Class III antiarrhythmic drugs

Class III antiarrhythmic drugs

Sotalol

Sotalol possesses both nonselective beta-blocking (class II) activity and class III antiarrhythmic action. Its adverse effects are those of a beta blocker , but it also has the potential to cause torsades de pointes, which is generally associated with high blood levels of sotalol. Nevertheless, usually the dosage is adjusted according to effects on heart rate. Sotalol can enhance proarrhythmic effects of other drugs that also block outward potassium currents, such as amiodarone, tricyclic antidepressants, phenothiazines, and macrolide antibiotics (in particular erythromycin).

Amiodarone

Amiodarone is classified as a class III drug, but it has a number of other actions that can contribute to both its antiarrhythmic and proarrhythmic potential. It is a noncompetitive antiadrenergic (beta-blocking) drug and demonstrates some degree of sodium channel blocking (class I) and calcium channel blocking (class IV) activity. The complicated pharmacokinetic profile, and adverse effects, of amiodarone limit its clinical use. It has variable oral bioavailability (20% to 80%) and the terminal elimination half-life is usually about 40 days, but it can exceed 100 days due to extensive tissue distribution. Parenteral administration requires the use of glucose 5% injection; it is incompatible with normal saline. Infusion must be via a large or central vein.

Adverse effects: Many of the adverse effects of amiodarone appear to be dose-related, but plasma concentration monitoring is of limited benefit. Adverse effects are slow to resolve when therapy is ceased because of the very long half-life. Adverse effects include hyperthyroidism or hypothyroidism, pulmonary alveolitis or fibrosis, benign corneal microdeposits, photosensitivity, skin pigmentation changes, nausea, peripheral neuropathy, elevation of hepatic transaminases, and central nervous system effects including nightmares and vivid dreams. Patients should wear sunscreen and sun-protective clothing while taking amiodarone. As amiodarone affects thyroxine metabolism, when requesting thyroid functions tests inform the laboratory that the patient is receiving amiodarone. There are a number of important drug interactions associated with amiodarone. In particular it may elevate digoxin levels and potentiate the actions of warfarin, thyroxine and cyclosporin.