure point in the patient's arm 11 are provided to generate acoustic pulses. This means in the embodiment illustrated takes the form of an acoustical conduit 22 with one end 23 in physical contact with the patient's arm and its other end terminating in a microphone 24, preferably mounted in sound insulation and vibration isolating material 25 to shield the microphone from ambient noise and perturbations. Acoustic pulses from the microphone are passed through a filter and threshold detector network 26 to provide quantized output electrical acoustic signals which are then passed through the other side of the coincidence circuit 21.

The coincidence circuit 21 will provide an output signal only when signals simultaneously occur at its first and second inputs. These output signals are passed to a measuring means 27 which may include a digital display and/or recording means.

Referring again to the output of the pressure transducer 18, it will be noted that the analog pressure signal is also passed to an analog-to-digital converter 28 which essentially converts the pressure values into digital coded signals. These coded signals are passed along a first line 29 into the measuring means 27, the measuring means being triggered to display or record the specific pressure coded signal fed thereto at the start of output signals received therein and at the termination of such output signals. Thus, the coded signals are stored and updated during each output signal. After termination of the output signal the first and last stored coded signals are displayed.

The same coded signals are also passed along a line 30 to a logic control circuit 31 connected to control the action of the inlet and outlet valves 14 and 17 through summing junctions as shown. It will also be noted that the output signals from the coincidence circuit 21 are also passed to the logic control circuit 31 by way of line 32 and that differentiated signals from circuit 19 are passed to the summing junctions. The proportional operation of the valves 14 and 17 are thus responsive to pressure rates of change feedback signals from the differentiating circuit 19.

In the upper graph of FIG. 2, there is illustrated at 33 the analog pressure signal appearing at the point A at the output of the pressure transducer 18. This plot represents the changes in pressure in millimeters of mercury occurring in the fluid chamber 12 of FIG. 1 as a function of time in seconds. It will be noted that the pressure is applied at a given rate over a given range which rate is fairly rapid; for example, 50 millimeters per second. After a given pressure range has been covered, the pressure is held for a given time interval or dwell period such as indicated at 34. The given rate of application is then again applied over a second given range, for example, 20 millimeters of mercury and then again held for the given time interval, the application and holding of the pressure being repeated as indicated. The respective steps or dwell periods might, for example, be one second in duration. At one of the dwell periods, the inlet valve 14 of FIG. 1 is automatically closed and the outlet valve 17 opened very slightly to permit a gradual decrease in the pressure as indicated by the curve 33. It will be noted that there are slight pressure fluctuations in the overall analog pressure signal 33. These fluctuations, corresponding to the heartbeats of the patient, are accentuated in the range between the systolic and diastolic pressures.

As described in conjunction with FIG. 1, these pressure fluctuations in the analog pressure curve are utilized to generate pressure pulses. These pressure pulses are shown in diagram B at 35 and correspond to the signals appearing at the lettered point B from the differentiating circuit 19.

Diagram C shows the pressure pulses after they have passed through the filter and threshold network 20 wherein it will be noted that they are essentially quantized to provide sharp individual pressure signals 36 of uniform amplitude.

Pulse diagram D in FIG. 2 illustrates the detected acoustic pulses some of which are shown at 37 from the microphone 24 of FIG. 1 which pulses are passed into the filter and threshold network 26. It will be noted that various extraneous noise signals and variations accompany the acoustic pulses and it is in the proper detection of the acoustic pulses resulting from a resumption of blood flow that is most important in providing accurate blood pressure measurements. As mentioned, the spurious signals which appear between the pulses have not been properly discriminated against in automatic apparatus heretofore proposed for making blood pressure measurements.

Diagram E shows the result of filtering and quantizing the acoustic pulses to provide uniform acoustic signals as at 38.

Pulse diagram F in FIG. 2 illustrates some of the output signals at 39 from the coincidence circuit 21. These output signals only occur upon time coincidence of the pressure signals 36 and the acoustic signals 38 and because of this correlation, it will be immediately evident that a substantial proportion of the extraneous unwanted signals are eliminated.

In the various plots A to F of FIG. 2, the abscissa time scales in seconds are identical. It will be noted that by projecting in time the start of the output signals 39 in diagram F up to the analog pressure curve 33, and thence over to the ordinate axis there will be provieed an indication of the systolic blood pressure. This blood pressure corresponds to the start of blood flow which is indicated by the acoustic pulses 37 in diagram D. When full normal flow is achieved as a result of the continued decrease in the pressure, these acoustic pulses 37 will no longer be evident and thus the output signals 39 will terminate. This projected point onto the analog pressure curve and thence over to the ordinate pressure axis will indicate the diastolic blood pressure.

