Effect of sex and pregnancy on the potency of intrathecal bupivacaine: determination of ED₅₀ for motor block with the up-down sequential allocation method.

BACKGROUND AND OBJECTIVE: The up-down sequential allocation model has been adapted to estimate the relative potency ratios for analgesia and motor block of the most commonly used epidural and intrathecal local anaesthetics. The aim of this study was to establish the median effective doses (ED50) for motor block with intrathecal bupivacaine and to estimate the ED50 ratios of these in male, female and pregnant patients.

METHODS: In this prospective, double-blind, parallel group, up-down sequential allocation study, we enrolled 30 male patients, 30 female, non-pregnant patients and 30 pregnant patients undergoing elective surgery under combined spinal-epidural anesthesia. The first two groups consisted of male or female patients undergoing elective lower limb surgery and the third group consisted of pregnant women at term (>36 and <41 weeks) with singleton pregnancies undergoing elective caesarean delivery. Patients received intrathecal isobaric bupivacaine 0.5% as part of the spinal-epidural anaesthesia technique. The initial dose was 4 mg and the testing interval was 1 mg with subsequent doses being determined by the outcome in the previous patient in the same group. The end point for efficacy was the occurrence of motor block in the lower limbs within 5 min.

RESULTS: There were significant (P < 0.0001) differences in ED50 estimates for motor block with intrathecal bupivacaine: 6.9 mg for men [95% confidence interval (CI), 5.2-8.6), 5.2 mg for women (95% CI, 4.5-5.8) and 3.4 mg for pregnant women (95% CI, 2.9-4.0).

CONCLUSION: We have demonstrated a hierarchy of potencies for motor block with intrathecal bupivacaine for men, women and pregnant women suggesting possible relevant differences owing to the effects of both sex and pregnancy.