Class IB antiarrhythmics
Class IB antiarrhythmics are used for treating acute ventricular ar-rhythmias. They include:
Mexiletine is absorbed from the GI tract after oral administration. Lidocaine is administered I.V. to prevent rapid metabolism by the liver after it enters the hepatic portal circulation.
Lidocaine is distributed widely throughout the body, including the brain. Lidocaine and mexiletine are moderately bound to plasma proteins. (Remember, only that portion of a drug that’s unbound can produce a response.)
Class IB antiarrhythmics are metabolized in the liver and ex-creted in urine. Mexiletine also appears in breast milk.
Class IB drugs work by blocking the rapid influx of sodium ions during the depolarization phase of the heart’s depolarization-repolarization cycle. This decreases the refractory period, which reduces the risk of arrhythmia. (See How lidocaine works.)
Because class IB antiarrhythmics especial-ly affect the Purkinje fibers (fibers in the conducting system of the heart) and my-ocardial cells in the ventricles, they’re used to treat only ventricular arrhythmias.
Class IB antiarrhythmics are used to treat ventricular ectopic beats, ventricular tachycardia, and ventricular fibrillation.
Class IB antiarrhythmics are usually the drug of choice in acute care because they don’t produce immediate serious adverse reactions.
Class IB antiarrhythmics may exhibit additive or antagonistic ef-fects when administered with other antiarrhythmics, such as phenytoin, propranolol, procainamide, and quinidine. Other drug interactions include the following:
Rifampin may reduce the effects of mexiletine
Theophylline levels increase when given with mexiletine.
Use of a beta-adrenergic blocker or disopyramide with mexile-tine may reduce the contractility of the heart. (See Adverse reac-tions to class IB antiarrhythmics.)