During the rapid pressure build up at the initial portion of the analog pressure curve 33 as shown in diagram A, it will be noted that slight pressure fluctuations will occur both during the build up period and the dwell period, such as indicated at 40, thereby assuring generation of the pressure pulses 35 in diagram B over the entire pressure range of interest. Further, it will be evident that until the pressure build up passes the systolic pressure point, there will be detected acoustic pulses by the microphone 24 of FIG. 1, as indicated at 41 in diagram D. Accordingly, coincidence of these acoustic pulses with the initial pressure pulses will give rise to output signals 42 from the coincidence circuit during the application of pressure.

The significance of the foregoing signals will be understood when considering the overall operation of the system which will now be described.

OPERATION

Referring to FIG. 1, the fluid chamber 12 is initially applied to a portion of the patient's body such as his arm. Further, the small cup 23 at the end of the acoustical conduit 22 may be attached to the chamber so that it is secured to the lower portion of the patient's upper arm in a position to detect acoustic pulses therein as a consequence of constricted blood flow.

The logic control circuit 31 for the inlet and outlet valves 14 and 17 is programmed and controlled by the fedback signals on the lines 30 and 32 and the differentiated signals from the circuit 19. Thus, upon starting a cycle of the equipment, the logic control 31 will open the inlet valve 14 to apply pressure fairly rapidly from the pressure source 15 to the fluid chamber 12.

Referring to diagram A of FIG. 2, the build up of pressure in the fluid chamber is shown by the analog pressure curve 33 wherein the build up rate is indicated at fifty millimeters per second. The programmed pressure control in the logic circuit 31 is such as to cause the valve 14 to close when 120 millimeters of mercury pressure is reached, so that the pressure is held for a dwell period of, for example, one second as indicated by the step 34. If any blood is still flowing to the lower portion of the patient's arm, acoustic pulses such as indicated at 41 in diagram D will be detected and output pulses 42 will be fed back into the logic control 31 through line 32 signalling that the pressure build up program is to be continued. The valve 14 will then be opened until the pressure increases, for example, 20 more millimeters of mercury to 140 at which point the valve will be closed for a second dwell period.

At the second dwell period, it will be noted that the pressure is above the represented systolic pressure and therefore flow has been blocked off and there will be no acoustic pulses corresponding to 41 in diagram D detected. Thus the output signals 42 will terminate. The pressure program in the logic control 31 is such as to effect one further pressure increase as a safety measure to make sure that blood flow is completely blocked. Thus, the pressure will increase in steps until two successive dwell periods have passed at which no acoustic pulses such as indicated at 41 are detected.

In the example of FIG. 2, this pressure is 160 millimeters of mercury but in the event that pressure pulses were detected during the dwell period at one-forty or one-sixty, the build up would continue as indicated by the dotted lines.

After the build up has been completed which in the example chosen is 160 millimeters of mercury, the inlet valve 14 is closed and the outlet valve 17 of FIG. 1 is opened slightly to permit a gradual decrease in pressure. This decrease may be at the rate, for example, of 2 millimeters per second.

During this decrease period, the small pressure fluctuations in the analog pressure curve 33 caused by the heartbeat are accentuated and generate the pressure pulses 35.

The gradually decreasing pressure will continue until a point is reached at which blood flow will start to the lower extremity of the arm. As mentioned, this pressure corresponds to the systolic blood pressure and in the example of FIG. 2 occurs at about 138 millimeters of mercury. The resumption of flow is marked by accentuation of pressure pulses as indicated at 33' giving rise to pressure signals as shown at 36'. Resumption of flow is also marked by the detection of the acoustic pulses 37 as shown in diagram D. An output electrical signal from the coincidence circuit will only occur when the acoustic pulses are in time synchronism with the pressure pulses 35 as correlated by coincidence of the pressure and acoustic signals 36 and 38. Thus, as also mentioned heretofore, spurious, extraneous acoustic pulses are substantially eliminated. The start of the output signals 39 triggers the measuring means 27 in FIG. 1 to record the systolic pressure as provided by the coded signals on line 29. This pressure could as well be visually displayed.

Termination of the output pulses 39 as will occur when the diastolic pressure is reached also triggers the measuring means 27 to record and/or display the corresponding diastolic pressure as provided by the coded signals and stored during the last output pulse.

It will thus be evident that the start and termination of the output signals from the coincidence circuit 21 define the indirect systolic and diastolic pressure points of the patient in the same manner that a doctor determines such pressures by taking readings when he listens for the initiation and termination of the acoustic pulses. However, in view of the provision of the correlation technique as described, accurate and consistent results can be achieved.

