Effects of supernormal capture on directly measured nonmodulated parasystolic cycles.

Pacemaker capture during the supernormal period was seen in a case of modulated parasystole where ectopic cycles occurred without any interposed nonparasystolic beat. This contrasts with previous clinical reports, since, in the latter, the parasystolic cycle length could not be measured directly. As in experimental studies, supernormal modulation resulted in a triphasic phase response curve. Although less likely, similar electrocardiographic changes could be produced by a conceptually different phenomenon, namely full parasystolic resetting due to loss of protection occurring exclusively early in the cycle, combined with modulation in the other (late) parts of the cycle.