Electrophysiologic study of tachycardia-dependent paroxysmal His bundle block in man.

Electrophysiologic study was performed in a patient with tachycardia-dependent paroxysmal atrioventricular block. The site of block was within the His bundle. The effective refractory period of the His bundle was markedly prolonged and it was comparable to the critical atrial cycle length producing type II His bundle block. The most likely mechanism of paroxysmal atrioventricular block was repetitive concealed penetration of the blocking zone by nonconducted impulses that reached the proximal His bundle. Enhancing the blocking ratio at the atrioventricular nodal level resulted in improvement of overall atrioventricular conduction.