Mo. Columb et G. Lyons, DETERMINATION OF THE MINIMUM LOCAL ANALGESIC CONCENTRATIONS OF EPIDURAL BUPIVACAINE AND LIDOCAINE IN LABOR, Anesthesia and analgesia, 81(4), 1995, pp. 833-837
The aim of this study was to devise a clinical model to determine the
effective concentrations in 50% of patients (EC(50)) for bupivacaine a
nd lidocaine in the first stage of labor and define EC(50) as the mini
mum local analgesic concentration (MLAC). This should allow the determ
ination of relative analgesic potency and, subsequently, the local ane
sthetic sparing efficacy of other epidural analgesics. Parturients not
exceeding 5 cm cervical dilation who requested epidural analgesia wer
e enrolled. The two studies involved 81 women (bupivacaine n = 41, lid
ocaine n = 40). After a lumbar epidural catheter was placed, 20 mL of
the concentration of local anesthetic being tested was given. The conc
entration was determined by the response of the previous patient to a
higher or lower concentration using double-blinded, up-down sequential
allocation. Efficacy was assessed using 100-mm visual analog pain sco
res with less than 10 mm within 1 h defined as effective. MLAC was det
ermined using the formula of Dixon and Massey. Results show MLAC bupiv
acaine 0.065% (95% confidence interval [CI] 0.045-0.085), MLAC lidocai
ne 0.37% (95% CI 0.32-0.42), equivalent to 2 and 14 mmol solutions, re
spectively. Thus bupivacaine was 5.7 times more potent than lidocaine
in weighted and 7 times more potent in molar ratios at analgesic EC(50
), in the volume of local anesthetic studied.