Study of retrograde conduction in complete heart block using His bundle recordings

Two patients with complete anterograde AV block without retrograde P ' waves are described in whom regular retrograde conduction across the blocked area was shown by recording the His bundle electrogram. When an atrial depolarization (A) occurred after the QRS complex within a certain period, A was followed either by no H potential or by a prolonged AH interval, indicating concealed retrograde penetration of the His bundle. Thefact that retrograde conductionaffectedanterograde conduction indicates that the anterogradeandretrograde pathways are not totally independent. Atropine significantly improved retrograde conduction; this suggests that the retrograde conduction occurs through the conduction system in a retrograde fashion rather than through a muscular pathway, bypassing a blocked area of the conduction system, as the latter should not be affected by atropine. It is well known that the conduction system is capable of transmitting impulses in both an antero-grade and retrograde fashion (Moe, Preston, and Burlington, I956; Rosenblueth, 1958). Failure to transmit impulses in one direction does not neces-sarily mean that conduction in the opposite direction is also blocked. Retrograde ventriculo-atrial conduction has been known to occur even in the presence of complete block of anterograde atrio-ventricular conduction (Wolferth and McMillan