An instantaneous heart-rate meter with digital display* J. A t h e r t o n Electrical Engineering Dept., BoRon Institute of Technology, Oeane Rd,, BoRon BL3 5AB, England
Abstract--A portable battery-driven instrument is described which displays the instantaneous heart rate in digital form. No controls are required except the on/oH switch and dry electrodes can often be used. Keywords--Heart rate meter, Dry e.c.g, electrodes
Introduction A LARGE number of medical and physiological investigations involve the measurement of heart rate. Heart-rate meters have been described by SLOMAN and TYSON (1962), McDONALD (1966) and CALDWELL (1970) amongst others, but these are often for connection to e.c.g, amplifiers, do not always have digital displays and suffer from the drawback that they require setting up even if only for sensitivity. It was therefore felt that a need could be fulfilled with an instrument which had the following specifications: (a) digital display of heart rate (b) display updated every beat to the value obtained over the preceding period (c) easily applied transducers (d) no controls except on/off switch (e) range of 20 to 199 beats p/min. ( f ) Accuracy of 5 ~ (g) Instrument to be battery driven and portable. This paper describes the principles and circuitry of such an instrument. Physiological considerations As the e.c.g, is the most readily available function it was decided to use this in a standard lead I configuration. To fulfil specification (c) largesurface-area handheld electrodes (brass or copper tubes) are used dry. A range of 20-199 beats/min was thought adequate for human subjects even in states of hradycardia and tachycardia. The QRS complex of the e.c.g, was considered as being the most suitable parameter with which to trigger the ratemeter. The height and width of QRS spikes were then considered, since, to satisfy specification (d), the ratemeter would have to include automatic controls to accommodate the difference between subjects. * First received 16th May and in final form 23rd May 1974
Medical and Biological Engineering
The automatic control must not, however, make severe adjustments between beats of the slowest subject or during the changes due to respiration. The Handbook of Biological Data (W. B. Saunders Co.) gives a range of 0.1 mV-2 mV and 50 m s lOOms for the height and width of the QRS complex for 959/00 of the population, and so this was taken to be the range of the automatic control. Also the instrument was made insensitive to polarity so that the electrodes could be attached to the subject either way around.
Description of rate meter
Basic principles The block diagram of the instrument is shown in Fig. 1. The e.c.g, is first amplified in a variable-sensitivity amplifier, shaped and made t.t.l, compatible in order that the QRS spike can be used to trigger a monostable circuit which resets the display. A ramp generator, which has been running since the previous QRS, is then sampled by a voltage controlled monostable, which produces a pulse corresponding to the ramp height (and therefore to the heart beat period). This allows clock pulses into the counter and display. The ramp is then quickly reset and allowed to sweep until the next QRS spike. A 'dead time' of 300 ms (corresponding to the periodic time at 200 beats/min) is incorporated in the instrument, during which time the circuit operation cannot be retriggered. The operation of the circuit may be summarised by the following equations: Ramp voltage V oc 7"1 where Tt is the input periodic time
V=K1Ta K1
September 1975
=
a constant. 669
This voltage is used to control the length of a pulse so that
and if this pulse of length 7"2 opens a gate and allows clock pulses through at a rate C hertz, then clock pulses admitted to counter
1 T2 (pulse length) oc - V
T2 ~
=CT2 CK2ft 60K t
K2 ---
V
and if CK2/6OK~ = 1 then pulses in counter = ft and these can be decoded and displayed directly.
