Saline volume and local anesthetic concentration modify the spread of epidural anesthesia

To examine the effects of the volume of saline and the concentration of local anesthetic on the quality of anesthetic level.

One hundred and fifty two patients received thoracic epidural anesthesia were allocated into two groups; mepivacaine 1% (75 patients) and 1.5% (77 patients). Each group was randomly divided into three subgroups depending on epidural saline volumes of 1 ml, 5 ml, or 10 ml. Fifteen minutes after the injection of 10 ml mepivacaine, the dermatome levels of hypesthesia to cold and pinprick were determined by an individual blinded to the saline volume.

The number of spinal segments with hypesthesia to cold in the three subgroups in the mepivacaine 1% group were 12.5 [6-20], 13 [8.5-20.5] and 12.5 [6.5-22], respectively (median [range]). The segments in the mepivacaine 1.5% group were 12 [7-18.5], 14 [8.5-19]* and 15 [6-23]*, respectively (*P < 0.05 vs 1-ml group). The number of spinal segments with hypesthesia for pinprick in the three subgroups in the 1% mepivacaine group were 10.5 [2-22], 10.5 [4-17] and 11 [4-19], respectively. The segments in the mepivacaine 1.5% group were 12 [7.5-16], 12 [7.5-17] and 11.5 [5-22.5], respectively. Saline volume did not alter the anesthetic level of the mepivacaine 1%, although it did affect the anesthetic spread of the mepivacaine 1.5%. In both groups, a differential nerve block was elicited in the 5 ml and 10 ml saline subgroups.

When a large volume of saline is administered prior to local anesthetic, more differential blockade and a greater extent of anesthesia may be elicited.