The utility of serum drug level monitoring during therapy with class III antiarrhythmic agents.

Serum drug level monitoring facilitates dosage optimization of agents with a narrow therapeutic index. Concentration-dependent antiarrhythmic and toxic effects have been demonstrated for several class I drugs, whereas the concentration-effect relationships of class III antiarrhythmics are less well established. The efficacy and toxicity of amiodarone, in particular, may be determined more by extensive tissue deposition than by its serum concentration. Furthermore, accumulation of the active metabolite desethylamiodarone seems to play an additional role. The antiarrhythmic serum concentration range of sotalol is wide; monitoring is not required. The clinical utility of serum concentration measurements with the newly developed class III antiarrhythmic agents remains to be evaluated in properly planned therapeutic trials in patients with supraventricular and ventricular arrhythmias.