K2 = a constant Then
Circuit description The complete circuit is shown in Figs. 2 and 3. The e.c.g, voltage from the subject is applied to the differential amplifier formed by Aa, A2 and A3. The differential input impedance is 1 M ~ and almost 6 M ~ to common-mode signals. The follower stages have high and low break points of 18 Hz and 2 Hz, respectively, and the differential stage 34 Hz and I Hz. R9 is to adjust the balance of the stage when driven with a common-mode voltage. The signal is then passed to the stage incorporating A+, which is an absolute value amplifier. Thus at the output of this stage are only positive-going signals which are applied directly to the noninverting terminal of As, which is used as a comparator. The peak value of the signal is also transferred to Cs and the inverting
K2 K, T~
T2=-but
6O T~ = input period = ~- seconds J1 where ]'1 is the heart rate in beats per minute. Therefore
T~-
K2fl 60 K 1
ampli f y ing, shaping,automatic sensitivitycontrol e
300ms ~ . , I [--
144_,0 ns _~ /
ramp voltage ramp
"dead tim~" I ~ [~igger
-- 9 monostoble ~ monostable ~
generator
voltage controlled[ monostable J
[--L
T
J-L
I
trigger
ramp resetting monostoble
trigger
reset
hundreds
,tens
units
:J-h resetl .__..lovor,,owH0+ H, oc,, o0o cour.~cade 1 bistable counter
enable
)unter
[de~ oerl Iooc',def. / / / / 670
I I /I I---/ I I 7 segment displays
Fig. I Basic block diagram
Medical and Biological Engineering
September 1975
emitter follower Tr7 from the ramp generator which is a constant-current source (Tr4) charging C1o in a linear fashion. The purpose of A6 is to measure the voltage dropped due to the base- emitter junctions of Try and Tr8 and re-apply it to the top of Raj to correct for the error which would otherwise result. The v.c.m, output pulse opens a gate and allows pulses from the 1 MHz crystal clock into the counter stages. When the v.c.m, pulse falls the ramp reset monostable emits a 3 ms pulse which turns on Tr6 and Trs, the latter discharging the ramp capacitor C~o. When this pulse is removed the ramp automatically restarts its sweep. The counter stages for the units and tens are conventional t.t.I, decade counters, The hundreds stage is a discrete flip-flop which lights the appropriate segment~ of the 7segment display to show 1. Thus the range maximum is 199 beats/min. The tens and units stages use a t.t,l, decoder for decoding binary coded decimal (b.c.d.) into a form for 7-segment displays. The clock is a crystal-controlled 1MHz oscillator with its negative half cycles removed by diode DT.
terminal of the comparator. Assuming Tra is OfF, t he capacitor is allowed to discharge in an exponential manner dictated by 821. Diode D4 is to replace the voltage dropped by D3. Thus the last peak voltage of the signal (decayed somewhat) is stored on Cs, and as the signal exceeds the stored value on the next QRS spike the comparator switches from Vsat to V § sat and back. The diode D5 is to prevent the comparator pulling the t.t.I, monostable input to below 0V. This first monostable then triggers as the comparator switches and provides a 'dead time' of 300 ms. On its rising edge, howeveL it triggers another monostable of about 140 ns duration. Whilst the output from this monostable is in the high state it resets the decade counters, and when it falls the voltage-control monostable is triggered. This voltagecontrol monostable (v.c.m.)has as its timing circuit a constant-current source formed by Tr8 and Ra~ so that, once fired, the duration of the output pulse is inversely proportional to the voltage on the base of Trs. This voltage has been passed through the
C4 ., 1R~. . O lOOjJF
C6 IOOpF .,
R12 470 kO R3 R5 75k0
18~1kI
%uF
to f i r s t monostable
)A47 R21 9~ . 18kf) R20 18k0
R16 R6
R23 2700
82k0
470kD
915V 1~176
C5 1-6k0 lOO,uF
C7 0.1jJF
R18
Tr 1 pN~7
R17
R24 lkQ C, Tr 5 o o
R32 1MO 1~
R33 1MQ 10
-15V
lOkO
Tr8
~2~ ~
R34 1M01"I,
16k@ 2Ol,
R2 6 F]
[1R27
2Olo
I
V
r"3~
5"/~ 11 10 voltage-controlled monostable 74121 4 5 6
[7 [ IKQ R29 c-~ lOkD
I
-15V
11 t0 ramp resetting monostable 74121 3 5 6
1
vc § 1-
[
. from short - monostable ~to clock gate
Fig. 2 Amplifier, ramp generator and voltage-controlled monostable NB Voltage suppfies to integrated circuits have not been shown for clarity
Medical and Biological Engineering
September 1975
671
1.8 kf~ in the collector of Tr3 brings the voltage on C8 down much more rapidly. In normal steadystate use, therefore, the fast decay circuit is inoperative. A prototype instrument has been constructed in a plastic box measuring approximately 140 x 90 • 60 mm with a cut-out window for the display, two sockets for the electrode leads, and a spring release 'press to read' battery switch. An invertor was built to provide i l 5 V for the amplifiers and + 5V for the logic, although this can be obtained from readily available batteries.
