Electrophysiologic observations in a patient with bradycardia-dependent atrioventricular block.

In a patient with atrioventricular (A-V) block distal to the His bundle (H), 1:1 A-V conduction with right bundle branch block and H-V interval of 70 msec was established with atrial pacing at rates of 120 to 150/min, suggesting that the A-V block was bradycardia-dependent. Advanced second degree A-V block distal to the H deflection occurred with atrial pacing at 160/min after completion of A-V nodal Wenckebach periodicity proximal to the H deflection because of the long H-H encompassing the blocked P wave. Atrial extrastimulus testing coupled with sinus rhythm (with A-V block) demonstrated that critical H1-H2 intervals of less than 545 msec allowed conduction to the ventricles. The H2-V2 interval shortened progressively from 290 to 70 msec with shortening of these critical H1-H2 intervals. Atrial extrastimulus testing coupled with an atrial driven cycle lenght of 500 mesc (with intact A-V conduction) revealed block of the H2 deflection with an H1-H2 interval longer than 540 msec. In conclusion, at critical diastolic intervals, impulses were blocked, creating a state of decreased responsiveness. If a cycle length of subsequent impulses was shorter than the critical diastolic blocking interval, membrane responsiveness gradually improved and conduction resumed. If a cycle length of subsequent impulses was longer than the critical blocking diastolic interval, A-V block was sustained. Blocked impulses continually penetrated to the site of block and reset the state of membrane responsiveness.