Electronic pacemaker models of parasystole. With special reference to artificial intermittent parasystole with phase 3 and phase 4 protection and to parasystolic modulation.

This study deals with a method of analysis of artificial pacemaker function which can be used to understand the electrocardiographic manifestations of some spontaneous rhythms. The modes of operation of 11 normally-functioning QRS-inhibited (VVI) pacemakers resembled those of spontaneous automatic nonprotected (nonparasystolic) rhythms. The function of 11 continuous asynchronous (fixed rate or VOO) pacemakers was similar to that of continuous-parasystolic rhythms. In 12 patients with malfunctioning QRS-inhibited (VVI) pacemakers, an abnormally-prolonged pacemaker refractory period was equivalent to intermittent parasystole with phase 3 protection; and non-sensing during the terminal portions of the cycle was the iatrogenic counterpart of intermittent parasystole with phase 4 protection. Premature beats occurring within the periods of phase 3 and phase 4 protection were "encompassed" by ectopic intervals equalling the ectopic cycle length, or twice the ectopic cycle length. Therefore, they were manifested differently from the "decelerating" and "accelerating" phases of modulation since premature beats occurring during the letter phases may be encompassed by ectopic intervals which are longer and shorter, respectively, than the ectopic cycle length. Because in previous reports the search for these phenomena was based on premises established "a priori," future studies should be designed to analyze tracings of "group beating" where no previous conclusions have been reached.