Electrophysiologic Effects of Verapamil in Children

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Abstract
The electrophysiologic effects of verapamil, a slow channel blocker, were studied during diagnostic cardiac catheterization in 24 children premedicated with lytic cocktail [meperidine, promothazine and chloropromazine]. The ages ranged from 50 days-12 yr. The children had congenital (20) and rheumatic heart disease (4). Surface ECG, high intra-atrial and His bundle electrograms were obtained in all before and 5 min after a single dose of verapamil (0.15 mg/kg, max [maximum] 5 mg i.v.). In 14 cases complete electrophysiologic studies were performed using the atrial pacing and extrastimulus technique. Due to variability of the resting heart rates and the effect of cycle length on refractory periods each patient was paced with identical S1-S1 interval before and after verapamil, allowing each case to serve as his own control. Verapamil prolonged the corrected AH [atrial-His] interval in all (mean .+-. SD; from 116 .+-. 37 to 152 .+-. 41 ms, P < 0.01) and shortened the HV [Hisial-ventricular] interval in 15/24 (mean .+-. SD: from 55 .+-. 13 to 47 .+-. 9.9 ms, P < 0.05). The effective and functional refractory periods of the total conduction system, the AV [atrioventricular] node (ERPAVN, effective refractory period in the AV node) and atrium (ERPA, effective refractory period in the atrium) increased significantly in 10/14. The most profound effect was on ERPAVN and (ERPA 25.54 .+-. 29 and 19.27 21.81% mean percent increase .+-. SD, respectively, P < 0.01 and P < 0.02). Verapamil apparently prolongs the effective and functional refractory periods of the cardiac conduction system with maximal effects on the AV node, which may be the mechanism of its effectiveness in the treatment of reentrant supraventricular arrhythmias.