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In the spontaneously beating isolated right atrium of the rabbit, premature beats were elicited by electrical stimulation. When the premature beat was elicited early in the atrial cycle, the postextrasystolic pause had a short duration and the sum of the pre- and postextrasystolic pause was about the same as the duration of a normal spontaneous interval. We have tried to demonstrate, using simultaneous multiple microelectrode impalements of SA node fibers, that by such an early atrial premature beat the atrium could be activated by a reentrant mechanism. The results of our experiments led us to the conclusion that the impulse of the premature beat, elicited early in the atrial cycle, discharges the SA node only fractionally and that the fibers in the neighborhood of the activated area are influenced electrotonically. This causes a change of both the site and the moment of the spontaneous impulse formation. The SA node discharges spontaneously after such an early premature beat. Reentry activation is likely to occur when a series of atrial premature beats is observed. A supraventricular tachvcardia might be caused by repeated atrial discharges following a stimulus very shortly after the atrial refractory period.

Effect of an Early Atrial Premature Beat on Activity of the Sinoatrial Node and Atrial Rhythm in the Rabbit