Background: Hemodynamic responses to vasopressors used during spinal anesthesia for elective Cesarean delivery, have not been well described. This study compared the effects of bolus phenylephrine and ephedrine on maternal cardiac output (CO). The hypothesis was that phenylephrine, but not ephedrine, decreases CO when administered in response to hypotension during spinal anesthesia.
Methods: Forty-three patients were randomized to receive 80 microg of phenylephrine or 10 mg of ephedrine. Both pulse wave form analysis and transthoracic bioimpedance changes were used to estimate stroke volume in each patient. Hemodynamic responses to spinal anesthesia and oxytocin were also recorded. A subgroup of 20 patients was randomized to receive oxytocin compared with oxytocin plus 80 microg of phenylephrine after delivery.
Results: Mean CO and maximum absolute response in CO were significantly lower during the 150 s after phenylephrine administration than after ephedrine (6.2 vs. 8.1 l/min, P = 0.001, and 5.2 vs. 9.0 l/min, P < 0.0001, respectively for pulse wave form analysis, and 5.2 vs. 6.3 l/min, P = 0.01 and 4.5 vs. 6.7 l/min, P = 0.0001, respectively for bioimpedance changes). CO changes correlated with heart rate changes. Coadministration of phenylephrine obtunded oxytocin-induced decreases in systemic vascular resistance and increases in heart rate and CO. Trends in CO change were similar using either monitor.
Conclusions: Bolus phenylephrine reduced maternal CO, and decreased CO when compared with ephedrine during elective spinal anesthesia for Cesarean delivery. CO changes correlated with heart rate changes after vasopressor administration, emphasizing the importance of heart rate as a surrogate indicator of CO. Coadministered phenylephrine obtunded hemodynamic responses to oxytocin.