Background: In this study we sought to determine and compare the minimum local anesthetic volumes (MLAV) and doses (MLAD) of two concentrations of bupivacaine for epidural pain relief in labor, and to quantify the effect on dose.
Methods: Eighty women were randomized in a double-blind manner to receive a first bolus of either plain bupivacaine 0.125% (w/v) or 0.25% (w/v). The arbitrary starting volume was 15 mL. Subsequent volumes were decided by sequential allocation according to analgesic efficacy. A visual analog pain score < or =10 (0-100) within 30 min, indicated effective analgesia. The next woman received a decrement of 2 mL. A failure of the visual analog pain score to reach < or =10 was followed by a 2 mL increment for the next woman.
Results: Using the formula of Dixon and Massey, MLAV and MLAD, with 95% confidence intervals (CI) were calculated for each concentration. MLAV was 13.6 mL (95% CI 12.4-14.8), with bupivacaine 0.125% (w/v), and 9.2 mL (95% CI 6.9-11.5) with bupivacaine 0.25% (w/v). The difference was highly significant (P = 0.002). MLAD for these volumes were 17.0 mg (95% CI 15.5-18.5), and 23.1 mg (17.2-28.9), respectively (P = 0.045).
Conclusions: Bupivacaine 0.125% (w/v) when compared with 0.25% (w/v) produced equivalent analgesia with a 50% increase in volume, but with a 25% reduction in dose. Any reduction in dose, without loss of efficacy, reduces risk of toxicity and improves safety.