Med. & Biol, Eng. & Comput., 1978, 16, 65-67

Display stabilising circuit for systolic-diastolic blood-pressure monitors S. C. Klevenhagen P. Storey Department of Medical Physics, The London Hospital, Whitechapel, London E1 1 BB, England.

Abs t rac t - -When using a digital panel meter to display blood pressure, i t is diff icult to achieve a stable, clinically usable display. A circuit is described, which allows the measured parameter to be displayed in a stable manner ignoring small beat-to-beat variations. The described circuit and a systolic-diastolic separator provide the complete circuitry for a low-cost instrument for the the numerical display of arterial blood pressure.

K e y w o r d s - - B l o o d pressure monitoring, Digital panel meter

1 Introduction

WHEN employing a digital technique for the display of systolic or diastolic pressure signals it is, in many cases, undesirable to have the beat-by-beat value displayed, since, even with a stable patient, the beat-to-beat variations of the monitored parameter cause the display to flicker. This makes the displayed number difficult to read, and, to overcome this problem, a circuit has been designed, which displays the measured parameter in a stable manner, ignoring variations of small magnitude while main- taining a fast response.

The circuit to be described is used as an auxiliary processor of signals derived from an analogue systolic-diastolic blood-pressure separator (McGouGn and MCDONALD, 1968; NAYLOR, 1971; SANDMAN and HILL, 1974; FRANCIS, 1974).

The circuit operates as follows: at the start of the monitoring procedure the incoming signal peak is displayed and simultaneously stored. From now on the circuit makes a continuous comparison of the instantaneous pressure signal arriving from the output of the separator with the stored value. The displayed value is held constant unless the incoming signal differs from the stored one by a set, but adjustable, margin, either positive or negative.

Thus the circuit has an insensitive window, the width of which is adjustable to suit a particular application. For the purpose of blood-pressure monitoring, the insensitivity threshold was set so as to ignore variations smaller than + 5 mm Hg.

2 Circuit description

The circuit of the display stabilising circuit is shown in Fig. 1. The input signal from a separator, either systolic or diastolic, is read and displayed

Received 24th March 1977

0140-0118/78/0689-0065 $1 �9 50/0

�9 IFMBE: 1978

on the digital panel meter (d.p.m.); at the same time the sample and hold (s/h) amplifier (AD782, Analog Devices) stores this value. The d.p.m. (XL99, Exel Electronics) has a 'hold-reading' facility which is used here to retain the read value on the display until a command is given to change it. This command pulse is produced by the comparator (type 319, RS Components) via the monostable multivibrator (type 74121, RS Components). The circuit continuously monitors, via the subtracter (Amplifier 747, RS Components), the difference between the signal held by the s /h amplifier and the instantaneous signal at the circuit input. The sub- tractor is designed to have a gain of 10 in order to amplify the difference between the detected signals. Since the instantaneous input signal may be larger or smaller than the held value, the detected difference is put through a rectifying circuit A1 (type 741, RS Components) so that the difference signal is always positive. The rectified voltage difference is then fed into a comparator (type 319, RS Components) and compared with a set, but adjustable, reference voltage, the magnitude of which defines the insensitive window of the circuit. When the difference signal exceeds the reference voltage the comparator changes state, producing an output pulse which triggers a monostable multivibrator (type 74121, RS Components). The output pulse of the mona- stable is the command signal for the d.p.m, and s /h amplifier, and causes the d.p.m, and the s /h amplifier to read and hold a new signal pressure value. The control pulse is fed to the d.p.m, by means of a c.m.o.s, switch (type 4016, RS Com- ponents).

