IntroductionThe antihistaminic antazoline (ANT) was reported to be highly effective and safe for rapid conversion of atrial fibrillation (AF). We therefore analyzed underlying mechanisms in an experimental whole-heart model. Methods and ResultsIsolated and retrogradely perfused rabbit hearts underwent a standardized protocol employing atrial burst pacing-induced AF in five of 20 hearts under baseline conditions (seven episodes). Thereafter, a combination of acetylcholine and isoproterenol was employed to enhance AF occurrence. Two monophasic action potential recordings on the left- and two on the right atrial epicardium showed a decrease in atrial action potential duration (aAPD, -25msec, P<.05) and atrial effective refractory period (aERP; -52msec, P<.01) after infusion of acetylcholine (1mol/L) and isoproterenol (1mol/L). This led to induction of AF in 14 of 20 hearts (145 episodes). Simultaneous infusion of ANT (20mol/L) led to a complete suppression of AF in all inducible hearts. Treatment with ANT also led to a significant increase in aAPD (+41msec, P<.01) and aERP (+74msec, P<.05), leading to a marked increase in atrial postrepolarization refractoriness (aPRR, +33msec, P<.01). Results were compared to 13 rabbits treated with flecainide. Flecainide induced a significant increase in aPRR and resulted in induction of AF in seven of 13 hearts (51 episodes) while 11 of 13 hearts were inducible with acetylcholine and isoproterenol (93 episodes). ConclusionAdministration of ANT was highly effective in suppressing AF. The antiarrhythmic effect could be explained by a significant increase in postrepolarization refractoriness as a result of a more marked increase in aERP as compared with aAPD.