The minimum effective dose of 0.5% hyperbaric spinal bupivacaine for cesarean section.

BACKGROUND: The aim of this prospective, blind study was to determine the minimum effective dose of hyperbaric bupivacaine required for cesarean section.

METHODS: With Ethical Committee approval and written consent, 24 healthy women undergoing elective cesarean section received a combined spinal epidural anesthesia. We sought to determine the minimum effective dose of spinal bupivacaine using a staircase method. In each patient an arbitrary dose of 0.5% hyperbaric bupivacaine in relation to patient height was used. The initial dose was 0.075 mg/cm height, while the outcome of each patient's response determined the dose for the subsequent patient. When successful spinal block (sensory level = or < T4 with complete motor blockade) was achieved within 20 min from spinal injection, the dose of spinal bupivacaine for the next patient was decreased by 0.01 mg/cm height. Conversely, when successful spinal block was not observed, the dose of spinal bupivacaine for the next patient was increased by 0.01 mg/cm height. Sensory and motor blocks were evaluated every 5 min by an independent, blinded observer. If successful spinal block was not achieved within the designed period, a 5-8 ml epidural bolus of 2% lidocaine was given to achieve adequate surgical anesthesia.

RESULTS: No complications were reported during the study, and all women delivered their baby uneventfully (APGAR scores 5 min after delivery ranged from 9 to 10) within 5 min from uterus incision. The duration of surgical procedure ranged from 30 to 48 minutes. The dose of hyperbaric bupivacaine providing adequate surgical anesthesia within 20 min from spinal injection in 50% of subjects was 0.036 mg/cm height (95% confidence intervals: 0.031-0.041 mg/cm height). The ED95 calculated from the probit transformation to provide effective spinal anesthesia for cesarean section was 0.06 mg/cm height.

CONCLUSIONS: This prospective, blind study demonstrated that a dose as low as 0.06 mg/cm height represents the dose of intrathecal bupivacaine providing effective spinal block in 95% of women undergoing elective cesarean section.