Myocardial Stimulation with Ultrashort Duration Current Pulses

In order to identify a practical short-duration limit for stimulating myocardium, theoretical and experimental studies were carried out using dog and turtle hearts. The strength-duration curves for current, charge and energy were derived from the standard excitable membrane model which employs a parallel resistance and capacitance. From these derivations, the predicted duration for minimum energy was identified. The experimentally measured strength-duration curves for two types of myocardium followed the predicted values closely. The duration for minimum energy was calculated to be 1.25 times the membrane time constant. The practical short-duration limit for a pacemaking stimulus is about 10% of the membrane time constant. For dog myocardium the average time constant was 2.4 ms. Therefore, a practical stimulus duration for minimum charge in the dog should be no longer than about two-tenths of a millisecond, although shorter duration stimuli are equally effective. This minimum charge criterion provides the minimum drain on the stimulator power supply.