An increase in the maximum level of sensory blockade by epidural 'top-up' in combined spinal epidural anesthesia may be explained by two mechanisms; a volume effect, compressing the dural sac, and a local anesthetic effect. This study was undertaken to investigate the relative importance of these factors.

Eighty patients about to undergo lower limb surgery under combined spinal epidural anesthesia were randomly assigned to four groups. Using the needle-through-needle technique, all patients received a subarachnoid injection of 8 mg of heavy bupivacaine through an epidural catheter. At 30 minutes after subarachnoid injection, an epidural top-up was given according to a randomization code. Experimental groups received 10 ml of saline, 10 ml of bupivacaine 0.25%, or 10 ml of bupivacaine 0.5%. The control group received no epidural top-up. The level of sensory blockade was checked by using the pinprick test at 5, 10, 15, 20, 25 and 30 minutes after subarachnoid injection, and at 5, 10, 15, 20, 25 and 30 minutes after epidural top-up. Blood pressure, heart rate and the incidence of side effects such as hypotension, bradycardia, nausea, and high block were analyzed.

There was no significant difference in maximum level of sensory block among the 10 ml saline, 10 ml bupivacaine 0.25% or 10 ml bupivacaine 0.5% treated groups during epidural phase. Although blood pressure and heart rate were decreased, there were no differences among experimental groups during epidural phase. The most common side effect during combined spinal epidural anesthesia was hypotension. The incidence of bradycardia and high sensory block (above T4 dermatome level) was similar among the groups.

After the maximum level of sensory blockade due to subarachnoid injection has been established, an epidural top-up with 10 ml of saline or 10 ml of either 0.25% or 0.5% bupivacaine did not significantly increase the level of subarachnoid block in patients with lower extremity surgery. The author concludes the there were no differences among groups with regard to the volume effect and local anesthetic effect in terms of the sensory blockade level during the epidural phase. However, blood pressure and heart rate in the epidural top-up groups reduced with operation time.