ED(50) of intrathecal ropivacaine for cesarean section under prophylactic infusion of phenylephrine: A consort study

BACKGROUND: Studies have reported that prophylactic continuous infusion of phenylephrine during spinal anesthesia for cesarean section can decrease the spread of local anesthetics. We investigated the ED(50) of intrathecal hyperbaric ropivacaine in parturient women undergoing cesarean section under prophylactic infusion of phenylephrine. METHODS: Sixty parturient women were allocated into 2 groups in this prospective study. Group P received 0.5 mL kg(−1) h(−1) of phenylephrine (5 mg/50 mL) at the start of intrathecal injection, and Group C (control group) received the same volume of saline. The dose of intrathecal ropivacaine for each subject was decided through up-down allocation method. The initial dose was set as 7.5 mg. Successful anesthesia was defined as the level of T6 or above achieved within 15 minutes after intrathecal injection and no additional epidural drug or venous analgesia to complete operation. The Massey formula was applied to calculate the ED(50) of intrathecal ropivacaine in both groups. RESULTS: The ED(50) of hyperbaric ropivacaine determined by up-and-down method was 7.2 mg (95% confidence interval (CI), 6.8–7.6 mg) in the Group P, and 6.8 mg (95% CI, 6.4–7.2 mg) in the Group C, there was significant difference between the 2 groups (P < .5). The ED(50) of intrathecal ropivacaine increases compared with Group C when phenylephrine is prophylactic infused to prevent spinal induced hypotension in cesarean section. CONCLUSION: The ED(50) of intrathecal hyperbaric ropivacaine is 7.2 mg when phenylephrine is prophylactic infused to prevent spinal induced hypotension in cesarean section, and more ropivacaine demands on spinal anesthesia for cesarean section (www.chictr.org.cn, registration number: ChiCTR-RIC-17011650).