Activation sequence during atrial flutter in dogs with surgically induced right atrial enlargement: I. Observations during sustained rhythms.

The atrial activation sequences during 15 episodes of sustained atrial flutters were determined in the isolated hearts of four dogs with surgically induced right atrial enlargement (TI/PS dogs). These sequences were compared with the activation sequences of six episodes of nonsustained atrial tachyarrhythmias induced in three control hearts. Total endocardial activation of both atria during normal sinus rhythm and during the arrhythmias was determined first by recording simultaneously from 192 pairs of recording electrodes positioned into egg-shaped electrode arrays, and then by determining the moment of activation from each of the recorded electrograms. Isochronal maps of total activation were constructed by computer. Nonsustained atrial rhythms inducible in control hearts were due to circus movement excitation either in the left atrium (two episodes) or in the right atrium (four episodes). On the other hand, all 15 episodes of sustained atrial flutter induced and mapped in the TI/PS dog hearts were due to reentrant excitation in the enlarged right atrium. The reentrant pattern could be in a clockwise or counterclockwise pattern. In these episodes of stable flutter an area of functional block was an essential component to the reentrant excitation.