An analysis of fast idioventricular rhythm in the dog.

Fast idioventricular rhythm was studied in dogs with and without recently-induced complete atrioventricular block. The following results were obtained. The fast idioventricular rhythm (1) has an average rate of 92 ± 7.7 beats/min, (2) is either intermittent or continuous, (3) originates from either ventricle, (4) is initiated suddenly by a beat that usually has a different electrocardiogram configuration, (5) may undergo a moderate deceleration before ceasing abruptly, (6) inhibits normal pacemaker activity, (7) when no longer present, can be brought back by a short period of fast driving, (8) can be suppressed by fast driving but the suppression is often preceded by a transient acceleration, (9) requires a longer period of driving than a normal idioventricular rhythm to be suppressed, (10) is accelerated by a short period of driving, (11) is "reset" by driving, (12) can be induced at progressively faster rates by repeated periods of driving during recovery from a prolonged overdrive, (13) is accelerated by sympathetic stimulation or norepinephrine administration, and (14) is accelerated by short periods of driving during submaximal sympathetic stimulation. We conclude that the fast idioventricular rhythm is a form of repetitive activity requiring initiating beats and thus is less sensitive to overdrive suppression. The behavior of this rhythm differs from that of normal idioventricular pacemakers and suggests that this rhythm does not result from an enhancement of a normal pacemaker process but rather from a different mechanism. This mechanism is affected by sympathetic stimulation and by norepinephrine administration and this may result in ventricular tachycardias.