The effect of amiodarone on defibrillation threshold

Reports in the literature suggest that amiodarone may increase the defibrillation threshold (DFT) in humans, though this is somewhat controversial. It has been anecdotally observed that this effect might become more marked the longer ventricular fibrillation (VF) persists. To substantiate this, 9 mongrel dogs were fed amiodarone, 400 mg/day for 1, 2, or 3 weeks, and 3 dogs on no drug served as controls. Using alternating current, repeated episodes of VF were induced and allowed to persist for periods of 3, 15, 30, and 50 sec. To determine DFT, serial defibrillations were performed through implanted intrapericardial patches using an external cardioverter-defibrillator delivering 1 to 40 J. Results showed that dogs fed amiodarone exhibited progressively increasing energy requirements for defibrillation over the 3 to 50 sec interval. Using linear regression analysis, amiodarone-treated dogs had increased DFT compared to the control (p < 0.005). Paired DFT analysis yielded significant increases at 50-sec VF duration in the 2-week (p < 0.005) and 3-week (p < 0.01) treated animals compared to control. We conclude that amiodarone increases DFT and that the effect is more marked the longer the VF persists.