DETERMINATION OF THE MINIMUM LOCAL ANALGESIC CONCENTRATION OF EPIDURAL CHLOROPROCAINE HYDROCHLORIDE IN LABOR

The aim was to determine the effective concentration in 50% of patient s (EC50) of chloroprocaine in the first stage of labor. A constant dos e modification of a model where EC50 was previously defined as the min imum local analgesic concentration (MLAC) was used. Parturients (n=36) requesting epidural analgesia in labor, at cervical dilatation not ex ceeding 7 cm, were enrolled into this prospective, double-blinded stud y, After placing a lumbar epidural catheter, chloroprocaine 150 mg dil uted to the concentration being evaluated was given. The concentration was determined by up-down sequential allocation. The volume of the bo lus ranged from 15 to 50 mi. Efficacy was assessed using 100 mm visual analogue pain scores with 10 mm or less within 30 min defined as effe ctive. MLAC (95%CI) was 0.42%w/v (0.34 to 0.5) using the formula of Di xon & Massey and as a sensitivity test was 0.4%w/v (0.35 to 0.46) usin g probit regression analysis. In conclusion, MLAC of chloroprocaine wa s 0.42%w/v in these parturients, equivalent to 14 millimolar solution. This study confirmed that concentration rather than dose could be use d as a measure of efficacy in this constant dose model.