Background - The effect of verapamil on long-term tachycardia-induced atrial electrical remodeling has not been reported. Methods and Results - Forty-eight dogs were randomly divided into verapamil and control groups. The dogs in the verapamil group received verapamil 120 mg every day, those in the control group did not receive verapamil. Atrial effective refractory period (AERP), inducibility of atrial fibrillation (AF), and duration of AF were assessed before and after complete atrioventricular junction ablation with 1- day, 1-week, or 6-week rapid atrial pacing (780 bpm). AERP shortening, AERP dispersion, AERP maladaptation, and inducibility of AF after 1-day pacing was significantly attenuated by verapamil. However, verapamil did not have any significant effect on these parameters in the dogs with 1-week or 6-week pacing. Verapamil did not have any significant effect on the conduction velocity in the dogs with 1-day, 1-week, or 6-week pacing. Before rapid atrial pacing, verapamil significantly prolonged the duration of AF. In the dogs with 1-day pacing, the duration of AF measured immediately after termination of pacing was similar between the control and verapamil groups. However, in the dogs with 1-week or 6-week pacing, the duration of AF after pacing was significantly longer in the verapamil group. ConClusions - verapamil cannot prevent long-term (1 and 6 weeks, respectively) tachycardia- induced changes of atrial electrophysiological properties. Furthermore, verapamil increases the duration of AF in the dogs either before or after long-term rapid atrial pacing.
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