Effects of diluent volume of a single dose of epidural bupivacaine in parturients during the first stage of labor

Background and Objectives. 0.1% bupivacaine for obstetric epidural analgesia is given by infusion, using a loading dose of a higher concentration alone or in combination with opioid analgesics. A single dose of 0.1% without any additive for relief of first-stage labor pain has not yet been documented. Methods. Fifty-eight primiparae in active labor and with less than 5 cm cervical dilatation received 20 mg epidural bupivacaine diluted in 4 mL. [group 1: 0.5% (I)], 10 mt [group 2: 0.2% (II)], or 20 mi, [group 3: 0.1% (III)]. Pain relief, dermatomal spread, and motor block were assessed. Results. Visual analog pain scale (VAS) was significantly lower in group 2 (0.88 +/- 1.34) and group 3 (0.25 +/- 0.61) than in group 1 (4.37 +/- 2.57). Onset and time to maximum analgesia was significantly shorter in group 2 than in group 3. Mean duration of analgesia was 120 +/- 21 minutes in group 3, 100 +/- 26 in group 2, and 43 +/- 21 in group 1. The mean numbers and upper limits of dermatomes blocked did not differ between groups 2 and 3, but were higher than in group 1. Motor blocks in groups 2 and 3 were more extensive than in group 1 with no difference between groups 2 and 3. Ten mZ. 0.2% or 20 mt 0.1% epidural bupivacaine results in a similar degree of pain relief, superior to that following 4 mt 0.5%, while duration was longest after 20 mt 0.1%. Conclusions. Analgesia lasts significantly longer following 20 mt 0.1% bupivacaine than following 10 mt 0.2% bupivacaine when given for first-stage labor pain. Four milliliters 0.5% bupivacaine results in inadequate pain relief.