Specific Antiarrhythmic Agents

SPECIFIC ANTIARRHYTHMIC AGENTS

The most widely used scheme for the classification of antiarrhyth-mic drug actions recognizes four classes:

1. Class 1 action is sodium channel blockade. Subclasses of this action reflect effects on the action potential duration (APD) and the kinetics of sodium channel blockade. Drugs with class 1A action prolong the APD and dissociate from the channel with intermediate kinetics; drugs with class 1B action shorten the APD in some tissues of the heart and dissociate from the chan-nel with rapid kinetics; and drugs with class 1C action have minimal effects on the APD and dissociate from the channel with slow kinetics.

2. Class 2 action is sympatholytic. Drugs with this action reduce β-adrenergic activity in the heart.

3. Class 3 action manifests as prolongation of the APD. Most drugs with this action block the rapid component of the delayed rectifier potassium current, I_Kr.

4. Class 4 action is blockade of the cardiac calcium current. This action slows conduction in regions where the action potential upstroke is calcium dependent, eg, the SA and AV nodes.

A given drug may have multiple classes of action as indicated by its membrane and electrocardiographic (ECG) effects (Tables 14–2 and 14–3). For example, amiodarone shares all four classes of action. Drugs are usually discussed according to the predomi-nant class of action. Certain antiarrhythmic agents, eg, adenosine and magnesium, do not fit readily into this scheme and are described separately.