It should be understood that in the event the raw signal-to-noise ratio is relatively low, further correlation techniques can be employed by utilizing more than the two individual channels. For example, it should be understood that the provision of the pressure pulses by differentiating the slight pressure fluctuations in the analog pressure curve 33 constitutes only one means of obtaining such correlation signals. Other techniques could be used such as arterial displacement, light transmission modulation at extremeties of the body, electrical potential fluctuation at the skin, and so forth.

The essence of the present invention resides in providing at least two independent signals corresponding, first, to the patient's heartbeat in the form of pressure signals, and second, to acoustic signals caused by constricted blood flow in the downstream portion of the circulatory system which is closed off. Time correlation of these signals then provides accurate output signals which are relatively clear of extraneous pressure pulses or spurious signals. The start and termination of the generated output signals are then utilized to trigger the proper display of the systolic and diastolic pressures or as mentioned automatically record such pressures.

From the foregoing description, it will thus be evident that the present invention has provided a vastly improved blood pressure measuring system wherein accurate and consistent measurements can be achieved without the necessity of skilled personnel.

Claims

What is claimed is:

1. A method of automatically measuring a patient's systolic and diastolic blood pressure comprising the steps of:

a. applying pressure to close off blood flow to a portion of the patient;

b. detecting pressure pulses corresponding to the patient's heartbeat;

c. decreasing the applied pressure gradually;

d. generating quantized pressure signals in response to said pressure pulses to provide individually defined pressure signals of uniform amplitude;

e. detecting acoustic pulses in said portion of the patient as the applied pressure is being decreased;

f. generating quantized signals in response to said acoustic pulses to provide individually defined acoustic signals of uniform amplitude;

g. correlating said pressure signals and acoustic signals to provide an output signal only when a pressure signal occurs simultaneously with an acoustic signal; and,

h. utilizing said output signals to provide indications of pressure being applied at the start and termination of the output signals, whereby the patient's systolic and diastolic blood pressures are respectively measured.

2. The method of claim 1, in which said pressure is applied at a given rate over a given range and thence held for a given time interval and thence again applied at said given rate over a second given range and held for said given time interval, the application and holding being repeated until there is an absence of said output signals detected during any two successive time intervals, said step of decreasing the applied pressure gradually then being initiated.

3. A system for automatically measuring the systolic and diastolic blood pressures of a patient, comprising, in combination:

a. means for applying pressure to close off blood flow to a portion of the patient's circulatory system;

b. means responsive to the patient's heartbeat to detect pressure pulses in substantial synchronism with said heatbeat;

c. means connected to said means for applying pressure for gradually decreasing the applied pressure;

d. pressure transducer means connected to said means for gradually decreasing the applied pressure to provide an analog pressure signal of the pressure;

e. means responsive to pulses in the downstream side of the close-off pressure point in the patient's circulatory system to detect acoustic pulses;

f. filtering and threshold detecting means for respectively filtering and detecting said pressure and acoustic pulses and respectively providing quantized individually defined electrical pressure and acoustic signals;

g. coincidence circuit means receiving said pressure and acoustic signals and providing an output signal only upon coincidence of a pressure signal and an acoustic signal; and

h. measuring means connected to receive the output signals and responsive to said analog pressure signal to provide indications of the pressure detected by said pressure transducer means at the start and termination of said output signals, whereby the systolic and diastolic blood pressures are automatically measured.

4. A system according to claim 3, in which said means for applying pressure includes a flexible fluid chamber arranged to surround said portion of the patient and having an inlet fluid line and associated inlet control valve, said means for gradually decreasing the applied pressure including an outlet fluid line from said chamber and associated outlet control valve, said means responsive to the patient's heartbeat to detect pressure pulses including a differentiating circuit receiving said analog pressure signal and providing said pressure pulses in response to pressure fluctuations in said analog pressure signal; an analog-to-digital converter responsive to said analog pressure signal to generate digital coded signals indicative of the pressure, said coded signals being applied to and stored in said measuring means; and a logic control circuit connected to said inlet and outlet control valves and connected to receive said coded signals and said output signals whereby automatic proportional opening and closing of the valves is controlled by the coded and output signals in accord with a programmed application of pressure and gradual decreasing of pressure to be provided in said fluid chamber.

5. A system according to claim 4, in which said means responsive to pulses downstream of the close-off pressure point includes an acoustical conduit with one end in physical contact with said portion of the patient and its other end terminating in a microphone for receiving sound in said conduit and converting the sound into said acoustic pulses, there being provided insulation material around said microphone to shield it from ambient noise and vibration.

6. A system according to claim 4, in which said measuring means includes means providing a visual digital display of the systolic and diastolic blood pressures.

7. A system according to claim 4, in which said measuring means includes means for automatically recording said systolic and diastolic blood pressures.