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

Purpose: To examine the effects of the volume of saline and the concentration of local anesthetic on the quality of anesthetic level. Methods: 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. Results: 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 .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. Conclusion: When a large volume of saline is administered prior to local anesthetic, more differential blockade and a greater extent of anesthesia may be elicited.