The purpose of Trl, Tr2 and Tr3 requires a little explanation. The decay of the voltage on C8 has to be such that it does not allow the comparator to switch due to the T wave of the e.c.g, or noise on the signal during the longest periodic time, which is 3 s (20 beats/min). However, at switch oN, the transients in the amplifiers take the voltage on the capacitor to Vs,~, and, if the subject has a very low amplitude, the settling time of the instrument could be unduly long. The ramp, however, reaches the bottom of its sweep after 3s, and so, when this happens, Trt and Tr2 switch OFF, Tr3 o~ and the
Vcc
R37~] 22kQU
[1 ,o 1
Vcc
'dea~ time monostoble 74121 4 6 6
5V
'ro *- I 1 empIPhfiers
~39 68k~
I
Tr,
Vcr C~4 100D111 I
~'
T
11 10 9 short monostclbl 74121
1MHz crystal 17
Clock
to vcm
vcm 123 67 decode counter
23
qued gate 7400
7490
Vcc 5V R41
IkO
E ,3r
TF9
1
BCIO
2
67
decoder
1S44F
I
, L L
7447 16151413 1211 lOCJ
,
I. Fig. 3 Logic circuits NB Voltage supp/ies to integrated circuits have not been shown for clarity
672
Medical and Biological Engineering
September 1975
Discussion
The instrument itself, when provided with a suitable signal, performs well. The problems, however, of providing a noise free signal from dry hand-held electrodes are considerable. First, as readings are required within a few seconds of electrode application, the material chosen for the electrodes must be one in which noise is negligible. Secondly, the skin resistance is an unknown factor and, for convenience, is preferably not minimised by electrode jelly. This is the mason for using large-surface-area electrodes and the low upper break point of the amplifiers. An imbalance in the electrode/skin resistance can cause a large differential signal to be produced from the commonmode signal (Fig. 4). R1 and R 2 in the amplifier are made equal and the amplifier balanced (using Rg) with a common-mode signal injected at the input, but if the electrode resistances, Re1 and Re2, are not equal, then, owing to the potential-divider
rs-u-~ec:ti
|i
Re 1
"'
amplifier
I RI
'JI
,
common-mode voltage
R3
R2
Re2
Fig. 4 Effect of skin~electrode resistance on commonmode rejection
effect, a different voltage appears across R~ than across R2. Thus we now have an unwanted differential signal together with the wanted one (the e.c.g.) Thirdly, the manner in which the subjects grip the electrodes varies considerably. Some people squeeze tightly creating considerable muscular noise, while others only just hold the electrodes creating a high skin/electrode resistance. When one considers this variation along with the possible
variation in skin grease and e.c.g, amplitude, one begins to see the possible spread in signals being presented to the instrument. It is this problem which the author feels is the main cause for the instrument's erratic performance on some subjects. Another type of electrode which has been tried is a flat copper rectangle attached to Velcro tape and fastened round the wrists of the subject. This relieves him of holding the electrodes, and the possibility of muscular noise is therefore reduced. This also has the advantage that it is suitable for unconscious subjects. Of course, conventional e.c.g. electrodes can be used and have been found to work well under adverse conditions when the dry electrode proved inoperable. It must be appreciated that certain abnormalities of the heart can cause the e.c.g, to alter substantially, but as long as there is a recurring component of distinctly separate amplitude, the rate meter will function. The output of the amplifier must not be taken to an oscilloscope or chart recorder other than for monitoring purposes, as any attempt at diagnosis may lead one to assume completely incorrect abnormalities due to the restricted bandwidth of the amplifier. The design of this instrument is the subject of a patent, and a commercial version is expected to be made available by Salford Electrical Industries Limited. Acknowledgments--The author would like to thank Salford Electrical Instruments Ltd. and Boiton Institute of Technology for their permission to publish this paper. Thanks are also due to Mrs. G. Atherton who prepared the manuscript. References CALDWELL, W. M., SMITH, L. D. and WILSON, M. F.
