Atrial tachysystole (flutter?) with apparent exit block.

A case of atrial tachysystole with apparent exit block is reported in which the basic cycle length of atrial activity is 260 msec. Simultaneous right intra-atrial electrograms show cycle lengths of 510 to 780 msec which are roughly multiples of 260 msec establishing the diagnosis of exit block. Rapid atrial stimulation (1,200/min) to overdrive the rapid ectopic atrial pacemaker was only transiently successful in converting the arrhythmia to atrial fibrillation with a slower ventricular response. Two days later the rhythm converted to normal sinus rhythm (cycle length, 600 msec) with periods of sino-atrial block measuring 2,400 and 1,240 msec. Atrial rate further slowed so that the patient ultimately required a permanent demand pacemaker. The case supports the theory of repetitive discharge from an ectopic atrial pacemaker to explain the mechanism producing atrial tachysystoles that simulate atrial flutter.