GB2069705A - Blood pressure measuring equipment - Google Patents

Abstract

Blood pressure measuring equipment comprises a sleeve provided with an inflatable air chamber and a microphone, an appliance connected with the sleeve for measuring and indicating systolic and diastolic blood pressure, and an automatic zero setting device (75) to cause pressure signals indicating fluid pressure in the chamber to be set to zero before any measurement is carried out. A pressure sensor (39) is connected to the air chamber to measure fluid pressure therein and comprises a piezo-resistive bridge circuit (91), which is connected with an amplifier (101). The zero setting device (75) comprises a differential amplifier (113) which is provided with a capacitive feedback and one input of which is connected via a transistor (117), which is switchable under the control of a pulse generator (81), with the output of the amplifier (101) following the bridge circuit (91). The output of the amplifier (113) with the capacitive feedback is connected with an input (91d) of the bridge circuit. When the appliance is switched on, the pulse generator (81) temporarily closes the transistor (117). The zero setting device (75) then operates as an integral regulator and automatically balances the bridge circuit while the air chamber is free of pressure and before actual pressure measurement is carried out. <IMAGE>

Description

SPECIFICATION Blood pressure measuring equipment The present invention relates to blood pressure measuring equipment.

Blood pressure measuring equipment described in US patent specification No,. 3 131 comprises a microphone which is connected by various electronic components with a logic circuit. The equipment also comprises an inflatable sleeve and a pressure sensor, the sensor being connected via an analogdigital converter, which can be switched on and off, and a gate circuit with a pressure recording device.

During blood pressure measurement, the sleeve is inflated to a pressure above systolic pressure and then slowly vented. In that case, Korotkoff tones are generated in a certain pressure region and converted by the microphone into electrical signals. The logic circuit controls the analog-digital converter and the gate circuit on the occurrence of each Korotkoff tone in such a manner that the instantaneous pressure measured by the pressure sensor is recorded in the pressure recording device. The first recorded pressure value corresponds to systolic pressure and the last recorded pressure value to diastolic pressure.

In blood pressure measuring equipment of that kind, due to drift of the pressure sensor from its zero point, measurement errors can arise, which according to the kind of the sensor used can reach an appreciable magnitude.

Although it would be possible to provide a multiple track potentiometer for setting or balancing the zero point, as is known for measuring devices of other kinds, such a balancing potentiometer is of little value in blood pressure measuring equipment intended to be used by the patients themselves, because in most cases balancing of the zero point would probably not be carried out by the patients.

There is therefore a need for blood pressure measuring equipment from which measurement errors, due to zero point drift of the pressure sensor, are substantially excluded.

According to the present invention there is provided blood pressure measuring equipment comprising measuring means attachable to a person and provided with a chamber inflatable by fluid, a pressure sensor for detecting fluid pressure in the chamber, electronic signal generating means elec fricaily connected to the sensor to generate electrical pressure signals indicative of the detected pressure, and zero setting means connected to at least one of the sensor and the signal generating means and operable in response to a command signal to cause the pressure signals to have a value indicative of zero fluid pressure in the chamber.

For clarification, it is noted that the references in the following description and claims to blood pressure and air chamber pressure are to be understood as denoting excess pressure measured with respect to ambient air pressure.

An embodiment of the present invention will now be more particularly described by way of example and with reference to the accompanying drawings in which Figure 1 is a schematic plan view of blood pressure measuring equipment according to the said embodiment, Figure 2 is a schematic block diagram of the principal electronic and pneumatic components of the equipment of Figure 1, Figure 3 is a circuit diagram of a pressure sensor, amplifier, zero balancing device and part of a control unit of the equipment, Figure 4 is a diagram illustrating the temporal course of a blood pressure measurement by the equipment; and Figure 5 is a circuit diagram of an amplifier with a zero balancing device in a modification of the equipment.

