Heart Rate Telemetry Using Micropower Integrated Circuits

Two heart rate/temperature telemetry transmitter circuits have been developed for estimating activity, physiological state or metabolic rate. Use of programmable micropower operational amplifiers permits total quiescent supply currents below 25 μA while retaining sufficient sensitivity for use on invertebrates (crabs). The first circuit responds to electrical activity of the myocardium or cardiac ganglion. It has the advantage of sensing an unambiguous all-or-nothing electrical signal which has an exact one-to-one correspondence with heart beat. Its disadvantages are low noise immunity due to high input impedance, sensitivity of other muscle potentials, possible signal loss due to electrode motion or degradation, and a requirement for critical electrode placement. The second circuit, slightly more complex, responds to a.c. impedance changes associated with heart movements. The signal waveform changes with heart rate and degree of filling and therefore is not as unambiguous as ECG. This circuit is, however, immune to both muscle potentials and external noise fields, shows very long electrode life, and does not require precise electrode placement. Both transmitters can be constructed with package weights below 5 g including batteries for two weeks operation.