Background Although intrathecal ropivacaine has been widely used for caesarean delivery, there are limited data for the use of ropivacaine for prophylactic cervical cerclage. We sought to determine the median effective dose of intrathecal ropivacaine for prophylactic cervical cerclage in 50% of patients (ED50) and the calculated dose required for successful block in 95% of patients (ED95).

Methods We included Chinese women scheduled for prophylactic cervical cerclage under combined spinal-epidural (CSE) anaesthesia in the first or second trimester. A predetermined dose of intrathecal isobaric ropivacaine was administered. If this determined dose achieved an effective block at a level not lower than T12, the next dose was decreased by 0.5 mg. Otherwise, the next dose was increased by 0.5 mg. The primary outcome was the ED50 of intrathecal ropivacaine. Secondary outcomes included the calculated ED95, time from CSE to the start of surgery and so on.

Results Forty patients were included in the study, 23 (57.5%) of 40 received an effective block only with intrathecal ropivacaine, while 17 (42.5%) patients needed extra epidural lidocaine to achieve a successful block. The ED50 of intrathecal ropivacaine confirmed by isotonic regression was 6.9 mg (95% CI, 6.68 to 7.12 mg), and the calculated ED95 was 7.8 mg (95% CI, 7.69 to 10.05 mg). When an effective block was achieved with intrathecal ropivacaine alone, the time to resolution of the sensory and motor blocks was 90 (75–100) min and 90 (60–100) min, respectively.

Conclusions The ED50 of intrathecal ropivacaine for prophylactic cervical cerclage was 6.9 mg. Intrathecal ropivacaine (7.8 mg) is likely to produce successful anaesthesia in 95% of patients undergoing prophylactic cervical cerclage.

Trial registration number ChiCTR2100051418.