Pharmacology

Phenylephrine

Phenylephrine is a selective α1 receptor agonist used in various settings for its vasoconstrictive properties. Whether this is as a decongestant (constricting vessels in the nasal/sinus mucosa) or a mydriatic to perform a fundoscopic exam, this medication’s selectivity gives me an idea of the expected response as well as potential side effects.

I most commonly use phenylephrine in the ICU and OR to treat acute hypotension caused by vasodilation from hypnotics, halogenated volatile anesthetics, or acute blood loss in tandem with ongoing resuscitation. I’m comfortable using a peripheral IV to bolus 40-120 mcg over a short period of time, but in the rare event of tissue infiltration, phenylephrine’s alpha agonist effects can be antagonized with medications like phentolamine administered subcutaneously. If the need for long term afterload enhancement arises, I almost always switch to a norepinephrine or vasopressin drip.

Acute increases in systemic vascular resistance (SVR) will activate baroreceptors (ie, the carotid sinuses) which, in turn, increase vagal tone to the heart. Since phenylephrine does not have any β-agonist properties, the unopposed increase in vagal tone is the mechanism by which the patient’s heart rate decreases in response to an acutely elevated SVR.

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