III. Drugs That Prolong Repolarization

Class III drugs currently in use include amiodarone, bretylium, dofetilide, and ibutilide (see Table 23-2). These drugs all exert their primary effects by prolonging repolarization in cardiac cells. Amiodarone, however, also appears to have some properties similar to drugs in other classes, and may help control arrhythmias by inhibiting sodium channel function (class I effect), by beta blockade (class II effect), or even by blocking calcium channels (class IV effect).5 

An initial increase in cardiac arrhythmias (proarrhythmic effect) may occur when class III drugs are instituted. The most important proarrhythmia is known as torsades de pointes, which is a form of ventricular tachycardia that can be fatal.11,40 Specific class III agents are associated with various other side effects. Amiodarone, for example, is associated with pulmonary toxicity and liver damage. Other class III drugs may have a more favorable side-effect profile but may not be as effective as amiodarone in controlling arrhythmias. Side effects of class HI drugs there- 

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