Relative potencies of bupivacaine, levobupivacaine, and ropivacaine for neonatal spinal anaesthesia

Comparing the relative potency of new local anaesthetics such as levobupivacaine and ropivacaine with bupivacaine by the minimum local analgesic concentration model has not been described for neonatal spinal anaesthesia. This information is important to compare agents and to determine the most effective spinal dose. We performed a two-stage study to determine the ED50, the ED95, and the relative analgesic potency of isobaric spinal bupivacaine, levobupivacaine, and ropivacaine in infants. In phase 1, 81 infants were randomized in a Dixon–Massey study to describe the minimum local analgesic dose. In phase 2, a further 70 patients were randomly allocated to receive spinal anaesthesia with doses in the upper dose–response range to define the ED95. The ED50 doses for bupivacaine, levobupivacaine, and ropivacaine were estimated by isotonic regression to be 0.30 mg kg−1 [95% confidence interval (CI) 0.25–0.43], 0.55 mg kg−1 (0.50–0.64), and 0.50 mg kg−1 (0.43–0.64), respectively. The ED95, respectively, of bupivacaine, levobupivacaine, and ropivacaine were 0.96 mg kg−1 (95% CI 0.83–0.98), 1.18 mg kg−1 (1.05–1.22), and 0.99 mg kg−1 (0.73–1.50). The relative potency ratios at the ED50 were bupivacaine:levobupivacaine 0.55 (95% CI 0.39–0.88), bupivacaine:ropivacaine 0.61 (0.41–1.00), and levobupivacaine:ropivacaine 1.09 (0.84–1.45). Appropriate doses for infant spinal anaesthesia are 1 mg kg−1 of isobaric 0.5% bupivacaine and ropivacaine and 1.2 mg kg−1 of isobaric 0.5% levobupivacaine.