Referring now to the accompanying drawings, in Figure 1 there is shown blood pressure measuring equipment comprising a sleeve 1 attachable to the arm of a person to be examined and an appliance indicated generally by 3. The sleeve 1 comprises a rubber bag defining a deformable and inflatable air chamber and contains a microphone. The sleeve 1 is detachably connected to the appliance 3 by a line 5, which comprises an air hose connected to the air chamber and a cable connected to the microphone, the line being provided at the appliance end with a coupling socket 7. The appliance 3 comprises a housing 9 provided with a threaded shank 9a to which a pump 13 with a substantially cylindrical rubber pump bellows is detachably fastened by means of a box nut 11.An air hose connection nipple 15 and an electrical connection pin 17, formed by a chassis plug, are provided on the housing 9 for coupling thereto of the socket 7. A connection element 19, also formed by a chassis plug, is included for the connection of a headphone. The appliance 3 also comprises three non-detenting press key switches 21,23 and 25, a three-place digital indicating unit 27 and various pneumatic and electronic components, as will be subsequently described, accomodated in the interior of the housing 9.

Figure 2 shows the inflatable air chamber, referenced 31, and the microphone, referenced 33, of the sleeve 1 as well as some of the pneumatic and electronic components in the appliance 3. The air chamber 31 is connected by the air hose in the line 5, and by air lines in the appliance 3, via a non-return valve 35 with the pump 13, an electrically controllable outflow valve 37 and a pressure sensor 39. The pump 13 is provided with an air inlet having a non-return valve 41. The two non-return valves 35 and 41 are so arranged that by alternating manual compression and release of the pump bellows air can be sucked from the ambient atmosphere and pumped into the air chamber 31.

The microphone 33 is connected by electrical conductors with the input of a filter means 51, the output of which is connected with the headphone connection element 19 and with a discriminator 53, which comprises a trimming potentiometer 54 for setting of a lower threshold value and a pulse shaper. The output of the pulse shaper is connected to a control unit 55.

The pressure sensor 39 comprises a measurement converter bridge circuit formed by piezo-resistive elements and is connected with the input of an amplifier 57, the output of which is connected via a differentiator 59, and via a parallel connection bridging the differentiator, with the control unit 55.

The control unit 55 and the amplifier 57 are also connected at outputs thereof with a device 75 for automatic zero balancing, the output of the device 75 being connected to the pressure sensor 39. The output of the differentiator 59 is also connected to the control unit 55 and additionally to an input of a regulator 61. The control unit 55 is connected to another input of the regulator 61, the output of which is connected with an electromagnetic actuating means of the outflow valve 37. The control unit 55 additionally has two connections which are connected to, respectively, two analog stores 63 and 65 each formed by a respective capacitor, and is connected to an indicating control device 67. The device 67 includes, amongst other things, an analogdigital converter and is in turn connected to the indicating unit 27.

A discriminator 69 is connected at its input to the output of the differentiator 59, and at its output to an input of a heartbeat frequency meter 71 and an input of the control unit 55. The meter 71 is connected at a control input thereof to an output of the control unit 55 and comprises an analog store connected at an output to an input of the control unit 55. The switch 21 is connected to the control unit 55 and the switch 23 to the indicating control device 67. Also present is a voltage source 73, which includes a battery and which is connected to supply voltage connections of the different operative components and to an earth connection. The switch 25 and also the control unit 55 are connected to the voltage source 73, which, apart from the battery, comprises logic elements and a regulator for stabilisation of the supply voltage.

The battery is accomodated in a battery compartment closable by a lid.

The construction of the zero balance device 75, the pressure sensor 39 connected therewith and the amplifier 57, the circuit diagrams of which are shown in Figure 3, will now be more closely explained. Also shown in Figure 3 is the control unit 55, of which, however, only a pulse generator 81 is illustrated in detail. The pulse generator 81 comprises a capacitor 83, one electrode of which is connected with the positive pole of the voltage source 73 and the other electrode of which is connected through a resistor 85 with earth and additionally with the input of an inverter 87. The output of the inverter 87 is connected with the base of a transistor 89, the emitter of which is connected with the positive pole of the voltage source 73 and the collector of which is connected with further elements of the control unit 55 and with the zero balance device 75.

