Estimation of the minimum local analgesic concentration (MLAC) of epidural lignocaine hydrochloride in labour

Determination of the minimum local analgesic concentration of epidural chloroprocaine hydrochloride in labor

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

The effect of epidural fentanyl on the minimum local analgesic concentration of epidural chloroprocaine in labor

The minimum local analgesic concentration (MLAC) has been defined as the median effective local analgesic concentration (EC50) in a 20-mL volume in the first stage of labor. The aim of this study was to determine the local anesthetic sparing efficacy of epidural fentanyl by its effect on the MLAC of chloroprocaine. Fifty-six parturients, not exceeding 7 cm cervical dilation, who requested epidural analgesia were allocated to one of two groups in this double-blind, randomized, prospective study. After placing a lumbar epidural catheter, 20 mL of the solution being tested was given: chloroprocaine (n = 28) or chloroprocaine with fentanyl 3 micrograms/mL (60 micrograms) (n = 28). The concentration of chloroprocaine was determined by the response of the previous patient to a higher or lower concentration using up-down sequential allocation. Analgesic efficacy was assessed using 100-mm visual analog pain scores with 10 mm or less within 30 min defined as effective. The MLAC of chloroprocaine was reduced from 0.43% wt/vol to 0.26% wt/vol by fentanyl (P = 0.023). Thus, the addition of epidural fentanyl 3 micrograms/mL (60 micrograms resulted in a significant 40% reduction in the MLAC of chloroprocaine in the first stage of labor.