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Beta-adrenergic agonists can be broadly categorized as having beta1 and beta2 receptor activity. This section describes the toxicity of beta2-selective agonists that are commonly available for oral use: albuterol (salbutamol), metaproterenol, and terbutaline (Table II–16). Clenbuterol, a potent beta2 agonist, is not approved for human use in the United States but is abused for its anabolic effects.

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Table II-16 Beta2-Selective Agonists
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  1. Mechanism of toxicity

    1. Stimulation of beta2 receptors results in relaxation of smooth muscles in the bronchi, uterus, and skeletal muscle vessels. At high toxic doses, selectivity for beta2 receptors may be lost, and beta1 effects may be seen.

    2. Pharmacokinetics. These agents are readily absorbed orally or by inhalation. Half-lives and other pharmacokinetic parameters are described in Table II–61.

  2. Toxic dose. Generally, a single ingestion of more than the total usual daily dose (see Table II–16) may be expected to produce signs and symptoms of toxicity. Pediatric ingestion of less than 1 mg/kg of albuterol is not likely to cause serious toxicity. Tonic-clonic seizures were observed 16 hours after ingestion of 4 mg/kg of albuterol in a 3-year old. A 22-year old woman developed acidosis, rhabdomyolysis, and acute renal failure following ingestion of 225 mg of terbutaline. Ingestion of 109 mcg of clenbuterol in a 31-year-old man resulted in supraventricular tachycardia and atrial fibrillation lasting 3 days. Dangerously exaggerated responses to therapeutic doses of terbutaline have been reported in pregnant women, presumably as a result of pregnancy-induced hemodynamic changes.

  3. Clinical presentation. Overdoses of these drugs affect primarily the cardiovascular system. Most overdoses, especially in children, result in only mild toxicity.

    1. Vasodilation results in reduced peripheral vascular resistance and can lead to significant hypotension. The diastolic pressure usually is reduced to a greater extent than is the systolic pressure, resulting in a wide pulse pressure and bounding pulse. Myocardial ischemia and infarction have been reported after IV administration of albuterol.

    2. Tachycardia is a common reflex response to vasodilation and may also be caused by direct stimulation of beta1 receptors as beta2 selectivity is lost in high doses. Supraventricular tachycardia or ventricular extrasystoles are reported occasionally.

    3. Agitation and skeletal muscle tremors are common. Rhabdomyolysis is possible. Seizures are rare.

    4. Metabolic effects include hypokalemia, hyperglycemia, and lactic acidosis. Delayed hypoglycemia may follow initial hyperglycemia. Hypokalemia is caused by an intracellular shift of potassium rather than true depletion.

  4. Diagnosis is based on the history of ingestion. The findings of tachycardia, hypotension with a wide pulse pressure, tremor, and hypokalemia ...

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