Pharmacologic alterations in human type I atrial flutter cycle length and monophasic action potential duration. Evidence of a fully excitable gap in the reentrant circuit.

OBJECTIVES This study compared the effect of changes in action potential duration versus conduction velocity on atrial flutter cycle length to determine whether there is a fully or partially excitable gap in atrial flutter. BACKGROUND In an excitable gap reentrant circuit, cycle length is proportional to conduction velocity. Action potential duration is not a direct determinant of cycle length when the gap is fully excitable. METHODS Right atrial monophasic action potentials were recorded from 41 patients during type I atrial flutter before and during pharmacologic interventions. RESULTS Adenosine (17 +/- 3 mg [mean +/- SD]) shortened (p < 0.001) action potential duration but did not change cycle length. Edrophonium (10 mg) had no significant effect on action potential duration or cycle length. Isoproterenol (0.03 microgram/kg body weight per min) shortened (p < 0.05) and procainamide (15 mg/kg, then 2 mg/min) prolonged (p < 0.001) action potential duration and cycle length. Alterations in cycle length were not correlated with changes in action potential duration. Procainamide's prolongation of action potential duration was reversed by adenosine without affecting cycle length. Procainamide's prolongation of action potential duration and cycle length was partially reversed by isoproterenol. Adenosine's and isoproterenol's shortening of action potential duration and isoproterenol's shortening of cycle length were enhanced by procainamide. CONCLUSIONS Atrial flutter cycle length is determined primarily by conduction velocity and does not depend directly on action potential duration. Atrial flutter has a fully excitable gap, and procainamide does not convert the gap from full to partial excitability. Adenosine and isoproterenol interact with procainamide such that their effects are enhanced and procainamide's effects are diminished.