3 Circuit details

The prototype was built using integrated circuits throughout and standard circuit configurations

Medical & Biological Engineering & Computing January 1978 65

where possible. The subtractor is a combination of two inverters to eliminate common-mode voltages. The gain is determined by the ratio of R2/R1 resistors. I t should be noted that the absolute

signal, thus allowing the calibration of the instru- ment. The calibration is best carried out together with the transducer amplifier by adjusting the attenuator A2 control to read, on the d.p.m., the correct value

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Fig. I Circuit diagram of the display stabiliser

values of resistors R2 and R1 are not important, but they should be matched to I X for accurate sub- traction. The amplifier A1, R3, R4 and D1 form the 'absolute value' rectifying circuit. For negative inputs, D1 is forward biased and the circuit operates as a unity-gain inverting amplifier. For positive inputs D1 is reverse biased and the signal follows the path R3, 1{4. In both cases the output is going positive. The comparator output is normally low and goes high when the difference between the incoming signal and the held values exceed the reference voltage. The monostable is wired to respond to a positive trigger and to produce a transition from high to low. The d.p.m, is wired so that the 'hold- reading' control must be switched to earth by the c.m.o.s, switch for at least 200 ms to initiate a new reading, and it was found necessary to make the monostable pulse of 400 ms duration to ensure correct operation of the d.p.m. The attenuator (VK2) is used between the s /h amplifier output and the d.p.m, to match the signal levels between the systolic-diastolic separator and the described circuit.

The switch S1 disconnects the control signal from the d.p.m, and the s /h amplifier. With the switch open, the digital panel meter follows the input

of the static pressure applied to the transducer. In the next step the reference voltage of the comparator must be set to determine the insensitive window of the described circuit.

The circuit is small and was built on an i.c. stripboard of size 100• 95 mm. For simplicity of use the front panel of the display unit has only three controls: a mains on/off switch, systolic/diastolic selector switch and a calibration switch. The power- supply requirements for the complete display unit are +15V, 100mA; - 5 V , 30 mA; and +5V, 500 mA.

The prototype of the display unit has been in clinical use for six months and was found by the operating room personnel to be an improvement over the conventional type of displays.

Acknowledgments--The authors wish to thank the staff of the Research Workshop, under the supervision of C. Ellis, for help in the mechanical construction of the instrument, and S. Gallagher for uszful discussions in the course of the circuit design.

66 Medical & Biological Engineering & Computing January 1978

References

FRANCIS, G. R. (1974) An improved systolic-diastolic pulse separator. Med. & Biol. Eng. 12, 105-108.

McGouGs, A. R. and McDONALD, R. D. (1968) Systolic- diastolic heart pressure monitor. Electron. Eng. 40, 673-677.

NAYLOR, W. S. (1971) An analogue preprocessor for use in monitoring arterial pressure. Biomed. En~. 6, 77-80.

SANDMAN, A. M. and HILL, D. W. (1974) An analogue preprocessor for the analysis of arterial blood pressure waveforms. Med. & Biol. Eng. 12, 360-363.

Circui t s tabi l isateur pour cadran d 'apparei l de mesure de pression systo l ique-d ias to l ique

Sommalre--Lorsqu'on utilise un compteur digital pour indiquer la pression sanguine, il est difficile d'obtenir une lecture stable pouvant 6tre utilis6e directement. Voici la description d'un circuit qui permet de montrer de mani~re stable le param~tre mesur6 sans tenir compte des faibles variations de chaque pulsation. La combinaison du circuit d6crit et d'un s6parateur systolique-diastolique fournit un ensemble complet correspondant h u n 6quipement d'un co=at peu 61ev6 et permettant d'atiicher num6riquement la pression art6rielle.

Anze ige-Stab i l is ie rungskre is f~ir systo l isch-d iasto l ische B l utd ruck- kont ro I li nstru m e n t e

Zusammenfasstmg--Bei der Verwendung eines Digital-Schalttafelmel3instrumentes zue Anzeige des Blutdruckes l~il~t sich eine stabile, ktinisch verwertbare Anzeige nur schwer verwirklichen. Beschrieben wird ein Kreis, mit dessen Hilfe der gemessene Parameter unter Vernachlassigung kleiner Unterschieden von eirtem Herzschlag zum anderen stabil angezeigt werden kann. Der hier beschriebene Kreis bildet usammen mit einem Systole-Diastole-Trenner den kompletten Schaltkreis fiir ein billiges Instrument zur numerischen Anzeige des Blutdruckes in den Arterien.

Medical & Biological Engineering & Computing January 1978 67