The 1,2,3,4 phenomenon. Atrioventricular nodal gap in the dog

This study confirms the facility with which the so-called 1,2,3,4 phenomenon can be reproduced in intact dog hearts. When a series of three atrial premature beats (A2, A3, A4) were delivered following a constant A1-A1 drive, we demonstrated a narrow zone of A1-A3 and A1-A4 intervals in which A4 conducted to the ventricles in the presence of A3 but not in its absence. We used His bundle and multiple atrial electrograms to produce the phenomenon in nine of 16 dogs. Facilitation of conduction of A4 (1) occurred above the His bundle, (2) occurred within narrow ranges of A1-A3 and A1-A4 intervals, (3) required penetration of A3 into the atrioventricular (AV) node, (4) required critically long A2-H2 intervals, (5) always was associated with prolonged A4-H4 intervals, and (6) was limited by atrial refractoriness. An apparent or pseudo-1,2,3,4 phenomenon demostrated in five of 16 dogs occurred with (1) latency between S4 and A4, (2) a sinus nodal or atrial reentry beat, or (3) an atrial escape beat. In the absence of A3, A4 could be made to conduct by preexciting the ventricle in advance of V2. We found no evidence for dual AV nodal pathways. Our results suggest that the underlying mechanism for the 1,2,3,4 phenomenon is analogous to the phenomenon of the gap in AV conduction.