Epidural volume extension and intrathecal dose requirement: plain versus hyperbaric bupivacaine

Epidural volume extension leads to an increase in sensory spread of local anesthetic, but whether this translates into lower dose requirements during combined spinal epidural (CSE) remains undetermined. Likewise, the influence of intrathecal drug baricity on the dose-sparing effect of epidural volume extension has not been investigated. We studied the dose-sparing effect of epidural volume extension and its relation to intrathecal drug baricity by comparing the ED(50) of plain and hyperbaric bupivacaine with and without epidural volume extension.

Eighty-eight adult male patients scheduled for lower limb orthopedic surgery under CSE in the sitting position were randomized to four groups (n = 22 each); intrathecal injection was made with plain bupivacaine in groups plain bupivacaine (PB) and plain bupivacaine with epidural volume extension (PBE), and hyperbaric bupivacaine in groups hyperbaric bupivacaine (HB) and hyperbaric bupivacaine with epidural volume extension (HBE). Fentanyl, 25 microg, was added to the intrathecal drug in all groups. Among these four groups, epidural volume extension was performed with 10 mL normal saline only for groups PBE and HBE. The dose of spinal bupivacaine was varied using the up-and-down sequential allocation method. The first patient of each group received 10 mg bupivacaine. A successful spinal block was defined as attainment of sensory level of at least T10 along with complete motor blockade within 20 min of the intrathecal injection. The dose of bupivacaine was sequentially increased or decreased by 1 mg depending on whether spinal block was a failure or success in the previous patient.

The addition of epidural volume extension to plain bupivacaine, i.e., group PBE versus group PB, resulted in a significant decrease in ED(50) (relative potency estimate: 1.2, 95% CI: 1.04-1.64) and increase in maximum sensory level (T6 vs T8, respectively, P < 0.05). These differences were not seen with hyperbaric bupivacaine (group HB vs HBE). Independent of the effect of epidural volume extension, the ED(50) of plain bupivacaine when compared with hyperbaric bupivacaine was significantly lower (relative potency estimate of group PB vs group HB: 0.78, 95% CI: 0.54-0.93; and for group PBE vs group HBE: 0.68, 95% CI: 0.37-0.87).

Administered with or without epidural volume extension, plain bupivacaine appears to be more effective, requiring a smaller dose and producing a higher sensory block with an earlier onset in comparison to hyperbaric bupivacaine. Epidural volume extension, when applied to intrathecal hyperbaric bupivacaine, fails to decrease the dose or raise the level of block.