Amiodarone hydrochloride is a relatively new antiarrhythmic agent, the properties of which differ in a significant manner electrophysiologically, pharmacokinetically and structurally from those of conventional as well as other investigational antidysrhythmic compounds. It is also pharmacologically unique in so far as its fundamental action on cardiac muscle following chronic
therapy differs markedly from that found during the
intravenous administration; its I.V. action is dominated by the lengthening of intranodal (AV) conduction time and the effective refractory period of the AV node, the electrophysiologic basis for which is unclear but accounts for the slowing of the ventricular response in
atrial flutter and fibrillation and the variable conversion rate of narrow QRS reentrant
paroxysmal supraventricular tachycardia. Intravenous
amiodarone is ineffective in most other arrhythmias; it does not lengthen repolarization, nor does it prolong the effective refractory period of atria, ventricle, His-Purkinje system or the accessory pathways of the heart in the
WPW syndrome. In contrast, chronically administered
amiodarone lengthens repolarization and the effective refractory period of all cardiac tissues as a function of dose and
duration of therapy consistent with its wide spectrum of antiarrhythmic activity in the prophylactic control of supraventricular and
ventricular tachyarrhythmias. The nature of the slow onset of action of the oral
drug is not well-understood; it may be due to the slow formation of active metabolites or the gradual and selective inhibition of T3 action on the myocardium since the effects of
amiodarone on cardiac repolarization are identical to those of
hypothyroidism and are negated by the concomitant administration of
thyroxine. Serum reverse T3 levels increase as a function of dose and duration of
amiodarone therapy and tentative data indicate that serial measurements of rT3 levels may provide a reliable index for gauging efficacy and toxicity of
amiodarone during chronic
therapy. The role of serum
drug and metabolite levels appears less reliable in this regard. The exceedingly long and variable elimination half-life of
amiodarone necessitates individualized loading and maintenance dosage regimens, and the latency of onset of antiarrhythmic action during oral
therapy is not shortened by intravenous bolus
injections or sustained infusions. However, the judicious choice of oral dosage as discussed herein permits the development of an effective prophylactic regimen for most patients with supraventricular and
ventricular tachyarrhythmias; when the lowest dosage regimen to control a particular
arrhythmia is identified, limiting side