Which Vasopressor Should Be Used To Treat Hypotension during Magnesium Sulfate Infusion and Epidural Anesthesia

Ephedrine restores and/or protects uterine blood flow and fetal well-being in laboratory animals. In contrast, α1-adrenergic agonists worsen uterine blood flow and fetal condition. We previously demonstrated that magnesium sulfate (MgSO4) attenuates the detrimental effects of phenylephrine on uterine vascular resistance in gravid ewes. Therefore, we performed this study to determine whether ephedrine or phenylephrine better restores and protects uterine blood flow and fetal oxygenation during epidural anesthesia-induced hypotension in hypermagnesemic gravid ewes. Twelve chronically instrumented gravid ewes were each used for three experiments: 1) ephedrine, 2) phenylephrine, and 3) normal saline (NS)-control. For each experiment the protocol was as follows: 1) at time zero, intravenous infusion of MgSO4 was begun; 2) at 150 min a thoracic level of epidural anesthesia was achieved with 2% lidocaine; and 3) at 165 min, an intravenous infusion of ephedrine, phenylephrine, or NS was begun and continued through 195 min. Epidural anesthesia uniformly decreased maternal mean arterial blood pressure (MAP), heart rate, cardiac output, uterine blood flow, and fetal PO2 in each of the three groups. Both ephedrine and phenylephrine restored maternal MAP to baseline, as expected from the experimental design. Ephedrine significantly increased cardiac output and uterine blood flow when compared with NS-control, but phenylephrine did not. Phenylephrine significantly increased uterine vascular resistance when compared with NS-control, but ephedrine did not. As a result, fetal pH and Po2 were significantly greater during ephedrine infusion than during infusion of NS-control. Fetal pH was stable during ephedrine infusion, but it continued to decrease during phenylephrine infusion. Ephedrine, but not phenylephrine, significantly increased fetal PO2 during treatment of hypotension. We conclude that, although ephedrine and phenylephrine provided similar restoration of maternal MAP, ephedrine was superior to phenylephrine in restoring uterine blood flow during epidural anesthesia-induced hypotension in hypermagnesemic gravid ewes. Furthermore, only ephedrine was clearly superior to NS-control in maintaining fetal pH and restoring fetal Po2 during treatment of hypotension.