The pressure sensor 39 comprises a piezoresistive bridge circuit 91 formed of silicon semiconductor material with four bridge branches, the resistance of which is dependent on pressure, and four connections 91a, 91 b, 91 c and 91 d. The connection 91a is connected with the inverting input of an amplifier 97 and through a resistor 95 with the negative pole of the stabilized voltage source 73. The non-inverting input of the amplifier 97 is connected to earth and the output of the amplifier to the connection 91 b of the bridge circuit 91. The resistor 95 and amplifier 97 together form a constant current source 93.

The amplifier 97 comprises a differential operational amplifier 101, which forms the actual amplifier, connected at its output through a feedback resistor 101 and a parallellyconnected capacitor 105 to the inverting input of the amplifier. The connection 91 c of the bridge circuit 91 is connected through a trimming potentiometer 99 and a fixed resistor 107 with the inverting input of the amplifier 101. The bridge circuit connection 91d is connected with the non-inverting input of the amplifier 101, the output of which is connected with the differentiator 59 and with the control unit 55.

The zero balance device 75 comprises a differential operational amplifier 113, the non-inverting input of which is connected to earth and the inverting input of which is connected through a resistor 115 and the source-drain path of a field-effect transistor 117 with the output of the amplifier 101. The gate of the transistor 117 is connected with the collector of the transistor 89 of the pulse generator 81. The output of the amplifier 113 is connected through a feedback capacitor 119 with the inverting amplifier input. The output of the amplifier 113 is additionally connected through a resistor 121 with the noninverting input of the amplifier 101 and the connection 91d of the bridge circuit 91.

The non-inverting input of the amplifier 101 is also connected through two resistors 123 and 125, which are connected in series, with earth. The junction of the two resistors 123 and 125 is connected through a switch 127 with the positive pole of the voltage source 73 and through a diode 129 with the indicating control device 67. The switch 127 is accomodated in the battery compartment so that it can be actuated only when the compartment is open.

The operation of the blood pressure measuring equipment will now be explained in detail.

For performance of a measurement, the sleeve 1 is connected by the line 5 with the appliance 3 and is attached to the arm of the person to be examined.

The dimensions of the appliance 3 are such that it can conveniently be held by one hand, for which purpose the pump 13 also serves as a handgrip.

In the diagram shown in Figure 4, the curve 151 shows the relationship of the pressure p of the air chamber 31, and the curve 153 the relationship of the magnitude of the cross-sectional flow area q of the outflow valve 37, to time t.

When the sleeve is attached, the appliance is made operationally ready at the instant to th roug h a brief depression of the operating element, i.e. of the key of the ON/OFF switch 25. As a result, the supply voltage source 73 is switched on so as to deliver a positive and a negative stabilized voltage with respect to earth.

The outflow valve 37, which is open before the switching-on of the supply voltage, remains open up to the start of pumping, i.e. up to the instant tl.

During the time interval from the instant to to the instant t1, the pressure p in the air chamber 31 therefore has the value zero. When the voltage supply is switched on, thus on the appearance of a positive supply voltage, the pulse generator 81 generates a pulse having duration which is determined by the size of the capacitor 83 and resistor 85, as well as the switching characteristic of the transistor 89, and is, for example, 0.3 to 1.5 seconds.

The switching-on of the supply voltage or the pulse generated by the pulse generator 81 represents a command for the zero balance device 75 to balance the zero point of the pressure sensor 39. The previously blocked transistor 117 serving as an electronic switching device is made conductive by the pulse. The amplifier 113 now starts to operate as an integral regulator and feeds a current to the connection 91d of the bridge circuit 91. This is varied until the voltage between the inputs of the amplifier 113 and consequently the voltage at the output of the amplifier 101 are both zero. The resistor 115 and the capacitor 119 are so dimensioned that the regulating time constant is, for example, about 0.05 seconds and is thus substantially shorter than the length of the pulse generated by the pulse transmitter 81.The regulator is therefore rapid enough to balance the pressure sensor 39 to zero during the duration of the pulse generated by the pulse gener atop 81.

