Ventricular parasystolic couplets originating in the pathway between the ventricle and the parasystolic pacemaker: Mechanism of "irregular" parasystole

This article explains the mechanism of "irregular" parasystole. Two theorie s have been suggested: "electrotonic modulation" and "type I second degree entrance block." This study attempts to clarify the mechanism of irregular parasystole in cases of true ventricular parasystole associated with ventri cular parasystolic couplets. Cases associated with ventricular parasystolic couplers were selected from 37 clinical cases of true ventricular parasyst ole in which one or more pure parasystolic cycles with no intervening nonec topic QRS complexes were found. Of the 37 cases of true ventricular parasys tole, ventricular parasystolic couplets were found in 4 cases. In none of t he other 33 cases, ventricular parasystolic couplets were found. In all the cases coexisting with ventricular parasystolic couplets, the latter ectopi c QRS complex of the couplet failed to reset the parasystolic rhythm. The a bove findings suggest that the latter ectopic QRS complex of the parasystol ic couplet originated not in the parasystolic pacemaker but in the pathway between the ventricle and the parasystolic pacemaker. It seems that when a sinus impulse fell late in the parasystolic cycle, it passed through the si te of second degree entrance block and that the parasystolic couplers origi nated from the reentrant pathway between the ventricle and the pacemaker. T his strengthens our previous suggestion that the mechanism of irregular par asystole is governed by "type I second degree entrance block" and not by "e lectrotonic modulation."