The mechanism of supraventricular tachycardia induced by a single premature beat in the isolated left atrium of the rabbit. I. Circus movement as a consequence of unidirectional block of the premature impulse.

In the isolated left atrium of the rabbit tachycardia could be elicited by a single stimulus shortly after the refractory period. The activation pattern of the preparation was mapped by means of multiple extracellular recordings. It could be demonstrated that a circus movement of the activation in a relatively small part of the atrial myocardium was responsible for the tachycardia. This phenomenon was studied on a cellular level with multiple microelectrode recordings. The impulse of the premature beat, when followed by a tachycardia, was one way only, while in the other directions the conduction failed completely or occurred with such decrement that the impulse died out. These differences in conductivity were connected with spatial differences in the refractory period. Therefore the excitability of the fibers surrounding the stimulating electrod was not restored to the same level at the moment the premature beat was elicited. Spatial dispersion in the refractory period was important for the occurrence of unidirectional block and thus for the initiation of circus movement and tachycardia.