Phenylephrine Reduces Maternal CO During Spinal Anesthesia for C-Section Hemodynamic Effects of Ephedrine, Phenylephrine, and the Coadministration of Phenylephrine With Oxytocin During Spinal Anesthesia for Elective Cesarean Delivery

Background: Spinal anesthesia is often used for caesarean sections and there can be considerable hemodynamic changes following its use. Objective: To compare the commonly used vasopressors ephedrine and phenylephrine as well as the secondary effects of the co-administration of ephedrine and phenylephrine along with oxytocin on maternal hemodynamics. Design/Methods: This was a double-blind randomized prospective study. Forty patients were divided into 2 groups: patients in group E received ephedrine and patients in group P received phenylephrine. The need for vasopressor was defined as a 20% decline in arterial BP. Twenty patients who had not received ephedrine were randomly assigned to receive oxytocin alone or with phenylephrine. Cardiac output (CO) was measured using the BioZ LiDCO instrument. Standard monitoring was applied. Data were recorded every 5 minutes. Systemic vascular resistance (SVR) and CO were derived using the LiDCOplus monitor. All patients received spinal anesthesia for C-section along with a standard colloid preload. One mL (80 μg phenylephrine or 10 mg ephedrine) of randomly assigned vasopressor was given if mean arterial pressure (MAP) dropped below 20% every 60 seconds until it returned to baseline values. Thirty seconds after delivery, 2.5 IU of oxytocin was given alone or with 80 μg of phenylephrine. Results: The primary outcome in this study was comparing the cardiac output after ephedrine and phenylephrine in response to hypotension after spinal anesthesia. The difference in heart rate was significant, but unlike ephedrine, phenylephrine induced bradycardia. Both groups had increase in MAP, though the time to peak MAP was shorter in the phenylephrine group. The mean peak of MAP was 8% lower than baseline in Group E and 8% more in Group P. In patients receiving the mixture of oxytocin and phenylephrine, absolute Heart rate and CO were lower, though MAP was higher. Discussion: The peak changes in CO and MAP were seen earlier in Group P than in Group E. After ephedrine, a phasic response was seen, increase in afterload, followed by a fall then a sustained increase in SVR. Atropine was used in 11 of 19 cases of bradycardia induced by phenylephrine, which made exact evaluation of CO difficult. This study showed that 80 μg of phenylephrine equals 10 mg of ephedrine, and a change in HR rather than MAP was a more important marker for CO during spinal anesthesia. It also demonstrated that coadministration of phenylephrine with oxytocin did not obtund the oxytocin response. Conclusions: A low dose of phenylephrine is the better first line treatment for hypotension to restore SVR and CO to baseline compared to ephedrine. Reviewer's Comments: This is a useful and interesting study of the commonly used vasopressors, and confirms other recent studies supporting the use of phenylephrine during subarachnoid block for C-section. In addition, ephedrine has been shown to result in more neonatal acidosis. (Reviewer-Sunita Goel, MD).