(1970) A wide-range linear beat-by-beat cardiotachometer. Med. & Biol. Engng. 8, 811-185. McDONALD, R. D. 0966) A heart rate meter for measurement of instantaneous and average rates.Meal. & Biol. Engng. 4, 291-298. SLOMAN, G. and TYSON,G. (1962) A heart rate meter for clinical use. Electron. Eng. 34, 626-627.
Un appareil de mesure instantande de la vitesse du coeur avec affichage numerique Sommaire--L'article d6crit un instrument portatif/~ pile servant/t afficher la vitesse instantan6e du coeur sous forme num6rique. Aucune commande n'est n6cessaire/~ part l'interrupteur de marche/arr6t. Des 61ectrodes s6ches peuvent fr6quemment 6tre utilisees.
Schnelle Herzschlagmessung mit Digitalanzeige Zusammenfassung--Ein tragbares, battericbetriebenes Instrument wird beschrieben, das den Herzschlag
sofort digital anzeigt. AuSer dem Ein-/Ausschalter sind keine weiteren Schalter erforderlich. Die Trockenelektroden k6nnen mehrfach Verwendung finden. Medical
and B i o l o g i c a l
Engineering
September
1975
673
Abstract--A portable battery-driven instrument is described which displays the instantaneous heart rate in digital form. No controls are required except the on/oH switch and dry electrodes can often be used. Keywords--Heart rate meter, Dry e.c.g, electrodes
Introduction A LARGE number of medical and physiological investigations involve the measurement of heart rate. Heart-rate meters have been described by SLOMAN and TYSON (1962), McDONALD (1966) and CALDWELL (1970) amongst others, but these are often for connection to e.c.g, amplifiers, do not always have digital displays and suffer from the drawback that they require setting up even if only for sensitivity. It was therefore felt that a need could be fulfilled with an instrument which had the following specifications: (a) digital display of heart rate (b) display updated every beat to the value obtained over the preceding period (c) easily applied transducers (d) no controls except on/off switch (e) range of 20 to 199 beats p/min. ( f ) Accuracy of 5 ~ (g) Instrument to be battery driven and portable. This paper describes the principles and circuitry of such an instrument. Physiological considerations As the e.c.g, is the most readily available function it was decided to use this in a standard lead I configuration. To fulfil specification (c) largesurface-area handheld electrodes (brass or copper tubes) are used dry. A range of 20-199 beats/min was thought adequate for human subjects even in states of hradycardia and tachycardia. The QRS complex of the e.c.g, was considered as being the most suitable parameter with which to trigger the ratemeter. The height and width of QRS spikes were then considered, since, to satisfy specification (d), the ratemeter would have to include automatic controls to accommodate the difference between subjects. * First received 16th May and in final form 23rd May 1974
Medical and Biological Engineering
The automatic control must not, however, make severe adjustments between beats of the slowest subject or during the changes due to respiration. The Handbook of Biological Data (W. B. Saunders Co.) gives a range of 0.1 mV-2 mV and 50 m s lOOms for the height and width of the QRS complex for 959/00 of the population, and so this was taken to be the range of the automatic control. Also the instrument was made insensitive to polarity so that the electrodes could be attached to the subject either way around.
