Concealed atrial bigeminy: mechanisms of disappearance and reappearance of manifest extrasystoles.

ECGs taken from four patients with concealed atrial bigeminy were analyzed to find the mechanisms of disappearance and reappearance of extrasystoles. Observations in three patients showing the classic form of concealed bigeminy suggest that when the extrasystole disappeared, the first sinus impulse after the last manifest extrasystole and its subsequent alternate sinus impulses passed through the reentrant pathway and thereafter became concealed extrasystoles. The other alternate sinus impulses appear to have been blocked within the reentrant pathway but after concealed conduction. It is suggested that such concealed conduction prolonged the conduction time of the next sinus impulse in the reentrant pathway and favored reappearance of the extrasystole. On the contrary, observations in one patient showing the even-number variant suggest that the first sinus impulse after a manifest extrasystole and its subsequent alternate sinus impulses were blocked in the reentrant pathway without appreciable concealed conduction.