Phenylephrine is a direct-acting sympathomimetic amine that functions as an alpha-1 adrenergic agonist. Its chemical structure is related to epinephrine and ephedrine and possesses potent vasoconstriction properties when given intravenously or applied directly to mucosal membranes. The overall effect of intravenous phenylephrine on cardiac output and end perfusion is likely more complex and variable based on bolus versus infusion dosing, volume status, baseline heart rate, autonomic tone, and cardiac pathology. These variations are attributable to the degree of venoconstriction which can temporarily increase preload, arterial constriction which will increase systemic vascular resistance and afterload, and the presence of reflex bradycardia leading to an overall mixed effect on cardiac output depending on the patient population.