Description of rate meter
Basic principles The block diagram of the instrument is shown in Fig. 1. The e.c.g, is first amplified in a variable-sensitivity amplifier, shaped and made t.t.l, compatible in order that the QRS spike can be used to trigger a monostable circuit which resets the display. A ramp generator, which has been running since the previous QRS, is then sampled by a voltage controlled monostable, which produces a pulse corresponding to the ramp height (and therefore to the heart beat period). This allows clock pulses into the counter and display. The ramp is then quickly reset and allowed to sweep until the next QRS spike. A 'dead time' of 300 ms (corresponding to the periodic time at 200 beats/min) is incorporated in the instrument, during which time the circuit operation cannot be retriggered. The operation of the circuit may be summarised by the following equations: Ramp voltage V oc 7"1 where Tt is the input periodic time
V=K1Ta K1
September 1975
=
a constant. 669
This voltage is used to control the length of a pulse so that
and if this pulse of length 7"2 opens a gate and allows clock pulses through at a rate C hertz, then clock pulses admitted to counter
1 T2 (pulse length) oc - V
T2 ~
=CT2 CK2ft 60K t
K2 ---
V
and if CK2/6OK~ = 1 then pulses in counter = ft and these can be decoded and displayed directly.
K2 = a constant Then
Circuit description The complete circuit is shown in Figs. 2 and 3. The e.c.g, voltage from the subject is applied to the differential amplifier formed by Aa, A2 and A3. The differential input impedance is 1 M ~ and almost 6 M ~ to common-mode signals. The follower stages have high and low break points of 18 Hz and 2 Hz, respectively, and the differential stage 34 Hz and I Hz. R9 is to adjust the balance of the stage when driven with a common-mode voltage. The signal is then passed to the stage incorporating A+, which is an absolute value amplifier. Thus at the output of this stage are only positive-going signals which are applied directly to the noninverting terminal of As, which is used as a comparator. The peak value of the signal is also transferred to Cs and the inverting
K2 K, T~
T2=-but
6O T~ = input period = ~- seconds J1 where ]'1 is the heart rate in beats per minute. Therefore
T~-
K2fl 60 K 1
ampli f y ing, shaping,automatic sensitivitycontrol e
300ms ~ . , I [--
144_,0 ns _~ /
ramp voltage ramp
"dead tim~" I ~ [~igger
-- 9 monostoble ~ monostable ~
generator
voltage controlled[ monostable J
[--L
T
J-L
I
trigger
ramp resetting monostoble
trigger
reset
hundreds
,tens
units
:J-h resetl .__..lovor,,owH0+ H, oc,, o0o cour.~cade 1 bistable counter
enable
)unter
[de~ oerl Iooc',def. / / / / 670
I I /I I---/ I I 7 segment displays
Fig. I Basic block diagram
Medical and Biological Engineering
September 1975
emitter follower Tr7 from the ramp generator which is a constant-current source (Tr4) charging C1o in a linear fashion. The purpose of A6 is to measure the voltage dropped due to the base- emitter junctions of Try and Tr8 and re-apply it to the top of Raj to correct for the error which would otherwise result. The v.c.m, output pulse opens a gate and allows pulses from the 1 MHz crystal clock into the counter stages. When the v.c.m, pulse falls the ramp reset monostable emits a 3 ms pulse which turns on Tr6 and Trs, the latter discharging the ramp capacitor C~o. When this pulse is removed the ramp automatically restarts its sweep. The counter stages for the units and tens are conventional t.t.I, decade counters, The hundreds stage is a discrete flip-flop which lights the appropriate segment~ of the 7segment display to show 1. Thus the range maximum is 199 beats/min. The tens and units stages use a t.t,l, decoder for decoding binary coded decimal (b.c.d.) into a form for 7-segment displays. The clock is a crystal-controlled 1MHz oscillator with its negative half cycles removed by diode DT.
