Parasystole and its variants.

Various mechanisms that cause deviations from the classical manifestations of a parasystolic rhythm are reviewed and illustrated by selected clinical electrocardiograms. They consist of: (1) Transient or continued fixed coupling of the ectopic beats, due to (a) synchronization of basic and parasystolic rhythms; (b) reversed coupling of the basic to the ectopic rhythm (unidirectional protection); (c) the operation of supernormal phase of excitability; and (d) intermittent parasystole, due to gap in the protection of the parasystolic center. (2) Irregularities in response to a regular parasystolic discharge may be caused by a second degree exit block, usually of Mobitz type II, rarely of type I. An electrophysiologic basis for the emergence and maintenance of parasystolic rhythms appears to be abnormal states of spontaneous diastolic (phase 4) depolarization in otherwise latent subsidiary cardiac pacemakers.