Amiodarone has an extremely complex and incompletely understood spectrum of actions. It blocks several K+ channels. It blocks both Na+ and Ca++ channels (class I and IV effects) and is a noncompetitive β receptor antagonist (class II effect).
The electrophysiologic action of class III antiarrhythmics includes the blockade of one or more of the three types of potassium channels involved in phase 3 of the action potential of myocardial cells.
The myocardial pacemaker cell's sodium channels are inactivated. Calcium influx is the initiator of the depolarization that results in a slow, more prolonged action potential.