terminal of the comparator. Assuming Tra is OfF, t he capacitor is allowed to discharge in an exponential manner dictated by 821. Diode D4 is to replace the voltage dropped by D3. Thus the last peak voltage of the signal (decayed somewhat) is stored on Cs, and as the signal exceeds the stored value on the next QRS spike the comparator switches from Vsat to V § sat and back. The diode D5 is to prevent the comparator pulling the t.t.I, monostable input to below 0V. This first monostable then triggers as the comparator switches and provides a 'dead time' of 300 ms. On its rising edge, howeveL it triggers another monostable of about 140 ns duration. Whilst the output from this monostable is in the high state it resets the decade counters, and when it falls the voltage-control monostable is triggered. This voltagecontrol monostable (v.c.m.)has as its timing circuit a constant-current source formed by Tr8 and Ra~ so that, once fired, the duration of the output pulse is inversely proportional to the voltage on the base of Trs. This voltage has been passed through the
C4 ., 1R~. . O lOOjJF
C6 IOOpF .,
R12 470 kO R3 R5 75k0
18~1kI
%uF
to f i r s t monostable
)A47 R21 9~ . 18kf) R20 18k0
R16 R6
R23 2700
82k0
470kD
915V 1~176
C5 1-6k0 lOO,uF
C7 0.1jJF
R18
Tr 1 pN~7
R17
R24 lkQ C, Tr 5 o o
R32 1MO 1~
R33 1MQ 10
-15V
lOkO
Tr8
~2~ ~
R34 1M01"I,
16k@ 2Ol,
R2 6 F]
[1R27
2Olo
I
V
r"3~
5"/~ 11 10 voltage-controlled monostable 74121 4 5 6
[7 [ IKQ R29 c-~ lOkD
I
-15V
11 t0 ramp resetting monostable 74121 3 5 6
1
vc § 1-
[
. from short - monostable ~to clock gate
Fig. 2 Amplifier, ramp generator and voltage-controlled monostable NB Voltage suppfies to integrated circuits have not been shown for clarity
Medical and Biological Engineering
September 1975
671
1.8 kf~ in the collector of Tr3 brings the voltage on C8 down much more rapidly. In normal steadystate use, therefore, the fast decay circuit is inoperative. A prototype instrument has been constructed in a plastic box measuring approximately 140 x 90 • 60 mm with a cut-out window for the display, two sockets for the electrode leads, and a spring release 'press to read' battery switch. An invertor was built to provide i l 5 V for the amplifiers and + 5V for the logic, although this can be obtained from readily available batteries.
The purpose of Trl, Tr2 and Tr3 requires a little explanation. The decay of the voltage on C8 has to be such that it does not allow the comparator to switch due to the T wave of the e.c.g, or noise on the signal during the longest periodic time, which is 3 s (20 beats/min). However, at switch oN, the transients in the amplifiers take the voltage on the capacitor to Vs,~, and, if the subject has a very low amplitude, the settling time of the instrument could be unduly long. The ramp, however, reaches the bottom of its sweep after 3s, and so, when this happens, Trt and Tr2 switch OFF, Tr3 o~ and the
Vcc
R37~] 22kQU
[1 ,o 1
Vcc
'dea~ time monostoble 74121 4 6 6
5V
'ro *- I 1 empIPhfiers
~39 68k~
I
Tr,
Vcr C~4 100D111 I
~'
T
11 10 9 short monostclbl 74121
1MHz crystal 17
Clock
to vcm
vcm 123 67 decode counter
23
qued gate 7400
7490
Vcc 5V R41
IkO
E ,3r
TF9
1
BCIO
2
67
decoder
1S44F
I
, L L
7447 16151413 1211 lOCJ
,
I. Fig. 3 Logic circuits NB Voltage supp/ies to integrated circuits have not been shown for clarity
672
Medical and Biological Engineering
September 1975
Discussion
