US3118440A - Automatic blood pressure meter - Google Patents

Description

J 1954 G. D. PH. DE DOBBELEER 4 AUTOMATIC BLOOD PRESSURE METER Filed July 13, 19.62 2 Sheets-Sheet l TRANSDUCER 3 PRESSURE IMPULSE DETECTOR l0 DAMPING CHAMBER ll AMPLIFIER THERMISTOR I3 Jan. 21, 1964 Filed July 13, 1962 VOL TA GE G. D. PH. DE DOBBELEER AUTOMATIC BLOOD PRESSURE METER 2 Sheets-Sheet 2 United States Patent 3,113,444) AUTOMATIC BLUUD PRESSURE METER Gilbert l). P. De Dobheleer, Heverlee, Belgium, assignor to Nil. Godart=Manimcx, De Bilt, Netherlands, a

Dutch company Filed .luly 13, 1962, Ser. No. 299,666 Claims priority, application Belgium July 18, 1961 7 Claims. (Cl. 1282.05)

The invention relates to the determination of the blood pressure of a patient, and in particular to an automatic blood pressure meter.

Blood pressure is usually determined by arranging an inflatable cuff, connected with a pressure meter, around the arm of the patient. The cuff is inflated to such a pressure that the blood flow in an artery of the patient is cut off. The system is provided with a narrow discharge aperture, so that the pressure in the cuff decreases gradually after the inflation; the rate of decrease is generally 3 mm. Hg per second. The pressure in the cuff is measured by means of a pressure meter at two moments, namely the moment at which the artery opens for the first time, and the moment at which the artery is no longer closed. These moments are determined by the physician with the aid of a stethoscope. The moment at which the artery opens for the first time is readily observable, because the admitted blood wave generates a clear sound impulse in the stethoscope. The moment at which the artery is no longer closed, i.e. remains open during the entire heart-beat cycle, is characterized by a decrease of the amplitude and a change of the timbre of the sound observed in the stethoscope. This latter observation is very subjective, because it is dependent on the aural sensitivity of the physician. The pressure read at the first moment is called the systolic blood pressure, and the pressure read at the second moment is called the diastolic blood pressure.

This known method is rather complicated, in particular when a continuous check on the blood pressure of the patient is desired during or after an operation. Additionally, the measurement of the diastolic blood pressure is not very reliable for the above-mentioned reasons. It has heretofore been proposed to automate the measure ment; for this purpose, a microphone was substituted for the stethoscope, and the sound picked up by the microphone was used to register the position of the pressure meter at the two above-mentioned moments. However, it was found in practice that the results of this automatic measurement were not very accurate, in particular with respect to the diastolic blood pressure, firstly, because the criterion for a continuous open condition of the artery is dimcult to observe, and secondly because the microphone picks up extraneous sounds, which cause a displacement of the thresholdvalue.

It is, therefore, an object of the invention to provide an improved device for the automatic determination of the blood pressure, by means of which both the systolic and the diastolic blood pressures may be read or registered with great accuracy.

Another object of the invention is to provide an automatic blood pressure meter by means of which the blood pressure of a patient may be continuously supervised.

Still another object of the invention is to provide an automatic blood pressure meter by means of which the systolic and diastolic blood pressure of a patient may be measured at constant intervals, and wherein the readings of the blood pressures are retained until the next measurement, so that the blood pressures may be read at any desired time.

A further object of the invention is to provide an automatic blood pressure meter making use of a flow sensitive element to determine the moment at which the diastolic blood pressure is to be measured.

According to a main feature of the invention, the moment at which the diastolic blood pressure is measured is determined by a member responding to the disappearance of the phase difference between the pressure impulses in two cuffs placed at a short distance from each other.

When the device according to the invention is used, two inflatable cuffs are arranged around the arm of the patient at a short distance from each other, and inflated to the same pressure. If the pressure in these cuffs lies between the systolic and the diastolic blood pressure, the artery is opened after each heart-beat first at the upper cuff, and slightly later at the lower cuff. In each cuff, a pressure impulse occurs when the artery is opened; thus, these pressure impulses have a phase difference as long as the pressure in the cuffs lies between the systolic and the diastolic blood pressure. However, this phase difference disappears at the moment at which the pressure in the cuffs descends below the diastolic blood pressure. At this moment, the artery remains open during the entire heart-beat cycle, so that the pressure exerted by the heartbeat does not meet with any obstruction in its propagation between the two cuffs; as a consequence, the two pressure impulses occur substantially simultaneously. Hence, the disappearance of the phase difference between the pressure impulses in the two cuffs is aclear criterion that the diastolic blood pressure has been reached; this criterion is used, according to the invention, to determine the moment at which the diastolic blood pressure is measured.

