The epidural "top-up" in combined spinal-epidural anesthesia: The effect of volume versus dose

The reinforcement of anesthesia by an epidural "top-up" in combined spinal- epidural anesthesia may be explained by a dual mechanism: a volume effect c ompressing the dural sac and a local anesthetic effect. The purpose of our study was to investigate the relative importance of each of these factors. Fifty patients scheduled for lower limb orthopedic surgery under combined s pinal-epidural anesthesia were randomly allocated to one of five groups com prising 10 patients each. Using a needle-through-needle technique, all pati ents received a subarachnoid injection of 10 mg of plain bupivacaine and an epidural catheter. After the maximal level of sensory blockade as a result of the subarachnoid injection had been established, an epidural top-up was given according to the randomization code. Patients in Group 1 received 10 mt of bupivacaine 0.25%; patients in Group 2 received 10 mt of saline; pat ients in Group 3 received 5 mL of bupivacaine 0.5%; patients in Group 4 rec eived 5 mL of saline; and patients in Group 5 received no epidural top-up. The maximal level of sensory blockade was then assessed for an additional 3 0 min. In Groups 1-4, the maximal level of sensory blockade increased signi ficantly, whereas there was no significant increase in Group 5. There was n o significant difference in the increase in the maximal level of sensory bl ockade among Groups 1-4. We conclude that, under the conditions of our stud y, there is no difference between 5 and 10 mt with regard to the volume eff ect of an epidural top-up in combined spinal-epidural anesthesia and that t o produce an additional local anesthetic effect with bupivacaine, the dose must be larger than 25 mg. Implications: In combined spinal-epidural anesth esia, an epidural "top-up" may increase the maximal level of sensory blocka de by means of a volume effect and a local anesthetic effect. In this study , volumes of 5 and 10 mt produced a similar increase, and 25 mg of bupivaca ine was insufficient to produce an additional local anesthetic effect.