Adverse Effects And Precautions

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The most common adverse effects of b receptor antagonists arise as pharmacological consequences of blockade of b receptors; serious adverse effects unrelated to b receptor blockade are rare.

CARDIOVASCULAR SYSTEM b receptor blockade may cause or exacerbate heart failure in patients with compensated heart failure, acute myocardial infarction, or cardiomegaly. Nonetheless, chronic administration of b receptor antagonists is efficacious in prolonging life in the therapy of heart failure in selected patients (see below and Chapter 33). The bradycardia caused by b antagonists may cause life-threatening bradyarrhythmias in patients with partial or complete AV conduction defects. Particular caution is indicated in patients who are taking other drugs, such as verapamil or various antiarrhythmic agents, which may impair sinus-node function or AV conduction.

Some patients complain of cold extremities while taking b blockers. Symptoms of peripheral vascular disease may worsen (this is uncommon), or Raynaud's phenomenon may develop.

After prolonged b blockade, there is enhanced sensitivity to b adrenergic stimulation when the b blocker is withdrawn abruptly, possibly related to upregulation of b receptors during b blockade. Thus, abrupt discontinuation of b receptor antagonists after long-term treatment can exacerbate angina and may increase the risk of sudden death. Optimal strategies for discontinuation of b blockers are not known, but it is prudent to decrease the dose gradually (over several weeks) and to restrict exercise during this period.

PULMONARY FUNCTION A major adverse effect of b receptor antagonists is the bronch-constriction resulting from blockade of b2 receptors in bronchial smooth muscle. b Blockers may cause a life-threatening increase in airway resistance in patients with bronchospastic disease. b1-selective antagonists or those with intrinsic sympathomimetic activity at b2 adrenergic receptors may be somewhat less Likely to induce bronchospasm; however, the selectivity of current b1 blockers is modest, and these drugs should be avoided if possible in patients with asthma.

CNS CNS-related adverse effects may include fatigue, sleep disturbances (including insomnia and nightmares), and depression. There is no clear correlation between the incidence of the adverse effects of b receptor antagonists and their lipophilicity.

METABOLISM b adrenergic blockade may blunt recognition of hypoglycemia and may delay recovery from insulin-induced hypoglycemia. b receptor antagonists should be used with caution in diabetic patients who are prone to hypoglycemic reactions; ^-selective agents may be preferable.

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