To the Editor:
I read with great interest the article by Ngan Kee et al .1It makes a significant contribution to our understanding of the fetal effects of ephedrine during spinal anesthesia for cesarean section. Placental transfer was found to be considerably greater for ephedrine than for phenylephrine, as evidenced by a markedly higher umbilical vein to maternal artery ratio with ephedrine. Interestingly, the umbilical vein to maternal artery ratio was greater than unity for ephedrine, which the authors suggest may have been caused by ion trapping. Could another factor have contributed to this (and to the magnitude of the difference between the groups)? The samples were taken at one point in time (delivery) during a dynamic situation. Ephedrine has a slower onset and a longer duration of action than phenylephrine. During spinal anesthesia for cesarean section, we have observed that ephedrine requirements decrease more rapidly over time than phenylephrine requirements.2During the second 15 min after spinal anesthesia, we observed ephedrine requirements to be 26% of those in the first 15 min compared with 79% for phenylephrine. If maternal ephedrine concentration was decreasing before delivery, this may have decreased, or even caused a reversal in, the maternal/fetal concentration gradient for ephedrine at the time of delivery.
James Cook University Hospital, Middlesbrough, Cleveland, United Kingdom. firstname.lastname@example.org