Lidocaine blocks sodium channels in nerve membranes, which inhibits sodium influx and prevents development of an action potential. For epidural anesthesia, we usually use lidocaine diluted in normal saline or distilled water. However, diluents containing sodium ions may affect lidocaine activity.

Fifty patients, ASA physical status class I or II, scheduled for elective epidural anesthesia were randomly divided into two groups. For epidural anesthesia, we used 14 ml of 2% lidocaine solution made with 7 ml of 4% lidocaine plus an equivalent of distilled water (Group DW) or normal saline (Group NS). In the right lateral decubitus position, a 19 G Arrow catheter was inserted 3 cm in the cephalad direction in the L3-4 intervertebral space. In the supine position, 3 ml of 2% lidocaine was injected as a test dose, and the remaining 11 ml of lidocaine was injected 2 minutes later. We used a pinprick test for evaluation of sensory block levels and a Bromage grade for degrees of motor block every 2 minutes during the first 30 minutes.

Levels of sensory block were higher in Group DW than Group NS at 12, 14 and 30 minutes. The degree of motor block was also higher in Group DW at 12 and 14 minutes.

Diluent containing sodium ions reduces sensory and motor block compared to sodium-free solution in epidural anesthesia.