Effect of i.v. phenylephrine or ephedrine on the ED50 of intrathecal bupivacaine with fentanyl for Caesarean section

Background. Prophylactic infusion of phenylephrine to prevent hypotension at Caesarean section has been shown to decrease the rostral spread of intrathecal plain levobupivacaine and intrathecal hyperbaric bupivacaine by a median of two dermatomes compared with ephedrine. The aim of this study was to determine the median effective dose (ED50) of intrathecal bupivacaine required to achieve a block to touch at the xiphisternum in patients undergoing Caesarean section when phenylephrine or ephedrine are used to prevent hypotension. Methods. Seventy women were randomized in two groups to receive either phenylephrine at a rate of 16.6 mg min 21 (concentration 1m gm l 21 ) or ephedrine at a rate of 1.5 mg min 21 (concentration 90 m gm l 21 ). Patients received varying doses of hyperbaric bupivacaine with fentanyl 25 mg using a double-blinded, up-down sequential allocation design. Effective doses were defined as anaesthesia to touch with ethyl chloride spray to the xiphisternum within 20 min. Results. The ED50 estimates of bupivacaine were similar in the two groups: 7.8 mg [95% confidence interval (CI) 6.7‐8.9] with phenylephrine and 7.6 mg (95% CI 6.8‐8.4) with ephedrine. Systolic blood pressure control was similar (P¼0.18) with vasopressors but heart rate was higher with ephedrine (P¼0.0014). Conclusions. Under the conditions of this study, we have shown that when phenylephrine or ephedrine were used to prevent post-spinal hypotension, the dosing requirement of hyperbaric bupivacaine was similar for intrathecal anaesthesia. Br J Anaesth 2009; 102: 806‐11