Effect of intravenous vasopressor on spread of spinal anaesthesia and fetal acid-base equilibrium

We previously found rostral spread of spinal plain levobupivacaine to be less with prophylactic i.v. phenylephrine than with ephedrine during Caesarean delivery. This study investigated whether rostral spread of spinal hyperbaric bupivacaine is also less with phenylephrine than with ephedrine.

The study was randomized and double blind. It compared phenylephrine 100 microg ml-1 (phenylephrine group, n=27), and ephedrine 4.5 mg ml-1 (ephedrine group, n=27), given by infusion during spinal anaesthesia for Caesarean delivery. Block height was assessed to cold and light touch sensation at 15, 30, 60, and 90-min after the spinal injection of 2.8 ml of hyperbaric 0.5% w/v bupivacaine, combined with 0.4 ml diamorphine (1 mg ml-1). Umbilical blood gas values were monitored during the study.

Block height was similar for both groups at all of the assessment times. Umbilical artery pH was higher with phenylephrine [median 7.32 (IQR 7.28-7.34)] than with ephedrine [7.20 (7.10-7.28)] (P<0.0001). There was a strong negative correlation between umbilical artery pH and spinal-delivery interval, but only with ephedrine: phenylephrine group, r2=0.09 (P=0.17), and ephedrine group, r2=0.53 (P<0.0001). Five-minute Apgar scores were higher with phenylephrine [10 (9-10)] than ephedrine [9 (9-9)] (P=0.009).

In contrast to its effect on spinal plain levobupivacaine, we did not find rostral spread of spinal hyperbaric bupivacaine to be less with prophylactic phenylephrine than with ephedrine. We observed an unexpectedly high incidence of fetal acidosis with ephedrine and found evidence that longer spinal-delivery intervals increase the risk of fetal acidosis developing with ephedrine, but not phenylephrine.