Introduction

These authors compared the effects of bolus phenylephrine and ephedrine on maternal cardiac output (CO) during spinal anaesthesia for elective Caesarean delivery. The hypothesis was that phenylephrine, but not ephedrine, decreases CO when administered in response to hypotension during spinal anaesthesia.

Methods

A total of 43 patients were randomised to receive 80 µg 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. Haemodynamic responses to spinal anaesthesia and oxytocin were also recorded. A subgroup of 20 patients was randomised to receive oxytocin compared with oxytocin plus 80 µg of phenylephrine after delivery.

Results

Mean CO and maximum absolute response in CO were found to be significantly lower during the 150 seconds after phenylephrine administration than after ephedrine (6.2 versus 8.1 L/min, p=0.001, and 5.2 versus 9.0 L/min, p<0.0001, respectively, for pulse wave form analysis, and 5.2 versus 6.3 L/min, p=0.01, and 4.5 versus 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

The authors conclude that bolus phenylephrine reduces maternal CO when compared with ephedrine during elective spinal anaesthesia for Caesarean delivery. CO changes correlated with heart rate changes after vasopressor administration, highlghting the importance of heart rate as a surrogate indicator of CO. Coadministered phenylephrine obtunded haemodynamic responses to oxytocin.