The electrical interaction between artificial pacemakers and patients, with applications to electrocardiography

Abstract

Eleven artificial pacemakers were calibrated prior to surgery. The current delivered by these units to human hearts was then measured at surgery with a current probe. Body surface pacemaker voltages were recorded subsequently in nine cases. The amplitude and shape of the current pulse depends upon the patient load impedance. Patient load impedance consists of myocardial electrode polarization impedance and tissue resistance. For the pacemaker and myocardial electrodes employed, the patient load impedance is well represented by an equivalent circuit consisting of a series parallel arrangement of two capacitors and two resistors. Values of these circuit parameters were determined in nine cases. Tissue resistance is represented by a fraction of the series resistor in the equivalent circuit. The remainder of the equivalent circuit is attributable to polarization. The voltage pulse measured at the body surface is related to the current pulse delivered to the myocardium by a transfer impedance. Since details of the current pulse, which had a rise time of less than a microsecond, were present in surface voltages, the transfer impedance is approximately resistive and independent of frequency up to a megacycle per second. It is concluded that no significant frequency distortion of electrocardiograms is caused by the electrical characteristics of body tissues. The value of the maximal scalar transfer impedance from these studies has been cast into the vectorial form employed by others in torso model studies. Under the conditions discussed, the vectorial transfer impedance has comparable magnitude whether obtained from model studies or from the present pacemaker study.