When the pulse generated by the pulse generator 81 ends, the transistor 117 returns to its blocked state. The amplifier 113 together with the capacitor 119 now form a store which maintains the previously regulated voltage at the amplifier output. The pressure sensor thus remains in its balanced state during the blocking of the transistor 117. In this connection it is pointed out that the resistor 121 has a resistance value which is at least ten times, for example about seventy times, the impedance of the bridge circuit 9 measured between the supply connections 91a and 91b. Moreover, the resistance value of the resistor 103 is at least a hundred times the bridge impedance. The amplifier 113 together with the resistor 121 therefore form a current source which, even when the bridge circuit 91 is detuned while measuring, feeds a practically constant current into this circuit.

The indicating control device 67 is constructed in such a manner that all three places of the indicating unit 27 have the value 8 at the instant of switchingon of the supply voltage and initially return this value for a short time interval, for example, until the pulse generated by the pulse generator 81 has ended. Thereafter, the pressure determined by the now balanced pressure sensor 39, i.e. the value zero, is indicated. This signals to the user that the pressure sensor has been properly balanced.

Between the instants t1 and t2, air is pumped intermittently into the air chamber 31 by the pump 13 so that an air pressure arises in the chamber which is greater than the systolic pressure. Shortly after the termination of the inflation process, the valve 37 is opened automatically at the instant B, so that air flows out of the air chamber 31 through the valve 37 and into the environment and the pressure in the air chamber falls. In that case, the pressure sensor feeds to the amplifier 57 a pressure signal, i.e.

a voltage, the magnitude of which is proportional to the pressure p, and the differential quotient dp/dt is determined by the differentiator 59. The regulator 61 regulates the outflow valve 37 in such a manner that the differential quotient dp/dt remains constant during the actual measurement phase apart from pressure fluctuations, which are caused by heart activity and which will be further explained.

The actual movement phase now runs broadly as follows: When the pressure p is now reduced from its maximum pressure lying above the systolic pressure p5, the pressure fluctuations caused by heartbeats occur from the instant4. These pressure fiuctuations are detected by the differentiator 59. The discriminator 69 then generates a pulse on each pressure fluctuation generated by a heartbeat for which the differential quotient dp/dt exceeds a predetermined threshold value of at least 100 Pascals per second, for example 400 Pascals per second.

This pulse sequence is designated by 155 in Figure 4.

When the pressure in the air chamber 31 is reduced to be in a certain range, blood flowing through the artery enclosed by the sleeve 1 generates noises, the so-called Korotkoff tones, on each blood stroke generated by a heartbeat. These Korotkoff tones are converted by the microphone 33 into electrical tone frequency signals and transmitted through the filter means 51, which preferably also amplifies the signals to the discriminator 53. When the voltages of the Korotkoff tone signals exceed the lower threshold value determined by the discriminator 53, the pulse shaper of the discriminator feeds a respective pulse to the congtrol unit 55. This pulse sequence is designated by 157 in Figure 4 and extends from the instant t5 to the instant t6.

In the control unit, the pulses generated through the pressure fluctuations and the pulses generated through the Korotkoff tones are fed to an AND-gate.

The AND-gate forms a coincidence circuit and opens a window for the pulses of the pulse sequence 157 during each pulse of the pulse sequence 155. Signals from the microphone are thus further processed only when they fall into a window opened by a pressure fluctuation, i.e. when a coincience exists between the tone signals and the pressure fluctuations. As a result, the Korotkoff tones can be distinguished from interfering noises and the latter be suppressed.

The control unit 55 includes an electronic switching device which connects the output of the amplifier 57 with the store 63 when the appliance is switched on.

The control unit 55 also includes means to ascertain the appearance of the first Korotkoff signal passing the afore-mentioned AND-gate. when the first Korotkoff signal arrives, the line from the amplifier 57 of the pressure-measuring channel is separated from the store 63. The store 63 accordingly stores the pressure value present on the arrival of the first Korotkoff tone signal, i.e. the systolic pressure.

As the pressure in the air chamber 31 drops, further Korotkoff tones follow the first Korotkoff tone. The control unit 55 comprises means which briefly connects the output of the amplifier 57 with the store 65 on each Korotkoff tone, i.e. on each pulse of the pulse sequence 125. A new pressure value is thus stored in the store 65 on each Korotkoff tone, these pressure values progressively reducing.

