Electrocardiographic observations in bradycardia and tachycardia-dependent atrioventricular block. Relationship to supernormal phase of intraventricular conduction.

This report describes the clinical course of a patient with bradycardia and tachycardia-dependent atrioventricular block. Bradycardia dependent A-V block (phase 4 block) was transient and precipitated by spontaneous slowing of the sinus rate, atrial and ventricular extrasystoles; The degree of slowing (critical RP interval) required to induce A-V block increased progressively over a three-day period. Bradycardia-dependent A-V block was terminated mostly by critically times spontaneous or paced ventricular escape beats, but normally conducted atrial impulses also appeared to restore A-V conduction on several occasions. The tachycardia-dependent component was manifested by an unusual fatigue phenomenon in the His-Purkinje system seen only at an atrial pacing rate of 150 per minute. These observations document the presence of both bradycardia and tachycardia-dependent A-V block in the presence of a normal H-V time and also illustrate the dynamic nature of both phase 4 block and the period of "supernormal" intraventricular conduction.