The distance between the two cuffs should be as small as possible, in order to avoid an unfavourable influence on the measurement by the occlusion of an amount of blood in the part of the artery between the two cuffs. Thus, the cuffs are preferably arranged in such a manner that they overlap each other; this overlapping arrangement may be ascertained by attaching the two cuffs to each other.

The member responding to the disappearance of the phase difference may be constructed in several Ways. For instance, the pressure in each cuff may be converted by means of a suitable transducer into an electric voltage, so that the pressure impulses are converted into voltage impulses. The phase difference between the two voltage impulses may be detected by means of a discriminator, and a relay may be energized when the phase diiference disappears, in order to operate the pressure meter.

In a preferred embodiment of the invention, the said member consists of a flow sensitive element, inserted in a conduit connecting the two cuffs. As long as a phase difference exists between the pressure impulses, a gas flow is generated upon occurrence of a pressure impulse from one cuff to the other, and this gas flow is detected by the flow sensitive element. When the pressure impulses coincide, no such gas flow is generated and this fact is used as a criterion for the measurement of the diastolic blood pressure. Since the pressure impulses occurring in the two cuffs at the moment of the diastolic pressure measurement may not always have the same amplitude, it is preferred to include a damping chamber in the conduit between one of the cuifs and the fiow sensitive element, whereby the pressure impulses are equalized, so that they may entirely compensate each other. The flow sensitive element is preferably a temperature sensitive resistor.

In a preferred embodiment, the envelope obtained by rectification of the voltage impulses occurring across the temperature sensitive resistor is compared with a threshold value occurring across a condenser with a large discharge time constant, to which a fraction of the impulse voltage is supplied through a rectifier. The diastolic lood pressure is measured at the moment: at which the envelope descends below the threshold value. In this manner, the said moment is rendered independent of the amplitude of the pressure impulses.

The moment at which the systolic blood pressure is measured is preferably determined by the occurrence of a pressure impulse in a third inflatable cutf arranged around the arm of the patient at some distance below the two first-mentioned cults. This pressure impulse may be detected by means of a transducer, or of a temperature sensitive resistor. The measurement of the pressure is preferably carried out for the systolic as well as for the diastolic blood pressure by means of a transducer responding to the pressure in one of the two first-mentioned cuffs, and generating an alternating voltage dependent on this pressure, from which a measuring voltage is obtained by rectification. At the moment at which the pressure in question is to be measured, this measuring voltage is supplied through a switch to a condenser with a large discharge time constant, which retains the reading during the time between two successive measurements. In this manner, it is rendered possible to read the two blood pressure values at any desired time.

The invention will be further explained by reference to the accompanying drawings, showing a preferred embodimerit.

FIG. 1 shows in which manner the cults are arranged around the arm of the patient, and also shows schematically the parts connected with the cufifs.

FIG. 2 shows the member responding to the disappearance of the phase difference between the two pressure impulses, by means of which the moment for measuring the diastolic blood pressure is determined.

FIG. 3 shows the electric circuit for processing the impulses obtained by means of the member shown in FIG. 2.

FIG. 4 shows the circuit for carrying out the pressure measurement.

FIG. 5 shows a diagram for explaining the operation.

As shown in FIG. 1, two cuffs 1 and 2 are arranged around the arm of the patient at a short distance from each other; these cuffs are attached to each other in overlapping positions. By means of conduits 3 and 4, the cuffs are each connected with a member 5, serving to detect a disappearance of the phase difference between the pressure impulses in the two cutfs. Member 5 in turn is connected with the transducer 6 for carrying out the pressure measurement. At some distance below the cuffs 1 and 2, a third cuff 7 is arranged around the arm of the patient, and this third cuff is connected with a member 8 for detecting a pressure impulse.

