Aluminum salts, cholestyramine, and colestipol may decrease absorption of b blockers. Pheny-toin, rifampin, and phenobarbital, as well as smoking, induce hepatic biotransformation enzymes and may decrease plasma concentrations of b receptor antagonists that are metabolized extensively (e.g., propranolol). Cimetidine and hydralazine may increase bioavailability of propranolol and metoprolol by affecting hepatic blood flow. b Receptor antagonists can impair the clearance oflidocaine.
Other drug interactions have pharmacodynamic explanations. For example, b antagonists and Ca2+ channel blockers have additive effects on the cardiac conducting system. Additive effects on blood pressure between b blockers and other antihypertensive agents often are employed to clinical advantage. The antihypertensive effects of b receptor antagonists can be opposed by indomethacin and other NSAIDs (see Chapter 26).
THERAPEUTIC USES Cardiovascular Diseases b Receptor antagonists are used extensively in the treatment of hypertension (see Chapter 32), angina and acute coronary syndromes (see Chapter 31), and congestive heart failure (see Chapter 33). These drugs also are used frequently in the treatment of supraventricular and ventricular arrhythmias (see Chapter 34).
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