Observations during clinical 2:1 and 3:1 A-V block below the A-V node. Evidence of partial penetration of atrial impulses into the His bundle

A clinical His bundle recording during 2:1 A-V block below the A-V node displayed RBBB, a prolonged H-V interval, and alternating amplitude and duration of the His potentials. The reduced amplitude of the non-conducted His potential suggests a lesser depth of penetration into the His tissue with subsequent block. The reduced His potential amplitude may be due to decremental conduction within the His bundle and/or prolonged refractoriness of the His tissue following atrioventricular conduction of the preceding atrial impulse.During 3:1 A-V block progressively deeper penetration of the atrial impulses into the His-Purkinje system occurred. Progressive penetration into the more proximal His-Purkinje system may have permitted recovery of a more distal area of refractoriness with subsequent atrioventricular conduction. This mechanisms appears similar to one of the mechanisms of 3:1 A-V block demonstrated experimentally, except that in this clinical record the major site of impaired conduction and progressive penetration is within the His-Purkinje system rather than within the A-V node.