This double-blind, randomized study was aimed at detecting the effect of three different volumes of intrathecal levobupivacaine on the minimum local analgesic dose in early labor.Ninety-three nulliparous women requesting combined spinal-epidural analgesia, at more than 37 weeks gestation, with spontaneous onset of labor, cervical dilatation from 2 to 5 cm, were enrolled. Parturients received 10 ml (group 10), 5 ml (group 5), or 2.5 ml (group 2.5) of the spinal solution containing plain levobupivacaine diluted with 0.9\% wt/vol saline to achieve the desired dose and volume at room temperature. A lumbar epidural catheter was then placed. The initial dose for each group was 2.0 mg, and the following doses were determined by the response of the previous patient using up-down sequential allocation. The authors required the test solution to achieve a visual analog pain score of 10 mm or less to be considered effective. The up-down sequences were analyzed using the Dixon and Massey formula and regression logistic model.The minimum local analgesic dose of spinal levobupivacaine in spontaneously laboring women was 1.35 mg (95\% confidence interval, 1.25-1.45 mg) in group 10, 1.63 mg (95\% confidence interval, 1.51-1.76 mg) in group 5, and 1.97 mg (95\% confidence interval, 1.89-2.05 mg) in group 2.5. A unit change in volume increased the odds of an effective response multiplicatively by a factor of 1.8.Analgesia can be achieved using lower doses and higher volumes even in subarachnoid space. The important role of the volume should be considered not only in epidural but also in spinal analgesia.
