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In the isolated blood-perfused canine heart we produced episodes of rapid atrial flutter by continuous infusion of acetylcholine and rapid pacing. The spread of excitation during atrial flutter was mapped with the aid of two endocavitary mapping electrodes containing 960 leads and recording from 192 different sites simultaneously. The flutter maps clearly showed that intra-atrial reentry was the mechanism responsible for the arrhythmia. However, the localization and size of the intra-atrial circuits differed from case to case even in the same heart. The orifices of the venae cavae or the atrioventricular ring did not serve as a central anatomic obstacle for circus movement. We also failed to identify a special role of the internodal pathways in the formation of the loop. Instead, the intra-atrial circuits could be found everywhere, provided sufficient atrial mass was available to accommodate the circuit. The diameter of the circuits varied between 1.5 and 3 cm at a cycle length between 65 and 155 msec. The average conduction velocity of the circulating impulse varied between 60 and 80 cm/sec. Spontaneous termination of atrial flutter frequently occurred and was based on local conduction block in a narrow part of the circuit. Another interesting aspect of these studies is the finding that during continuous circus movement of the impulse, the amount of myocardium that is activated may vary considerably. This marked periodicity in excited tissue mass during atrial flutter could adequately explain the continuously undulating baseline or typical sawtoothlike F waves as seen in the surface electrocardiogram during atrial flutter.

Intra-atrial reentry as a mechanism for atrial flutter induced by acetylcholine and rapid pacing in the dog.