Abstract

Vasopressors used in obstetrics, as adjuvants to prevent or treat hypotension secondary to spinal or epidural anesthesia, not only maintain or increase arterial blood pressure but may also increase uterine vascular and myometrial tone, thereby increasing uterine vascular resistance, which would reduce uterine blood flow. However, some vasopressors increase uterine perfusion pressure more than they increase uterine vascular resistance; thus blood flow would be maintained at normal levels.

Most pressor drugs are sympathomimetic amines, and their site and form of action depend on whether alpha- or beta-adrenergic receptors are stimulated. The pregnant human myometrium has both: stimulation of the alpha receptors increases uterine activity; beta-receptor stimulation decreases uterine activity. Vasopressor drugs that are solely or predominantly alpha-receptor activators (methoxamine, phenyleprine hydrochloride, levarterenol bitartrate) produce uterine artery vasoconstriction with increased muscle tone and tetanic contractions. Therefore, it would appear more prudent to use ephedrine, mephentermine, or dilute metaramine bitartrate, which produce their effects primarily by stimulating the heart while causing only mild vasoconstriction and, to date, have not been shown to have adverse effects in obstetrics.