Two unusual cases of intra-atrial gap.

Two cases are described in which, during investigation of rhythm disturbances, it was shown that, by delivering atrial stimuli of increasing prematurity, a point was reached where the artificial stimulus failed to induce an atrial response. However, with even more premature stimulations, atrial responses were again observed. These observations are best explained by a gap phenomenon in which very early stimuli delivered during the effective refractory period of the atria were delayed long enough in the exit area around the pacing electrode to allow atrial recovery.