Dose-dependent electrophysiologic effects of amiodarone in the immature canine heart

The electrophysiologic effects of incremental doses of intravenous amiodarone were studied in the intact neonatal canine heart and were compared with the responses observed in the adult. Seven neonatal puppies aged 5 to 14 days, and 6 adult dogs were studied. Assessment of sinus and atrioventricular (AV) nodal function and atrial and ventricular refractory periods was performed using standard His bundle recording techniques and programmed extrastimulation before and after doses of 2.5, 5 and 10 mg/kg of intravenous amiodarone. Amiodarone depressed sinus node cycle length, sinus node recovery time and AV nodal conduction in both groups. Atrial and ventricular refractory periods were also prolonged in a dose-dependent fashion in both the neonatal and adult dogs. Although similar responses to amiodarone were observed in both groups, the immature dogs were more sensitive to amiodarone in prolongation of atrial refractory periods and depression of sinus node recovery time. The neonatal group, however, demonstrated more resistance to amiodarone-induced depression of AV nodal conduction. Thus, intravenous amiodarone produces dose-dependent electrophysiologic changes in the neonate similar to those in the adult, although the significant differences in drug sensitivity may be clinically important.