The Class IA drugs block the fast sodium channel and delayed potassium channels. Therefore they slow phase 0 depolarization and phase 3 repolarization in ventricular tissue (Fig. 14-2). These actions decrease the ventricular conduction velocity and prolong the ventricular action potential duration and refractory period (Table 14-1). On the ECG, this increases the QRS duration and prolongs the QT interval. Class IA drugs suppress abnormal (ectopic) automaticity, but they usually do not significantly affect SA node automaticity and the heart rate. TABLE 14-1 Electrophysiologic Properties of Selected Antiarrhythmic Drugs*
Antiarrhythmic Drugs
Overview
Sodium Channel Blockers
Class IA Drugs
Drug Properties
ATRIOVENTRICULAR (AV) NODE
HIS-PURKINJE SYSTEM AND VENTRICLE
ELECTROCARDIOGRAM
Drug
Ectopic automaticity
Conduction velocity
Refractory period
Conduction velocity
Refractory period
Heart rate
PR interval
QRS duration
QT interval
Class I Drugs
Quinidine
↓
↑ or ↓
→ or ↑
↓↓
↑↑
↑ or ↓
↑ or ↓
↑↑
↑↑
Lidocaine
↓
→
→
→ or ↓
↑ or ↓
→
→
→
→ or ↓
Flecainide
↓
↓
→ or ↑
↓↓
↑
→
↑
↑↑
→ or ↑
Class II Drugs
Propranolol
↓
↓↓
↑↑
→
→
↓↓
↑↑
→
→ or ↓
Class III Drugs
Amiodarone
↓
↓
↑
↓
↑↑
↓
↑
↑
↑↑
Dofetilide
→
→ or ↓
↑
→
↑↑
→
→
→
↑↑
Sotalol
↓
↓
↑
→
↑↑
↓
↑
→
↑↑
Class IV Drugs
Verapamil, diltiazem
↓
↓↓
↑↑
→
→
↓↓
↑↑
→
→
Other Drugs
Adenosine
↓
↓↓
↑↑
→
→
↑
↑↑
→
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Antiarrhythmic Drugs
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