Pronounced effect of procainamide on clockwise right atrial isthmus conduction compared with counterclockwise conduction: possible mechanism of the greater incidence of common atrial flutter during antiarrhythmic therapy.

INTRODUCTION

It has been shown that the induction rates of common and reversed common atrial flutter are comparable during baseline control study, whereas the rate is significantly greater for common flutter than reversed common flutter during administration of antiarrhythmic agents. The mechanism of this discrepancy is not known.

METHODS AND RESULTS

The study consisted of 15 patients (group 1) with clinically documented common atrial flutter either with (n = 10) or without (n = 5) Class I antiarrhythmic therapy, and 15 patients with paroxysmal supraventricular tachycardia (group 2). Bidirectional conduction velocity and minimal pacing cycle length of 1:1 conduction during incremental pacing from both the low lateral right atrium and coronary sinus were assessed. The response of these variables to procainamide was analyzed in correlation with the induction rate of each type of flutter during the pacing protocol. Conduction velocity in the clockwise (CW) direction was significantly slower for all pacing cycle lengths than conduction velocity in the counterclockwise (CCW) direction in group 1 but was similar in group 2. Minimal pacing cycle length of 1:1 conduction did not differ between CW and CCW conduction in either group. However, in group 1, minimal pacing cycle length of 1:1 conduction of CW conduction was prolonged to a greater degree after procainamide than that of CCW conduction. There also was a significant increase in the induction rate of common flutter. This preferential effect of procainamide on CW conduction was not observed in group 2.

CONCLUSION

CW conduction over the isthmus is preferentially influenced by procainamide compared with CCW conduction, which may explain the greater incidence and induction probability of common flutter during antiarrhythmic therapy.