Dose-Response Curves for Intrathecal Bupivacaine, Levobupivacaine, and Ropivacaine Given for Labor Analgesia in Nulliparous Women

Background and Objectives: Bupivacaine, levobupivacaine, and ropivacaine are often given intrathecally for labor analgesia, but limited data are available for their dose-response properties in this context. The objective of this study was to describe the dose-response curves of these local anesthetics when given intrathecally for labor analgesia, to determine values for D-50 (dose producing a 50% response) and to compare the calculated values of D-50 for levobupivacaine and ropivacaine with those for bupivacaine. Methods: With ethics approval and written consent, we randomized 270 nulliparous laboring patients requesting neuraxial analgesia at 5-cm cervical dilation or less to receive a single dose of intrathecal local anesthetic without opioid as part of a combined spinal-epidural technique. Patients received either bupivacaine, levobupivacaine, or ropivacaine at a dose of 0.625, 1.0, 1.5, 2.5, 4.0, or 6.25 mg (n = 15 per group). Visual analog scale pain scores were measured for 15 minutes, after which further analgesia and management were at the clinician's discretion. The primary end point was percentage reduction of pain score at 15 minutes. Logistic sigmoidal dose-response curves were fitted to the data using nonlinear regression, and D-50 values were calculated for each drug. Results: Data were analyzed from 270 patients. Patient characteristics were similar between groups. The calculated D-50 and 95% confidence interval values were as follows: bupivacaine, 1.56 mg (1.25-1.94 mg); ropivacaine, 1.95 mg (1.57-2.43 mg); and levobupivacaine, 2.20 mg (1.76-2.73 mg). Conclusions: The results of this study support previouswork showing that intrathecal levobupivacaine and ropivacaine are less potent than bupivacaine.