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  1. Pharmacology. Labetalol is a mixed alpha- and beta-adrenergic antagonist; after intravenous administration, the nonselective beta-antagonist properties are approximately sevenfold greater than the alpha1 antagonist activity. Hemodynamic effects generally include decreases in heart rate, blood pressure, and systemic vascular resistance. Atrioventricular conduction velocity may be decreased. After intravenous injection, hypotensive effects are maximal within 10–15 minutes and persist for about 2–4 hours. The drug is eliminated by hepatic metabolism and has a half-life of 5–6 hours.

  2. Indications. Labetalol may be used to treat hypertension accompanied by tachycardia associated with stimulant drug overdose (eg, cocaine or amphetamines) and clonidine withdrawal. Note: Hypertension with bradycardia suggests excessive alpha-mediated vasoconstriction (Hypertension, Pseudoephedrine, Phenylephrine, and Other Decongestants); in this case, a pure alpha blocker such as phentolamine is preferable because the reversal of beta2-mediated vasodilation may worsen hypertension. In addition, it may have an unpredictable effect on coronary vascular tone; other agents, such as nitroglycerin, may be preferable for stimulant-induced coronary vasoconstriction.

  3. Contraindications

    1. Asthma.

    2. Congestive heart failure.

    3. Atrioventricular block.

    4. Known hypersensitivity to the drug.

  4. Adverse effects

    1. Paradoxical hypertension may result when labetalol is used in the presence of stimulant intoxicants that have strong mixed alpha- and beta-adrenergic agonist properties (eg, cocaine, amphetamines) and in patients with pheochromocytoma owing to the relatively weak alpha-antagonist properties of labetalol compared with its beta-blocking ability. (Note: This has been reported with propranolol but not with labetalol.)

    2. Orthostatic hypotension and negative inotropic effects may occur.

    3. Dyspnea and bronchospasm may result, particularly in patients with asthma.

    4. Nausea, abdominal pain, diarrhea, tremors, dizziness, and lethargy have been reported.

    5. Labetalol may mask physiologic responses to hypoglycemia (tremor, tachycardia, and glycogenolysis) and, therefore, should be used with caution in patients with diabetes.

    6. Use in pregnancy. FDA Category C (indeterminate). This does not preclude its acute, short-term use for a seriously symptomatic patient (Introduction).

  5. Drug or laboratory interactions

    1. Additive blood pressure lowering with other antihypertensive agents, halothane, calcium channel antagonists, or nitroglycerin.

    2. Cimetidine increases the oral bioavailability of labetalol.

    3. Labetalol is incompatible with 5% sodium bicarbonate injection (forms a precipitate).

    4. Labetalol may cause false-positive elevation of urinary catecholamine levels and can produce a false-positive test for amphetamines on urine drug screening.

  6. Dosage and method of administration

    1. Adult. Give 20-mg slow (over 2 minutes) IV bolus initially; repeat with 40-80-mg doses at 10-minute intervals until blood pressure is controlled or a cumulative dose of 300 mg is achieved (most patients will respond to a total dose of 50–200 mg). Alternatively, administer a constant infusion of 0.5–2 mg/min (adjust rate) until blood pressure is controlled or a 300-mg cumulative dose is reached. After this, give oral labetalol starting at 100 mg twice daily.

    2. Children (off-label dosing). Initial dose of 0.2–1 mg/kg is given intravenously over 2 minutes (maximum dose, 40 mg). May repeat every 10 minutes as needed.

  7. Formulations

    1. Parenteral. Labetalol hydrochloride (Normodyne, Trandate, others), 5 mg/mL, 20- and 40-mL multiple-dose vials (with EDTA and parabens as preservatives), and 4-and ...

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