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The present experiments describe effects of transient increases in heart rate on ventricular pacemakers during digitalization. Fifteen anesthetized, vagotomized dogs were studied during ouabain infusion or after recovery from ouabain-induced ventricular tachycardia. Vagal stimulation was used to assess baseline ventricular automaticity and 30-second periods of atrial pacing were utilized to evaluate the response of ventricular pacemakers following different increments in rate. After an average dose of 45 ± 3 (SE) μg/kg of ouabain, vagal stimulation unmasked an automatic ventricular focus in 14 animals and pacing was followed by ventricular acceleration in all 15 animals. The Degrees of ventricular acceleration varied directly with the pacing rate. The increment in ventricular rate above the level unmasked by vagal stimulation averaged 59 ± 4% of the increment in atrial rate. At the higher driving rates, a transient postpacing depression was present before the emergence of a rapid ventricular focus. These findings were unaffected by beta-receptor blockade with 4-(2-hydroxy-3-isopropylamino propoxy) acetanilide (AY21,011). The results indicate that transient increases in heart rate may accelerate rather than depress ventricular pacemakers during digitalization.

Acceleration of Ventricular Pacemakers by Transient Increases in Heart Rate in Dogs during Ouabain Administration