A ventricular parasystolic focus capable of generating manifest ectopic beats should not be totally insulated from the electrical events that accompany depolarization in the surrounding tissue; the intrinsic cycle length of the ectopic discharge may be modulated by electrotonic influences transmitted across the zone of "protection." To study the nature of the interaction, response patterns were examined in a mathematical model programmed to simulate an ectopic pacemaker protected, but not divorced from ventricular responses to the normal pacemaker. Computer runs covered a wide range of heart rates, and a wide range of magnitudes of the simulated electrotonic influence. Application of the results obtained in the model to published examples of complex arrhythmias revealed a remarkably close fit to many clinical examples. This findings suggests that many patterns attributed to a re-entrant "extrasystolic" rhythm may, in fact, represent the modulated activity of a parasystolic focus.

An ineffective stimulus applied to cardiac tissue within the relative refractory period can alter the response to an immediately subsequent stimulus. We observed three response patterns that can coexist at different sites of stimulation in the same heart. In the first pattern, a stimulus of two to ten times diastolic threshold, applied too early to elicit a propagated response, becomes effective when a stimulus of equal strength is delivered 10 msec earlier. In the second pattern, a stimulus applied just late enough to evoke a response fails to do so when a stimulus of equal strenght precedes it by as much as 30 msec. Finally, in the third pattern, two stimuli, separated by 10 msec, both of which are late enough to be effective when they are given alone, fail to yield a propagated response when they are applied together. These results have a bearing on the use of trains of stimuli to assess the ventricular fibrillation threshold. Possible interpretations are based on the temporal dispersion of recovery from the refractory state.

No abstract