Class III antiarrhythmics
antiarrhythmics are used to
treat ventricular arrhyth-mias. The drugs in this class are amiodarone,
dofetilide, ibutilide, and sotalol.
Sotalol is a nonselective beta-adrenergic blocker
(class II drug) that also has class III properties. Nonselective means that the
drug doesn’t have a specific affinity for a receptor. Although sotalol is a
class II drug, its class III antiarrhythmic effects are more predomi-nant,
especially at higher doses. Therefore, it’s usually listed as a class III
The absorption of these antiarrhythmics varies
After oral administration, amiodarone is absorbed
slowly at wide-ly varying rates. The drug is distributed extensively and
accumu-lates in many sites, especially in organs with a rich blood supply and
fatty tissue. It’s highly protein-bound in plasma, mainly to al-bumin.
Dofetilide is very well absorbed from the GI tract,
with almost 100% overall absorption. Of that, about 70% is bound to plasma
proteins. Ibutilide, which is administered only by I.V., has an ab-sorption of
100%. Sotalol’s absorption is slow and varies between 60% and 100%, with
Although the exact mechanism of action isn’t known,
class III an-tiarrhythmics are thought to suppress arrhythmias by converting a
unidirectional block to a bidirectional block. Class III antiarrhyth-mics have
little or no effect on depolarization. Rather, these drugs slow repolarization,
prolonging the refractory period and duration of the action potential.
Class III antiarrhythmics are used for
life-threatening arrhythmias. Amiodarone is the first-line drug of choice for
ventricular tachy-cardia and ventricular fibrillation.
§ Amiodarone increases phenytoin, procainamide,
and quinidine levels.
§ Amiodarone also increases the risk of digoxin
§ Ibutilide shouldn’t be administered within 4
hours of class I or other class III antiarrhythmics because it increases the
potential for a prolonged refractory period.
§ Dofetilide shouldn’t be administered with
cimetidine, ketocona-zole, megestrol, prochlorperazine, sulfamethoxazole,
trimetho-prim, or verapamil because of their potential to induce
§ Sotalol shouldn’t be administered with
dolasetron or droperidol because of the increased risk of life-threatening
§ Concomitant use of amiodarone and
fluoroquinolones, macrolide antibiotics, and azole antifungals may cause
prolonga-tion of the QTc interval, leading to cardiac arrhythmias, including
torsades de pointes.
Severe hypotension may develop from too-rapid I.V.
administra-tion of amiodarone. (See Adverse
reactions to class III antiar-rhythmics.)