Minimum Current Requirements for Epidural Stimulation Test Confirmation of Epidural and Intrathecal Catheter Placement

Abstract

The typical blind insertion of a catheter into the epidural space risks catheter misplacement into the intrathecal space. The epidural stimulation test is designed to confirm the correct epidural location of a catheter but may also detect unintended intrathecal catheter placement by evaluating the minimum electrical current required for appropriate motor stimulation. Using this test, we observed the minimum current requirements for appropriate motor stimulation of catheters placed in the epidural and intrathecal spaces. In this prospective observational study, patients scheduled for epidural catheter placement and intrathecal catheter placement were evaluated by the epidural stimulation test. The epidural space was localized by using a loss-of-resistance technique with normal saline, and the intrathecal space was identified by advancing a Tuohy needle until a continuous flow of clear cerebrospinal fluid was obtained. Afterward, a catheter was placed in the appropriate space, and a nerve stimulator delivered progressively, increasing electrical current until an appropriate muscle contraction was palpated. The minimum milliamperage required for this muscle contraction was our primary outcome measure. Of 37 catheters intentionally placed in the epidural space, the mean current required to produce an appropriate palpable motor contraction was 7.8 +/- 3.3 mA with a range of 2 to 14 mA. Of the 11 catheters intentionally placed in the intrathecal space, the mean current required to produce an appropriate palpable motor contraction was 1.3 +/- 0.8 mA with a range of 0.05 to 2.4 mA. We conclude that the epidural stimulation test minimum electrical current requirement seems to be lower for intrathecal compared with epidural catheter placement.