• Ban C H Tsui, Alese Wagner, and Brendan Finucane.
• Department of Anesthesiology and Pain Medicine, University of Alberta Hospitals, 3B2.32 Walter Mackenzie Health Science Centre, 8440-112 Street, Edmonton, Alberta T6G 2B7, Canada. btsui@ualberta.ca
• Can J Anaesth. 2004 Aug 1;51(7):690-5.

PurposeUsing electrical epidural stimulation, a current of 1 to 10 mA is required to confirm the presence of the tip of an epidural catheter in the epidural space. The purpose of this study was to examine the hypothesis that the threshold current required to elicit a motor response in the intrathecal space is significantly lower than that in the epidural space in a porcine model.MethodsFour 20-kg pigs were used in this experiment. Eighteen gauge, insulated, Tuohy needles were advanced into the epidural space using the loss of resistance technique at five different spinal levels in each pig. When the epidural space was entered, an electrical current was applied to the needle and increased progressively until a motor response was elicited. The needle was then further advanced until cerebrospinal fluid (CSF) was observed or until the needle had been advanced a maximum of 1 cm. At this point, the current was reapplied and increased until motor activity was evident.ResultsA total of 20 needles were inserted in four pigs. The current required to produce a motor response in the epidural space was 3.45 +/- 0.73 mA (mean +/- SD). The current required to produce a motor response in the intrathecal space (entry confirmed by the presence of CSF) was 0.38 +/- 0.19 mA (mean +/- SD). Two needles were advanced 1 cm without obtaining CSF but the current thresholds were similar to those obtained when CSF was evident (0.4 mA and 0.3 mA, respectively).ConclusionThe threshold current of an insulated needle required to elicit a motor response in the intrathecal space, was significantly (P < 0.01) lower than that in the epidural space in a porcine model.

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