Clinical and electrophysiologic studies in two patients with chronic persistent atrial tachycardia unresponsive to drug or electroconversion therapy are reported. In both, exercise increased the atrial rate and decreased the atrioventricular nodal conduction time. The opposite effect occurred after f3-adrenergic blockade. Single electrically-induced atrial depolarizations (A2) at progressively premature times resulted in a linear increase in the return cycle (A2A3) until plateau levels were achieved but failed to produce sinus rhythm. Abrupt cessation of continuous atrial pacing at rates up to 600 beats/min in one patient and up to 300 beats/min in the other resulted in a pause that was similar to the plateau levels of A2A3 achieved during atrial scanning with single impulses. Our findings suggest that an ectopic atrial focus is responsible for this arrhythmia. The electrophysiologic properties of the ectopic pacemaker appear to be similar to the sinoatrial node in some respects but to differ in terms of spontaneous rate and in relative immunity from the effects of overdrive pacing.

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