Concealed bypasses of the atrioventricular mode in patients with paroxysmal supraventricular tachycardia revealed by intracardiac electrical stimulation and verapamil

Thirteen patients with paroxysmal Supraventricular tachycardia were studied with use of His bundle electrograms and programmed intracardiac stimulation. No patient had evidence of either the Wolff-Parkinson-White or Lown-Ganong-Levine syndrome. During ventricular pacing at a rate of 90 to 180 beats/min retrograde conduction time increased by an average of 80 msec in eight patients; in the remaining five patients the average increase was only 9 msec. The tachycardia was terminated in all 13 patients after intravenous administration of verapamil, 10 mg. This drug acts predominantly on the atrioventricular (A-V) node, and during termination of an A-V nodal reciprocal tachycardia both the antegrade and retrograde conduction times would be expected to be prolonged. During termination of the tachycardia antegrade conduction was prolonged by an average of 43 msec and retrograde conduction by an average of 79 msec in eight patients. However, in five patients antegrade conduction was prolonged by an average of 101 msec and retrograde conduction by an average of only 3 msec. The minimal effect of this drug on retrograde conduction and the minimal increase in retrograde conduction during ventricular pacing in these five patients is strong evidence for the presence of an A-V nodal bypass that was not apparent from the surface electrocardiogram. The potential hazards should atrlal fibrillation occur and allow rapid antegrade conduction in an A-V nodal bypass are discussed.