Med. & Biol.Eng. & Comput., 1977, 15, 700-702

Technical note H e a r t - or respiration-rate calculator Keywords--Heart rate, Measurement. Respiration rate multiply functions are then used to calculate the rate, which is read-out on the built-in 8-digit display. The design of the present instrument is accurate to within one beat at 100 beats per minute. The accuracy increases for lower rates and decreases to about 2~o at 200 beats per minute. This could be improved by using a faster clock or by increasing the number of beats used to determine the interval for calculation. It is limited only by the

Tins NOTE describes a heart-rate meter which uses a commercial 5-function calculator to determine the time necessary to detect the occurrence of eight heart beats and to automatically calculate the rate in beats per minute. The calculator is shown to be readily adapted to this task, or to the similar one of calculating respiration rate. The internal clock measures the time interval by using the adding function as a counter. 'The divide and

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Medical & Biological Engineering & Computing

November 1977

precision of the calculator. The principle demonstrated is useful for other rate calculations where the interval between events is relatively long, or where a measuring interval less than a minute is desired. An extra feature of this design is that the instrument functions as a conventional calculator when not measuring heart rate. The logical design of the instrument is shown in Fig. 1. A Rockwell 5-function calculator which uses the B5000 m.o.s./l.s.i, circuit was used although other calculators would do as well. Connections were wire 0Red with the keyboard matrix and are indicated along the right edge of the diagram with the prefixes STR for strobe and KB for keyboard return. The c.o.s./m.o.s. logic used interfaces readily with the calculator through the 4016 bilateral switches shown on the right. The algorithm used for the counting routine and rate calculation is determined by the pulse-rate formula: R a t e = (f)(8)(60)/n, where f is the frequency of the clock and n is the number of clock pulses counted in an 8-pulse interval. A detailed description of the circuit follows. The clock and timing pulses for the counting operation are derived from the STR8 pulses of the calculator. They are shaped by the 4093 Schmitt trigger at the upper left and divided by the 4024 binary counter to an appropriate rate for counting and calculating. The 4049 and 4081 inverts and combines the outputs from the divide-by-8 and divide-by-16 stages to provide the counting pulses, Cp. The divide-by-32 output calculate pulse is used to step the 4017 state counter through its routine, and is combined with its sequential outputs to provide gating levels for the 4016 switches bridged across the keyboard switches to enter numbers and start the arithmetic operations of the calculator. Amplified pulses from a finger pulse or respiration detector or e.c.g. are shaped by the 4093 at the lower left which drives the pulse indicator and clock input of the 4024 pulse counter. The calculator counting routine starts when the pulse counter is reset. This also resets the 4013 count/

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calculate flip-flop to the count mode and the 4013 clock enable flip-flop which in turn resets the state counter to state 0. The combination of Cp and 0 state causes a 1 to be entered in the calculator via the 4016 in the upper right-hand corner. The next calculate pulse steps the state counter to state- 1, the output of which when ANDed with the count level from the count/calculate flip-flop enables the Cp pulses to strobe the +function of the calculator via the second 4016 in the right-hand column, causing one count to be added to the 1 in the calculator. The counting process repeats because the + 1 enable level disables the clock input to the state counter holding it in state-1. When the pulse counter counts the eighth pulse, the count/calculate flip-flop is set to calculate and the clock input t o the state counter is re-enabled causing it to step to state-2 with the next calculate pulse. The = function is then strobed and the calculator is ready for the next operation. The 5-function calculator does not have a reciprocal key but states 3, 4 and 5 perform this operation with the -- ~ , and = functions resulting in the reciprocal of the count multiplied by 100. State-6, state-3 and state-8 multiply this result by a factor determined by the clock frequency. State-9 sets the clock enable flip-flop to disable the clock and leaves the calculated rate to be displayed. The twelfth pulse detected stores the rate in the calculator memory and the fourteenth pulse resets the registers and clears the calculator for the next rate calculation. The finger pulse detector and amplifier used in this version of the calculator are shown in Fig. 2. The pulse sensor is a Monsanto MCA?, combination l.e.d, infrared source and transistor photodetector used as a photoplethysmograph. It is followed by a 2-stage amplifier with a gain 1000 a n d bandwidth from 0"4 to 6 Hz. The RCA 3130T operational amplifiers are used because of their c.o.s./m.o.s, design and requirement for only a single supply voltage. E.C.G. electrodes or a respiration

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Medical & Biological Engineering & Computing

November 1977

701

sensor with an appropriate amplifier may be used as well. The instrument described above is useful for heart rates from 20 to 200 beats per minute. Its accuracy is limited by the built-in clock of the calculator which requires a regulated power supply for the stated accuracy of + 1% at 100 beats per minute. The lower rate limit can be extended by increasing the size of the coupling capacitors in the pulse amplifier. For very low pulse rates a d.c. amplifier would he required. The high rate is limited by the counting speed of the calculator. This could be improved by increasing the number of beats counted between calculations or by using a faster counter. A crystal-controlled clock would be necessary for greater accuracy. The time necessary for a rate calculation by a commercial calculator limits the speed at which the rate measurement can be repeated to about once every 15 s. An analogue signal can be derived from the digital display of the calculator for recorder readout, but it was not considered practical for this version of the instrument. H. LUDWIG

702

Medical Electronics Laboratory University o f Wisconsin Center for Health Sciences Room 80 Medical Sciences Building 1215 Linden Drive Madison Wis. 53706, USA

References ATSERTON, J. A. (1975) An instantaneous heart-rate meter with digital display, Med. & Biol. Eng. 13, 669-673. CALDWELL, W. M., SMITH, L. D. and WILSON, M. F. (1970) A wide-range linear beat-by-beat cardiotachometer, Ibid. 8, 181-185. LUDWIG, H. and YAN-KtT NG (1967) Heart rate meter with digital timing and linear beat-to-beat readout. Ibid. 5, 615-621. TAYLOR, K. D. and MANDELBER6, M. (1975) Precision digital instrument for calculation of heart rate and R-R interval. IEEE Trans. B M E ~ V 2 2 , 255-256~

Medical & Biological Engineering & Computing

November 1977