In FIG. 2, the member 5 is shown in detail. This member comprises a block 9 made of a plastic material, and containing a plurality of channels and a damping chamber 10. The block 9 contains six vertical channels 11A-11F, each ending in the upper surface of the block. Culfs 1 and 2 are connected respectively to channels 11C and 11D, and channel 11E is connected with the pump (not shown) for inflating the cuffs. Through channel 11F, the air in the cuffs is gradually discharged, whereby the pressure in the ends decreases at a rate of about 3 mm. Hg per second. Channel 11B is connected with the transducer 6. Channels 11B and 11C communicate with an inclined channel 12 ending in channel 11A. A thermistor 13 is placed at the intersection of channels 11A and 12, and provided with supply conductors led through channel 11A. The lower end of channel 11A communicates through a forwardly projecting channel 14 with the damping chamber 10. Channel 11D is likewise connected with the damping chamber through a forwardly projecting channel 15. Channels 11C-11F are mutually connected by a horizontal transverse channel 16 with which they communicate through narrow channels 17, having a small diameter, so that the pressure impulses occurring in the cuffs 1 and 2 are not propagataed through these channels.

As appears from FIG. 3, the thermistor 13 is connected to a voltage source in series with a fixed resistor 18. The thermistor is heated by the electric current to a relatively high temperature, for instance of C. When the thermistor is cooled by a passing gas how, its resistance is considerably increased, so that a much higher voltage occurs across the thermistor. The voltage impulses generated in this manner are supplied through condenser 19 to an amplifier 20. The output impulses of this amplifier are supplied to a rectifier 21, so as to generate a rectified voltage across the resistor 22, corresponding with the envelope of the successive impulses. This rectified voltage is transmitted by the triode 23, operating as a cathode follower, to the resistor 24.

By means of a potentiometer 25, a predetermined fraction of the output voltage of amplifier 20, for instance 20-30% of this voltage, is supplied to a rectifier 26, having a condenser 27 in its output circuit; no leakage resistor is connected in parallel with the condenser 27, so that its discharge time constant has a very high value. Thus, the voltage across condenser 27 follows the envelope of the output impulses of amplifier 20 as long as the amplitude of these impulses increases. If the amplitude decreases, the voltage across the condenser 27 remains constant. By means of a triode 28, operating as a cathode follower, the voltage across condenser 27 is transmitted to a resistor 29. Thus, a threshold voltage occurs across resistor 29, amounting to 20-30% of the maximum amplitude of the output impulses of amplifier 20.

By means of a member 30, the moment is determined at which the voltage occurring across resistor 24, and corresponding with the envelope of the output impulses of amplifier 20, descends below the threshold voltage occurring across resistor 29. At this moment, the diastolic blood pressure is measured. For the sake of simplicity, the member 30 has been represented in the drawing as a galvanometer. It may be constructed, for instance, as a galvanometric relay, but it can also be formed by the input circuit of an amplifier controlling an electro-magnetic relay.

FIG. 4 shows the measuring circuit used for measuring the pressure in the cuff 1. This pressure is converted into an electric voltage by means of a transducer comprising a primary winding 31 fed with an alternating current, a secondary winding 32, and a core 33. The core 33 is mechanically coupled with a membrane subjected to the pressure in the cuff 1, so that the position of the core is dependent on this pressure. Thus, the voltage induced in the secondary winding 32 is likewise dependent on the pressure in the cuff 1. This voltage is amplified by means of the triode 34, and the amplified voltage is supplied to a resistor 35, followed by a filter 36, consisting of a resistor and two condensers. A direct voltage proportional to the pressure in the cuff 1 is obtained at the output terminals of filter 36.

At the moment at which the pressure is to be measured, a switch 37 is closed for a short time, whereby the output voltage of filter 36 is transmitted to a condenser 38, having no leakage resistor connected in parallel, so that it has a high discharge time constant. The voltage across condenser 38 is transmitted by means of a triode 39, operating as a cathode follower to a resistor 40, so as to be compared with a fixed voltage occurring across the cathode resistor 41 of a triode 42. A galvanometer 43 is connected between the cathodes of triodes 39 and 42 and indicates the blood pressure. Due to the high discharge time constant of condenser 38, its charging voltage is retained during the interval between two successive measurements, so that the meter 43 may be read at any desired time. Means for recording the blood pressure may be connected in parallel with meter 43.

