Letters to the Editor: Letters & Announcements
We would like to thank Dr. Tsui (1) for his interest in our work (2) and his insightful comments. We agree that the upper current limit used for a positive epidural stimulation test should be increased to at least 15 mA, with an appropriate elicited motor response.
We do not agree with the assertion that “when a catheter is situated properly within the epidural space, muscle twitches are typically elicited with a current much greater than 1 mA.” In our study (2), of the 167 patients with a positive epidural stimulation test, 9 had unilateral motor response with current less than or equal to 1 mA and 19 had positive unilateral motor response at current less than or equal to 1.5 mA. All of these patients had adequate postoperative analgesia. None had evidence of subdural or intrathecal catheter placement. Hogan (3) demonstrated that epidural catheter tips were more often found lateral in the epidural space than in the intervertebral foramen. With increasing volume of injectate, the epidural spread becomes more symmetric. We believe that as long as the motor response is unilateral and segmental, the catheter tip is most likely in close proximity to a nerve root, probably in the intervertebral foramen. Bilateral or multi-segmental motor response at low current should warn the clinician of possible intrathecal or subdural catheter placement.
Radiological confirmation remains the “gold standard” for assessing adequate epidural catheter placement and localization. As such confirmation was not feasible in our institution, our “gold standard” was our studied population's appropriate postoperative analgesic response, the ultimate reason we insert epidural catheters in surgical patients. The lidocaine test was our usual presurgical test for epidural localization before the introduction of the epidural stimulation test and epidural pressure waveform analysis.
Dr. Tsui should be commended for his work with the epidural stimulation test, a method that improves the success of epidural placement, confirms adequate dermatomal level, and increases the efficacy of epidural anesthesia and/or analgesia.
Etienne de Medicis, MD, MSC, FRCP(C)
Rene Martin, MD, FRCP(C)
Jean-Pierre Tetrault, MD, MSC, FRCP(C)
Centre Hospitalier Universitaire de Sherbrooke
Sherbrooke, Quebec, Canada
1. Tsui BC. Epidural stimulation test criteria? Anesth Analg 2006;103:775–6.
2. de Medicis E, Tetrault JP, Martin R, et al. A prospective comparative study of two indirect methods for confirming the localization of an epidural catheter for postoperative analgesia. Anesth Analg 2005;101:1830–3.
3. Hogan Q. Epidural catheter tip position and distribution of injectate evaluated by computed tomography. Anesthesiology 1999;90:964–70.