Our primary aim was to determine the minimum local anaesthetic dose (MLAD) defined as the median effective local anesthetic dose for spinal anesthesia in neonates. Secondary aims were to determine the dose–response curve for spinal anesthesia including the clinically relevant ED95 dose and to describe the duration of motor block following ropivacaine spinal anesthesia.Methods
Fifty neonates less than 55 weeks postmenstrual age (PMA) having inguinal hernia repair were enrolled. The study was performed in two stages. The MLAD dose of ropivacaine was determined by up–down sequential allocation. An expanded dose ranging study was then performed with dose escalation to doses predicted to fall in the ED50–95 range dose. Probit and logit analysis on all patients was used to compare ED50 with the value described by the Dixon–Massey method and to determine doses up to the ED95.Results
Fifty neonates with a mean PMA of 43 weeks and mean weight of 3.7 kg were enrolled. The motor block MLAD (95% confidence interval) determined by the Dixon–Massey method was 0.51 (0.38–0.64) mg·kg−1. A logistic regression analysis of the results of all 50 patients confirmed the Dixon–Massey result. The ED50 (95% CI) was 0.50 (0.39–0.63) mg·kg−1 with an estimated ED95 was 1.08 (0.70–1.67) mg·kg−1. These values were confirmed by probit analysis. Overall the mean duration (95% confidence interval) of lower limb motor blockade was 60.0 min (51.5–68.5 min). The duration of anesthesia far exceeded surgical duration in all successful cases.Conclusions
Ropivacaine is an effective agent for spinal anesthesia in neonates at a recommended dose of 1.08 mg·kg−1. The motor block duration, however, is significantly shorter than equivalent agents and highly variable in duration.