The measuring circuit as shown in FIG. 4 must be provided for the systolic blood pressure as well as for the diastolic blood pressure; the switch 47 being operated by detector 8 or by member 30 respectively. If desired, the part to the left of switch 37 in FIG. 4 may be common to the two measuring circuits.

The detector 8 may be constructed in any desired manner. For instance, the pressure impulse occurring in the cuff 7 may be detected by means of a thermistor responding to the air current caused by the pressure impulse, or by means of a transducer of the kind shown in FIG. 4, connected with the cuff 7. After the first impulse, the detector must be switched off for the remaining part of the cycle, so that later pressure impulses cannot influence the measurement any more.

The operation of the above-described arrangement is as follows. The cuffs 1, 2 and 7 are attached around the arm of the patient, and inflated to such a pressure that the blood flow in the artery is cut off. The inflation is carried out by means of channel 11E for cuffs 1 and 2, and by means of a separate air supply channel (not shown) for cuff 7. After the inflation, the air leaks away from the cuffs, whereby the pressure gradually decreases, preferably at a rate of about 3 mm. Hg per second.

After some time, the pressure in the cuffs has decreased to such an extent that the artery is opened during the maximum of the pressure caused by the heart-beat. A blood wave is passed, which generates a pressure impulse in the cuff 7. This pressure impulse is detected by means of the member 8, so that the switch 37 of the measuring circuit for the systolic pressure is closed for a short time, and the pressure is measured. After that, the detector 8 is switched off.

In the subsequent time interval, the artery is opened and closed during each heart-beat cycle. At the cuff 1, the artery is opened earlier than at the cuff 2, so that the pressure impulses generated in the cuffs have a certain phase difference. During the occurrence of the pressure impulse in cuff 1, an air current flows from channel 11C through channels 12 and 14 to chamber 10, and thence through channel 15 to channel 11D, so that the thermistor 13 is cooled. Immediately afterwards, the pressure impulse in cuff 2 occurs, whereby the direction of the air current is reversed and the cooling of the thermistor is continued. Thus, a voltage impulse occurs across the thermistor 13 during each heart-beat cycle. These impulses are indicated at 44 in FIG. 5; the envelope of the impulses appearing across resistor 24 is indicated at 45. The threshold value across resistor 2g is indicated at 46. This threshold value increases proportionally to the envelope 45 as long as the amplitude of the impulses increases; as soon as the amplitude of the impulses begins to decrease, the threshold value remains constant.

After some time, the pressure in the cuffs has decreased to such an extent that the artery remains open during the entire heart-beat cycle. The pressure caused by the heart-beat is now freely propagated through the artery, so that the pressure impulses in cuffs 1 and 2 coincide, and their amplitude decreases at the same time. It has been found in practice that the amplitude of the pressure impulse in cuff 2 is larger at this time than the amplitude of the pressure impulse in cuff 1. In order to ascertain an accurate compensation of the two pressure impulses, the pressure impulse in cuff 1 is directly transmitted to the thermistor 13 through the channel 12, whereas the pressure impulse in cuff 2 reaches the thermistor through chamber 19 which causes a predetermined damping. The amplitudes of the pressure impulses at the thermistor 13 are thereby equalized, so that the thermistor is no longer subjected to an air current. As a consequence, the amplitude of the voltage impulses across the thermistor decreases sharply, so that the envelope decreases at the same time. As soon as the envelope descends below the threshold voltage 46 the member 30 is operated, so that the switch in the measuring circuit for the diastolic pressure is closed, and the pressure is measured.

The above-described operation may be periodically repeated by means of a suitable program switch. This is of importance if the blood pressure of the patient is to be continuously checked for a period of time. If desired, the thermistor 13 may be replaced by a different flow sensitive element.

Although the invention has been described hereinbefore with reference to a specific embodiment thereof, it is to be understood that the invention is not restricted to this embodiment, and that many modifications and alterations are possible within the scope of the invention as set forth in the appended claims.

I claim:

1. A device for determining the blood pressure of a patient, comprising two inflatable cuffs adapted to be arranged around an arm of the patient at a short distance from each other, means for inflating the said cuffs to such a pressure that the blood flow in an artery of the patient is cut off, discharge means whereby the pressure in the said cuffs is gradually decreased after their inflation, a pressure meter connected with one of the said cuffs, a conduit interconnecting the said cuffs, a flow sensitive element inserted in said conduit, and means controlled by said fiow sensitive element for operating said pressure meter at the moment at which the phase difference between the pressure impulses occurring in the said cuffs disappears.