The instrument itself, when provided with a suitable signal, performs well. The problems, however, of providing a noise free signal from dry hand-held electrodes are considerable. First, as readings are required within a few seconds of electrode application, the material chosen for the electrodes must be one in which noise is negligible. Secondly, the skin resistance is an unknown factor and, for convenience, is preferably not minimised by electrode jelly. This is the mason for using large-surface-area electrodes and the low upper break point of the amplifiers. An imbalance in the electrode/skin resistance can cause a large differential signal to be produced from the commonmode signal (Fig. 4). R1 and R 2 in the amplifier are made equal and the amplifier balanced (using Rg) with a common-mode signal injected at the input, but if the electrode resistances, Re1 and Re2, are not equal, then, owing to the potential-divider
rs-u-~ec:ti
|i
Re 1
"'
amplifier
I RI
'JI
,
common-mode voltage
R3
R2
Re2
Fig. 4 Effect of skin~electrode resistance on commonmode rejection
effect, a different voltage appears across R~ than across R2. Thus we now have an unwanted differential signal together with the wanted one (the e.c.g.) Thirdly, the manner in which the subjects grip the electrodes varies considerably. Some people squeeze tightly creating considerable muscular noise, while others only just hold the electrodes creating a high skin/electrode resistance. When one considers this variation along with the possible
variation in skin grease and e.c.g, amplitude, one begins to see the possible spread in signals being presented to the instrument. It is this problem which the author feels is the main cause for the instrument's erratic performance on some subjects. Another type of electrode which has been tried is a flat copper rectangle attached to Velcro tape and fastened round the wrists of the subject. This relieves him of holding the electrodes, and the possibility of muscular noise is therefore reduced. This also has the advantage that it is suitable for unconscious subjects. Of course, conventional e.c.g. electrodes can be used and have been found to work well under adverse conditions when the dry electrode proved inoperable. It must be appreciated that certain abnormalities of the heart can cause the e.c.g, to alter substantially, but as long as there is a recurring component of distinctly separate amplitude, the rate meter will function. The output of the amplifier must not be taken to an oscilloscope or chart recorder other than for monitoring purposes, as any attempt at diagnosis may lead one to assume completely incorrect abnormalities due to the restricted bandwidth of the amplifier. The design of this instrument is the subject of a patent, and a commercial version is expected to be made available by Salford Electrical Industries Limited. Acknowledgments--The author would like to thank Salford Electrical Instruments Ltd. and Boiton Institute of Technology for their permission to publish this paper. Thanks are also due to Mrs. G. Atherton who prepared the manuscript. References CALDWELL, W. M., SMITH, L. D. and WILSON, M. F.
(1970) A wide-range linear beat-by-beat cardiotachometer. Med. & Biol. Engng. 8, 811-185. McDONALD, R. D. 0966) A heart rate meter for measurement of instantaneous and average rates.Meal. & Biol. Engng. 4, 291-298. SLOMAN, G. and TYSON,G. (1962) A heart rate meter for clinical use. Electron. Eng. 34, 626-627.
Un appareil de mesure instantande de la vitesse du coeur avec affichage numerique Sommaire--L'article d6crit un instrument portatif/~ pile servant/t afficher la vitesse instantan6e du coeur sous forme num6rique. Aucune commande n'est n6cessaire/~ part l'interrupteur de marche/arr6t. Des 61ectrodes s6ches peuvent fr6quemment 6tre utilisees.
Schnelle Herzschlagmessung mit Digitalanzeige Zusammenfassung--Ein tragbares, battericbetriebenes Instrument wird beschrieben, das den Herzschlag
sofort digital anzeigt. AuSer dem Ein-/Ausschalter sind keine weiteren Schalter erforderlich. Die Trockenelektroden k6nnen mehrfach Verwendung finden. Medical
and B i o l o g i c a l
Engineering
September
1975
673
