The effect of adrenergic enhancement on overdrive excitation

The effect of adrenergic enhancement (stimulation of a stellate ganglion or administration of norepinephrine) on arrhythmias induced by ventricular overdrive ("overdirve excitation") was studied in dogs with complete atrioventricular block. The following results were obtained. (1) Adrenergic enhancement may lead to abnormal ventricular rhythms. (2) A brief drive during adrenergic enhancement induced or enhanced fast ventricular rhythms. (3) The induced rhythms were characterized by an abrupt onset, a fast rate of discharge, and a moderate degree of slowing before an abrupt cessation. (4) The induced rhythms were accelerated by further drive. (5) Increasing the duration of overdrive during adrenergic enhancement resulted in overdrive suppression and not in overdrive excitation. (6) Prolonged drive could induce a few beats (instead of suppression) but at a rate below control ("inhibited excitation"). (7) The arrhythmogenic effects of overdrive and adrenergic enhancement were potentiated by the simultaneous administration of calcium. It is concluded that interventions which increase the inward calcium current (overdrive, adrenergic enhancement and higher [Ca]o) favor the onset and the maintenance of overdrive excitation.