2. A device as claimed in claim 1, wherein the said cuffs are attached to each other in overlapping arrangement.

3. A device as claimed in claim 1, further comprising a damping chamber in said conduit between one of the said cuffs and said flow sensitive element for equalizing the amplitudes of the pressure impulses in the said cuffs at the moment at which said pressure meter is operated.

4. A device for determining the blood pressure of a patient, comprising two inflatable cuffs adapted to be arranged around an arm of the patient at a short distance from each other, means for inflating the said cuffs to such a pressure that the blood how in an artery of the patient is cut off, discharge means whereby the pressure in the said ends is gradually decreased after the inflation, a pressure meter connected with one of the said cuffs, a conduit interconnecting the said cuffs, a temperature sensitive resistor inserted in said conduit, a fixed resistor connected in series with said temperature sensitive resistor, a voltage source feeding the series connection of the said resistors, and means controlled by the voltage across said temperature sensitive resistor for operating said pressure meter at the moment at which the phase difference between the pressure impulses in the said cuffs disappears.

5. A device as claimed in claim 4, further comprising a first rectifier for deriving an envelope voltage from the voltage impulses occurring across said temperature sensitive resistor, a second rectifier receiving a predetermined fraction of the impulse voltage occurring across said temperature sensitive resistor, a condenser fed by said second rectifier and having a large discharge time constant, means for comparing said envelope voltage with the voltage across said condenser, and means for operating said pressure meter when said envelope voltage descends below the voltage across said condenser.

6. A device for measuring the diastolic and systolic blood pressures of a patient, comprising two inflatable cuffs adapted to be arranged around an arm of the patient at a short distance from each other, a third inflatable cuff adapted to be arranged around said arm at some distance below the said first-mentioned cuffs, means for inflating the said cuffs to such a pressure that the blood flow in an artery of the patient is cut off, discharge means whereby the pressure in the said cuffs is gradually decreased after their inflation, a first pressure meter for measuring the diastolic blood pressure connected with one of the said first-mentioned cuffs, a second pressure meter for measuring the systolic blood pressure connected with one of the said first-mentioned cuffs, a member responsive to the phase difference between the pressure impulses occurring in the said first-mentioned cuifs, means controlled by said member for operating said first pressure meter at the moment at which said phase difference disappears, and means controlled by the first pressure impulse occurring in said third cufl for operating said second pressure meter.

7. A device for determining the blood pressure of a patient, comprising two inflatable cuffs adapted to be arranged around an arm of the patient at a short distance from each other, means for inflating the said criti s to such a pressure that the blood flow in an artery of the patient is cut off, discharge means whereby the pressure in the said cuffs is gradually decreased after their inflation, a transducer connected with one of the said cufls and converting the pressure occurring therein into an alternating voltage, a rectifier converting said alternating voltage into a direct measuring voltage, a condenser having a large discharge time constant, a switch interconnecting said rectifier and said condenser, a member responding to the phase difference between the pressure impulses occurring in the said cuffs, means controlled by said member for closing said switch at the moment at which said phase difference disappears, whereby said measuring voltage is transferred to said condenser, and a measuring instrument indicating the voltage across said condenser.

References Cited in the file of this patent UNITED STATES PATENTS Colegrave Aug. 9, Sheer Nov. 10,

Claims (1)

1. A DEVICE FOR DETERMINING THE BLOOD PRESSURE OF A PATIENT, COMPRISING TWO INFLATABLE CUFFS ADAPTED TO BE ARRANGED AROUND AN ARM OF THE PATIENT AT A SHORT DISTANCE FROM EACH OTHER, MEANS FOR INFLATING THE SAID CUFFS TO SUCH A PRESSURE THAT THE BLOOD FLOW IN AN ARTERY OF THE PATIENT IS CUT OFF, DISCHARGE MEANS WHEREBY THE PRESSURE IN THE SAID CUFFS IS GRADUALLY DECREASED AFTER THEIR INFLATION, A PRESSURE METER CONNECTED WITH ONE OF THE SAID

Images