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Introduction

  1. Pharmacology. Phentolamine is a competitive presynaptic and postsynaptic alpha-adrenergic receptor blocker that produces peripheral vasodilation. By acting on both venous and arterial vessels, it decreases total peripheral resistance and venous return. It also may stimulate beta-adrenergic receptors, causing cardiac stimulation. Phentolamine has a rapid onset of action (usually 2 minutes) and a short duration of effect (approximately 15-20 minutes).
  2. Indications
    1. Hypertensive crisis associated with stimulant drug overdose (eg, amphetamines, cocaine, or ephedrine). Also, an adjunct for cocaine-induced acute coronary syndrome to reverse coronary artery vasoconstriction.
    2. Hypertensive crisis resulting from interaction between monoamine oxidase inhibitors and tyramine or other sympathomimetic amines.
    3. Hypertensive crisis associated with sudden withdrawal of sympatholytic antihypertensive drugs (eg, clonidine).
    4. Extravasation of vasoconstrictive agents (eg, epinephrine, norepinephrine, and dopamine).
    5. Reversal of local anesthesia containing a vasoconstrictor.
  3. Contraindications. Use with extreme caution in patients who have intracranial hemorrhage, ischemic stroke, or severe coronary insufficiency; excessive lowering of blood pressure may aggravate injury.
  4. Adverse effects
    1. Hypotension and tachycardia may occur from excessive doses.
    2. Anginal chest pain and cardiac arrhythmias may occur.
    3. Slow intravenous infusion (≤0.3 mg/min) may result in transiently increased blood pressure caused by stimulation of beta-adrenergic receptors.
    4. Use in pregnancy. FDA Category C (Introduction). Phentolamine was used to manage pheochromocytoma during a delivery, with no adverse effects to the newborn attributable to the drug.
  5. Drug or laboratory interactions. Additive or synergistic effects may occur with other antihypertensive agents, especially other alpha-adrenergic antagonists (eg, prazosin, terazosin).
  6. Dosage and method of administration
    1. Parenteral. Give 1-5 mg IV (children: 0.02-0.1 mg/kg up to a maximum of 2.5 mg) as a bolus; repeat at 5- to 10-minute intervals as needed to lower blood pressure to a desired level (usually 90-100 mm Hg diastolic in adults and 70-80 mm Hg diastolic in children, but this may vary with the clinical situation). Dose range for adults with pheochromocytoma is up to 20-30 mg. Once hypertension is controlled, repeat every 2-4 hours as needed.
    2. Catecholamine extravasation. Infiltrate small amounts of a 0.5% (5 mg/mL) solution into the affected area with a fine (25-27-gauge) hypodermic needle; improvement is evidenced by hyperemia and return to normal temperature. Maximum dose 10 mg (children: 0.1 mg/kg). Should be administered as soon as possible, and within 12 hours of extravasation.