A Comparison of Minimum Local Anesthetic Volumes and Doses of Epidural Bupivacaine (0.125% w/v and 0.25% w/v) for Analgesia in Labor Gordon R. Lyons, MD, FRCA* Mitko G. Kocarev, MD, DEAA* Rowan C. Wilson, MRCP, FRCA* Malachy O. Columb, FRCA† BACKGROUND: In this study we sought to determine and compare the minimum local anesthetic volumes (MLAV) and doses (MLAD) of two concentrations of bupiva- caine for epidural pain relief in labor, and to quantify the effect on dose. METHODS: Eighty women were randomized in a double-blind manner to receive a first bolus of either plain bupivacaine 0.125% (w/v) or 0.25% (w/v). The arbitrary starting volume was 15 mL. Subsequent volumes were decided by sequential allocation according to analgesic efficacy. A visual analog pain score 10 (0 –100) within 30 min, indicated effective analgesia. The next woman received a decrement of 2 mL. A failure of the visual analog pain score to reach 10 was followed by a 2 mL increment for the next woman. RESULTS: Using the formula of Dixon and Massey, MLAV and MLAD, with 95% confidence intervals (CI) were calculated for each concentration. MLAV was 13.6 mL (95% CI 12.4 –14.8), with bupivacaine 0.125% (w/v), and 9.2 mL (95% CI 6.9 –11.5) with bupivacaine 0.25% (w/v). The difference was highly significant (P = 0.002). MLAD for these volumes were 17.0 mg (95% CI 15.5–18.5), and 23.1 mg (17.2–28.9), respectively (P = 0.045). CONCLUSIONS: Bupivacaine 0.125% (w/v) when compared with 0.25% (w/v) pro- duced equivalent analgesia with a 50% increase in volume, but with a 25% reduction in dose. Any reduction in dose, without loss of efficacy, reduces risk of toxicity and improves safety. (Anesth Analg 2007;104:412–5) The concentration of local anesthetic injected into the epidural space that is required to block conduction of an action potential is influenced by the length of nerve root exposed to local anesthetic which, in turn, is limited by the confines of the epidural space. High concentrations of local anesthetics require only limited exposure to the nerve root, but if the pharmacological sleeve is extended to bathe all the available nerve root within the epidural space, then a lower concentration of local anesthetic will be sufficient to block nerve transmission (1). Extending the pharmacological sleeve requires that the volume of injectate is increased at the expense of concentration. Christiaens et al. (2) showed, in a fixed dose study, that pain relief in labor was improved if the volume of local anesthetic was increased from 4 to 20 mL. Benefits have also been seen with continuous and patient-controlled epidural analgesia, again with fixed dose models (3,4). These studies show that higher volumes and lower concentrations are more effective for nerve blockade when compared with lower volumes of higher concentration local anesthet- ics. There are also potential benefits in differential blockade, such as motor sparing, when higher concen- trations are exchanged for lower concentrations given in larger volumes. Dose is the product of concentration and volume, and there is a potential for a reduction in dose when lower volume-higher concentration solutions are sub- stituted with higher volume-lower concentration so- lutions. This reduction, however, will not be apparent when comparisons are made using a fixed-dose model. This study was therefore designed to quantify any dose reduction which might have potential impli- cations for safety. The aim was to determine the median effective volumes and doses, which were defined as the minimum local analgesic volume (MLAV) and minimum local analgesic dose (MLAD) of bupivacaine 0.125% (w/v) and 0.25% (w/v), respec- tively, for epidural bupivacaine given as the first bolus to women for analgesia in labor. The two concentra- tions chosen for this comparison were both of a magnitude that might be expected to produce conduc- tion blockade without any need for adjuncts. From the *Department of Obstetric Anaesthesia, St James Uni- versity Hospital, Leeds, UK; and †Intensive Care Unit, South Manchester University Hospital, Wythenshawe, UK. Accepted for publication October 13, 2006. Address for correspondence and reprint requests to Mitko Kocarev, MD, DEAA, Department of Obstetric Anaesthesia, Deliv- ery Suite, Gledhow Wing, St. James University Hospital, Leeds LS9 7TF, UK. Address e-mail to mkocarev@yahoo.com. Copyright © 2006 International Anesthesia Research Society DOI: 10.1213/01.ane.0000252458.20912.ef Vol. 104, No. 2, February 2007 412