Epidural saline solution prior to local anaesthetic produces differential nerve block

The loss-of-resistance technique is generally used to identify the epidural space usually with normal saline. However, the effect of epidural saline on anaesthetic spread has not been demonstrated. The purpose of this study was to determine whether epidural saline affected the anaesthetic level and the quality of analgesia.

Seventy patients undergoing upper abdominal surgery received thoracic epidural anaesthesia combined with general anaesthesia. The patients were randomly allocated into three groups using epidural saline volumes of 1, 5, or 10 ml. Ten minutes after epidural injection of 12 ml mepivacaine 1%, the dermatome levels of hypaesthesia for cold and pinprick were determined by an individual blinded to the amount of saline administered.

The levels of hypaesthesia for cold sensation were not different among the three groups (23 [14-44] total bilateral dermatomes blocked with 1 ml saline, 22 [16-41] dermatomes with 5 ml, and 24 [10-36] dermatomes with 10 ml (median [range])). However, the larger volume of saline produced decreased spread of hypaesthesia for pinprick (22 [4-41] dermatomes blocked bilaterally for 1 ml vs 16 [8-24] dermatomes for 10 ml group, P < 0.004).

Our results suggest that a large volume of saline solution injected in the epidural space to elicit loss-of-resistance dilutes the local anaesthetic solution, resulting in reduced spread of the block to pinprick.