BackgroundIt was reported that epidural volume extension could decrease the ED50 of intrathecal plain bupivacaine. In this study, we investigated the ED50 of intrathecal hyperbaric ropivacaine followed by epidural normal saline bolus for cesarean section.MethodsSixty parturients were allocated into two groups in this prospective study. About 10 mL of epidural normal saline was given after the intrathecal dose of hyperbaric ropivacaine in the Group S (normal saline group), and no epidural injection of normal saline was given after the intrathecal ropivacainve injection in the Group C (control group). The dose of intrathecal ropivacaine for each parturient was decided by up-down allocation method. The initial dose was set as 10 mg. Effective anesthesia was defined as the level of T6 or above achieved within 10 minutes after intrathecal injection and no additional epidural drug to complete operation. The Massey formula was applied to calculate the ED50 of intrathecal ropivacaine.ResultsThe ED50 of intrathecal ropivacaine for cesarean section determined by up-and-down method was 7.51 mg (95% CI, 7.09–7.93 mg) in the Group S and 8.29 mg (95% CI, 7.73–8.85 mg) in the Group C, and there was a significant difference in ED50 of ropivacaine between the two groups (P<0.05). Compared with the Group C, the ED50 of intrathecal ropivacaine decreased when followed by epidural normal saline bolus.ConclusionThe ED50 of intrathecal hyperbaric ropivacaine for cesarean section is 8.29 mg, and it is reduced when followed by epidural normal saline bolus (www.chictr.org.cn, registration number: ChiCTR-ROC-17013382).