As already mentioned, the pulse sequence 157 extends to the instant teAs no further pulses occur after the instant6, the value of the pressure p measured at the instant t6 remains stored in the store 65 until the appliance is switched off. This storage value then represents the diastolic pressure The control unit 55 also comprises circuit means by which it can be ascertained when no further Korotkoff tone has occurred during a predetermined time interval of 2 to 10, for example 5, seconds. At the end of this time interval, namely at the instant t7, the control unit 55 delivers to the regulator 61 a signal which has the effect that the valve 37 is fully opened.

The pressure p then drops very rapidly and at the instant t8 is again at the value zero, i.e. the ambient air pressure.

The control unit 55 is so constructed that it connects the output of the amplifier 57 with the indicating control device 67 at regular time intervals of, for example, 0.3 seconds up to the instant t7. The indicating unit 27 then indicates the instantaneous pressure each time. The control unit 55 could, however, also be constructed in such a manner that the pressure would be indicated on each pulse of the pulse sequence 123 in the time interval between the instants t4 and t7.

The control unit also switches the heartbeat frequency meter 71 on temporarily so that this measures the heartbeat frequency during the occurrence of the pulse sequence 123 and determines a mean value. This is stored in the store of the meter 71 until the appliance is switched off.

The nature of the control unit 55 is such that the store 63, the store 65 or the store of the mete 71 can be cyclically interrogated by a brief depression of the switch 21. The relevant storage value stored in analog form is then fed to the indicating control device 67 and converted by this into a digital signal.

This is fed to the indicating unit 27 so that the unit thus selectably indicates systolic or diastolic pressure or heartbeat frequency. The indicating control device 67 includes a network which can be switched over by the switch 23 and connected between the feed lines of the stores 63 and 65 and the analogdigital converters. This makes possible selectable presentation of the pressure indication in kilopascals of torrs, the switching between these alternative forms of indication being effected by a brief depression of the switch 23.

When all three storage values have been read off, the appliance 3 can be switched off by depressing the ONIOFF switch 25, whereby the measurement is concluded.

The appliance 3 also includes a time switch. This switches the appliance off when no pumping has been carried out for a predetermined time of, for example, one minute and, when a measurement is performed, at the latest one minute after the end of the measurement. This ensures that the applicance is switched off even if the user forgets as the end of the measurement, or on an interruption of the measurement, to depress the ON/OFF switch 25.

The pressure sensor 39 is thus automatically balanced to zero in response to the requisite actuation of the ON/OFF switch 25 at the start of every measurement. Since this balancing takes place shortly before the actual pressure measurement, any zero point drift of the bridge circuit 91 is eliminated in all measurements. The zero balance device 75 also makes it possible to balance the bridge circuit 91 even when the zero balance deviation thereof amounts to more than 100 % of the provided pressure measurement range.

Now that the general mode of operation of the equipment has been described, it will be explained how the pressure measuring channel can be tested.

For this purpose, the switch 127 is closed with the air chamber 31 free of pressure or, better still, with the appliance 3 separated from the sleeve 1, but switched on. As a result, the bridge circuit 91 is detuned in defined manner. The indicating unit now indicates a pressure which is to agree with a test value, which may be stated on a data sheet. On closure of the switch 127, a voltage is fed through the diode 129 to the indicating control device 67 and causes the indicating unit 27, possibly by indication of the letter "T", to signal that the switch 127 is disposed in the test position, i.e. is closed. When the switch 127 is opened, normal measuring can be again be carried out.

A modified amplifier 257 with zero balance device 275, in lieu of the amplifier 57 and zero balance device 75, is illustrated in Figure 5. The amplifier 257 comprises a differential operational amplifier 301, provided with a feedback, as the actual amplifier and has inputs connected with the pressure sensor (not shown). The zero balance device 275 comprises an amplifier 313, which is provided with full feedback and serves as an impedance converter. The noninverting input of the amplifier 313 is connected through a capacitor 319 with the output 301. The non-inverting input of the amplifier 313 is also connected through a switching device 317 with earth. The output of the amplifier 313 is connected with the following differentiator 59 as well as with the control unit 55.

