Effect of metoprolol on atrial fibrillatory rate, atrioventricular nodal concealed conduction, and ventricular response during atrial fibrillation in pigs.

We wished to elucidate the effect of beta-blockade on fibrillatory rate and atrioventricular (AV) nodal concealed conduction during atrial fibrillation (AF). Subsequent to determination of the effect on atrial functional refractoriness with the extrastimulus technique (basic cycle length 400 ms), the effect of metoprolol (0.3 mg/kg) on atrial fibrillatory rate was determined in 8 open-chest pigs with metacholine-facilitated AF. Once stable AF was established, fibrillatory rate was recorded with a bipolar epicardial electrode, together with the ventricular response during 500 ventricular intervals. For each episode of AF, three indexes were calculated to determine the degree of concealed conduction: the ratio of the longest to the shortest ventricular interval, the ratio of the median ventricular interval to the median atrial interval, and the coefficient of variation of the ventricular intervals. Metoprolol decreased fibrillatory rate (571-432 beats/min, p < 0.01), suggesting a proportionate increase (+32%) in atrial functional refractoriness during AF that far exceeded the increase (+7%) during sinus rhythm (217-233 ms, p < 0.05). None of the indexes of concealed conduction was affected by metoprolol. Metoprolol decreases fibrillatory rate in AF, possibly due in part to its class I effect, causing rate-dependent prolongation of atrial refractoriness. Despite reducing fibrillatory rate, metoprolol does not affect AV nodal concealed conduction measurably. Our results support the assumption that the reducing effect of beta-blockers on ventricular rate during AF is due to direct prolongation of AV nodal refractoriness.