A His bundle extrasystole can both induce and reverse 2:1 atrioventricular block.

An 84 year-old man with history of recurrent dizziness presented with first degree atrio-ventricular block (1° AVB) and periods of 2:1 AVB. An electrophysiological study revealed a predominant 1:1 AV conduction with markedly prolonged AH interval and frequent His bundle extrasystoles (H). A properly timed H could induce periods of 2:1 AV nodal block and 1:1 AV conduction could only resume following another properly timed H. Procainamide suppressed H. However, because of persistence of the patient symptoms, a permanent pacemaker was eventually inserted. The case illustrates a hitherto not described manifestation of H.