Circus movement atrial flutter in the canine sterile pericarditis model. Differential effects of procainamide on the components of the reentrant pathway. To evaluate the mechanisms of action of procainamide on the components of the reentrant pathway, drug-induced changes in activation patterns, effective refractory periods (ERPs), and stimulation thresholds were analyzed in nine dogs with sterile pericarditis and sustained atrial flutter. Activation maps were based on 127 close bipolar recordings from a special "jacket" electrode. From the control map, 22 +/- 2 sites covering the slow zone and the normal zone of the reentrant circuit were selected to measure ERPs and thresholds. The excitable gap was estimated from the longest ERP during pacing at the tachycardia cycle length. During atrial flutter, epicardial activation proceeded as a single wave around an arc of functional conduction block in the proximity of the atrioventricular (AV) ring or around a combined functional/anatomic obstacle, with the arc being contiguous with one of the venae cavae. An area of slow conduction, which accounted for 53 +/- 15% of the revolution time within 35 +/- 15% of the total length of the reentrant pathway, was bordered by the arc of block and the AV ring or a caval vein and the AV ring, respectively. Procainamide (5-10 mg/kg i.v.) prolonged the cycle length of atrial flutter from 144 +/- 17 to 190 +/- 24 msec (p less than 0.05) and then terminated the arrhythmia in all studies. The increase in cycle length was due to an increase in conduction time in the slow zone by 37 +/- 11 msec (86 +/- 17% of the total cycle length increase). During the last reentrant beat, conduction failed in the slow zone, with the arc of block joining the AV ring. At termination, procainamide had prolonged conduction time, stimulation threshold, and ERP in the normal zone by 11 +/- 18%, 40 +/- 80%, and 5 +/- 15%, respectively, compared with 51 +/- 16%, 86 +/- 93%, and 14 +/- 21%, respectively, in the slow zone (p less than 0.05 for all three parameters). The duration of the excitable gap did not change significantly. We conclude that procainamide preferentially affected the slow zone of single loop reentrant circuits. The drug terminated circus movement atrial flutter without abolishing the excitable gap, and its effect on conduction seemed the major determinant of the antiarrhythmic action. 