Adrenergic Agonists: Phenylephrine

PHENYLEPHRINE

Clinical Considerations

Phenylephrine is a noncatecholamine with predominantly selective α₁-agonist activity. The pri-mary effect of phenylephrine is peripheralvasoconstriction with a concomitant rise in systemic vascular resistance and arterial blood pressure. Reflex bradycardia mediated by the vagus nerve can reduce cardiac output. Phenylephrine is also used topically as a decongestant and a mydriatic agent.

Dosing & Packaging

Small intravenous boluses of 50–100 µg (0.5– 1 mcg/kg) of phenylephrine rapidly reverse reduc-tions in blood pressure caused by peripheral vasodi-lation (eg, spinal anesthesia). The duration of action is short, lasting approximately 15 min after admin-istration of a single dose. A continuous infusion (100 mcg/mL at a rate of 0.25–1 mcg/kg/min) will maintain arterial blood pressure, but at the expense of renal blood flow. Tachyphylaxis occurs with phen-ylephrine infusions requiring upward titration of the infusion. Phenylephrine must be diluted from a 1% solution (10 mg/1-mL ampule), usually to a 100 mcg/mL solution.