The Effect of Intravenous Amiodarone on Heart Rate in Patients with Acute Myocardial Infarction or Ischemia and Sinus Tachycardia: Discussion

Effect of Amiodarone on Sinus Node Function
It has been widely shown that the administration of oral amiodarone increases sinus cycle length and usually prolongs the sinus node recovery time and the sinoatrial conduction time. Moreover, the majority of published series on oral amiodarone have reported among its side effects the development of severe sinus bradycardia, sinus pauses, or sinoatrial block requiring the discontinuation of the medication or the need of permanent pacing. The effect of IV amiodarone on heart rate is less clear. Several studies have shown little or no effect of IV amiodarone on sinus cycle length. Most of these patients had heart rates within the normal range. However, similar studies have demonstrated a significant reduction m heart rates following the administration of amiodarone. Source

Experimental work on dogs and rabbits has shown a significant negative chronotropic effect of IV amiodarone. Gloor et al injected amiodarone IV selectively into the sinus node artery. The heart rate slowed 3.4 to 33.7 beats/min with increasing doses of amiodarone (2.5 μg/ml to 50 μg/ml). In addition, the administration of amiodarone following autonomic blockade slowed the heart rate from 112 ± 3 to 77 ± 19 beats/min, suggesting that autonomic blockade had little or no influence on the negative chronotropic effect of amiodarone. Pickoff et al gave IV amiodarone to canine hearts and showed significant dose-dependent increase in sinus cycle length and corrected sinus node recovery time. Goupil and Lenfant, studying spontaneously beating isolated right atria of rabbits, showed that amiodarone acts on sinus node activity by increasing the action potential duration of the sinoatrial node cells and by decreasing the slope of diastolic depolarization. In the same study, amiodarone reduced the adrenergic effects on the sinus node activity, in contrast to (3-blocking agents that abolished the adrenergic effects.