Electrocardiographic observations on the termination of supraventricular tachycardia by verapamil

The effect of intravenous verapamil on the termination of supraventricular tachycardia (SVT) was studied by continuous electrocardiographic monitoring of 27 episodes of SVT. Progressive increase of the cycle length heralded conversion in eight episodes while cycle-length alternation preceded cessation of the arrhythmia in 13 episodes. In five patients the arrhythmia was either stopped or closely followed by a ventricular premature beat (VPB), followed by further VPBs in three. Runs of bizarre ventricular tachycardia followed initial sinus-beats in two patients. Sinus standstill, lasting 30 seconds, was observed in one patient. The first post SVT beats had an aberrant QRS configuration with a normal P-R interval in four cases and an aberrant QRS complex with a short P-R interval, resembling Wolff-Parkinson-White complexes, in a further seven patients. The possible mechanisms causing this variability of pre- and post-conversion period are discussed. It is suggested that some aspects of verapamil action may be explained by a parasympaticomimetic effect on the myocardium.