Relationship of Heart Rate to Ventricular Automaticity in Dogs During Ouabain Administration

Spontaneous changes in ventricular automaticity were determined from changes in idioventricular rate in eight dogs with heart block. The mediating effects of imposed rate were assessed in terms of (1) changes in post-pacing depression after ventricular pacing in 14 dogs and (2) changes in vagally induced ventricular escape time after atrial pacing in five dogs with intact AV conduction. Spontaneous ventricular automaticity decreased throughout the major portion of digitalization. The transition to a ventricular tachycardia was late and abrupt. Transient increases in rate transiently enhanced ventricular automaticity throughout digitalization. In animals with heart block, this was recognized at 38 ± 3% (SE) of the toxic dose of ouabain as blunting of postpacing depression, i.e., a significant decrease in the depression of the first post-pacing beat. Blunting was progressive and was replaced at 72 ± 3% of the toxic dose by post-pacing acceleration, i.e., the cycle length of the first post-pacing beat became shorter than the average pre-pacing cycle length. In vagal stimulation experiments ventricular escape time decreased earlier during atrial pacing at 180-190 beats/min (74 ± 3% of the toxic dose) than during atrial pacing at 140-150 beats/min (92 ± 3%). Early ventricular escape was not seen after pacing at 100 beats/min. These findings indicate that heart rate is a critical determinant of early increases in ventricular automaticity during digitalization.