For carrying out zero balancing, the switching device 317 is temporarily closed with the air chamber free of pressure. As a result, the electrode of the capacitor 319 which is connected with the amplifier 313 is electrically brought to earth potential. In that case, a voltage difference arises between the two capacitor electrodes and remains in being on reopening of the switching device 317.

The switching device 317 is shown as a switch and could be formed by, for example, a separate, non-detenting press key switch or by a contact of the non-detenting ON/OFF press key switch 25. The switching device 317 could, however, be formed bya switching transistor controlled analogously to the transistor 117 through a pulse generator.

It is also mentioned that the transistor 117 could be replaced by a contact of a manually actuable switch, preferably a non-detenting press key switch, or by a relay.

The command for balancing of the zero point can also be given through the manual actuation of a switch, particularly the ON/OFF switch 25. It would, however, be possible to provide the equipment with a a pressure-sensitive switching device, for example a diaphragm switch. This could then be connected by fluid with the air chamber and electrically with the zero balance device, so that the zero balance device, when the air chamber is free of pressure, performs a zero balance either continuously or in certain phases in which other conditions are fulfilled.

The equipment could also be modified in other respects. For example, instead of a piezo-resistive bridge circuit the pressure sensor could comprise another type of measurement transducer, for example an inductive or capacitive measurement transducer. It would be possible in this case to carry out an automatic zero balance with a zero balance device directly on the measurement transducer or on a component of the pressure measuring channel connected downstream of the transducer.

Moreover, the pressure sensor could be connected in terms of fluid with the air chamber not through lines but through direct incorporation therein.

It will be apparent that various ones of the electronic components shown in Figures 2,3 and 5 could be assembled into integrated switching cir cuit.

In summary, the command for the zero balancing can be given either by manual actuation of a switch, particularly a non-detenting key switch, or, without the user's participation, by a switching device of the equipment. The person operating the equipment therefore needs to actuate at most one switch for the zero balance, but does not need to set a potentiometer or other infinitely adjustable operating elements.

It is also pointed out that, instead of a separate microphone and a separate pressure sensor, a combined sound-pressure pick-up could be provided, the pick-up serving for the detection of the tones generated by the blood as well as also for the detection of the quasi-static blood pressure and the pressure modulation produced by the heartbeats.

The pick-up could be arranged either in the inflatable sleeve or in the appliance. Electrical signals delivered by the pick-up could then be split up by a frequency band dividing filter and fed, as appropriate, to the sound channel and the pressure channel of the electronic system.

Finally, reference is made to the specifications of Swiss patent specifications Nos. 1296/80, 1297/80 and 1299/80 of the applicant in which further details of such blood pressure measuring equipment are described.

Claims (9)

1. Blood pressure measuring equipment comprising measuring means attachable to a person and provided with a chamber inflatable by fluid, a pressure sensor for detecting fluid pressure in the chamber, electronic signal generating means electrically connected to the sensor to generate electrical signals indicative of the detected pressure, and zero setting means connected to at least one of the sensor and the signal generating means and operable in response to a command signal to cause the pressure signals to have a value indicative of zero fluid pressure in the chamber.

2. Equipment as claimed in claim 1, comprising switching means for providing the command signal.

3. Equipment as claimed in claim 2, the switching means being manually operable.

4. Equipment as claimed in claim 3, comprising a voltage source connected to the switching means, the switching means being adapted to control the supply of operating voltage in the equipment

5. Equipment as claimed in claim 1, comprising a pulse generator for generating a pulse to serve as the command signal, the zero setting means being adapted to set the pressure signals to said zero pressure value within the duration of the command signal pulse.

6. Equipment as claimed in claim 5, wherein the zero setting means comprises regulating the pressure signals at said zero pressure value for a period longer than the duration of the command signal pulse.

7. Equipment as claimed in claim 6, wherein the pressure sensor comprises a bridge circuit having at least one branch comprising a pressure-dependent electrical resistance, the regulating means being connected at an output thereof to said one branch.

8. Blood pressure measuring equipment substantially as hereinbefore described with reference to Figures 1 to 4 of the accompanying drawings.

9. Equipment as claimed in claim 8 and modified substantially as hereinbefore described with reference to Figure 5 of the accompanying drawings.