Letters to the Editor: Letters & Announcements
To the Editor:
de Medicis et al. (1) should be commended for the manner in which they systematically evaluated two newly developed epidural confirmation methods (epidural stimulation test [EST] and epidural pressure waveform analysis) as well as confirming that many patients will require higher than 10 mA to elicit a positive EST in the epidural space. However, the authors' statement “a segmental unilateral motor response <1 mA…was considered positive for adequate epidural space catheterization” is incorrect. In addition, although their suggestions that “the inclusion of sensory response in the appropriate dermatome at a current <10 mA as a criterion for adequate epidural catheter localization for EST testing” and “ the sensitivity of EST could be increased” are true, these would be less desirable for maintaining the objectivity of the confirmation test. To clarify the EST test criteria, I would like to address the original intention of the current range (1–10mA) described in our first report of the technique (2).
Electrical stimulation is intended for use as a reliable real-time and objective technique to confirm epidural catheter placement (2–6). The EST criteria define correct placement of the epidural catheter tip (typically 1–2 cm from the nerve roots) by elicited motor responses (either unilateral or bilateral) with a current between 1–10 mA. However, as stated in our original article, “the milliamperage current settings are intended as guidelines and may require adjustment as experience increases.” (2) Obviously, there was no prior magic about the absolute numerical value of 1 mA (lower limit) or 10 mA (upper limit). The main reason for using this range was the practical applications of ease of memory and technical use.
Regarding the lower limit current 1 mA, when a catheter is situated properly within the epidural space, muscle twitches are typically elicited with a current much greater than 1 mA. Likewise, although not an absolute limit, any motor response (particularly unilateral) observed at <1 mA or barely above 1 mA may suggest catheter placement in the subarachnoid or subdural space or in close proximity to a nerve root (7,8). Thus, this response should be regarded as an important warning and any response below 1 mA is indicative of a negative, rather than positive, response, as the authors state in the criteria.
In regards to the upper limit, experience has suggested that currents >10 mA are required for motor responses in many cases and can be considered positive tests without signaling faulty placement. For example, in a previous article published in this journal (9), we had accepted a positive motor test with up to 15 mA. In another study, we also demonstrated the need for a range of current 6–17 mA for a motor response when using an insulated needle, depending on how deep the needle was embedded into the epidural space (10).
My experience with the test suggests that the distribution of an elicited motor response is more important than the actual current required, as long as it is well above 1 mA. Typical positive stimulation should be elicited as corresponding muscle twitches at the upper limb (cervical), anterior intercostals/ abdominal muscle (thoracic) or lower limb (lumbar), rather than with local muscles twitches under the grounding electrode (these are negative tests). Thus, not only do muscle twitches elicited by current stimulation confirm the position of the catheter in the epidural space but the specific motor muscle response can also predict the level of the catheter tip (7,11,12). Subjective sensory information cannot always reliably produce this accuracy.
Beyond the above clarification, the authors' use of the 2% lidocaine dose may not be definitive and is questionable to be used as “gold standard” to assess the accuracy of both indirect confirmation methods. This test dose can be prone to many confounding factors, from actual epidural spread via paravertebral space injection to simply a placebo effect from the injection, and may have contributed to both indirect methods having poor negative predictive value. Both newly developed methods strive to improve on the accuracy of subjective test doses while providing practical usefulness without either cumbersome equipment involved with imaging techniques or timely procedures. Clearly, evaluating these objective tests with subjective methodology seems inappropriate.
In summary, positive responses within the range of our initial criteria are associated with high positive prediction (100%), indicating catheter placement in the epidural space. Negative responses are associated with a large percentage of false negative predictions, likely related to multiple confounding factors.
Ban C. H. Tsui, MD, MSC, FRCP(C)
Department of Anesthesiology and Pain Medicine
University of Alberta Hospital
1. 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.
2. Tsui BC, Gupta S, Finucane B. Confirmation of epidural catheter placement using nerve stimulation. Can J Anaesth 1998;45:640–4.
3. Tsui BC, Gupta S, Finucane B. Determination of epidural catheter placement using nerve stimulation in obstetric patients. Reg Anesth Pain Med 1999;24:17–23.
4. Tsui BC, Seal R, Koller J, et al. Thoracic epidural analgesia via the caudal approach in pediatric patients undergoing fundoplication using nerve stimulation guidance. Anesth Analg 2001;93:1152–5.
5. Tsui BC, Wagner A, Cave D, Kearney R. Thoracic and lumbar epidural analgesia via the caudal approach using electrical stimulation guidance in pediatric patients: a review of 289 patients. Anesthesiology 2004;100:683–9.
6. Tsui BC, Bateman K, Bouliane M, Finucane B. Cervical epidural analgesia via a thoracic approach using nerve stimulation guidance in an adult patient undergoing elbow surgery. Reg Anesth Pain Med 2004;29:355–60.
7. Tsui BC, Gupta S, Finucane B. Detection of subarachnoid and intravascular epidural catheter placement. Can J Anaesth 1999;46:675–8.
8. Tsui BC, Gupta S, Emery D, Finucane B. Detection of subdural placement of epidural catheter using nerve stimulation. Can J Anaesth 2000;47:471–3.
9. Goobie SM, Montgomery CJ, Basu R, et al. Confirmation of direct epidural catheter placement using nerve stimulation in pediatric anesthesia. Anesth Analg 2003;97:984–8.
10. Tsui BC, Wagner A, Cave D, Seal R. Threshold current for an insulated epidural needle in pediatric patients. Anesth Analg 2004;99:694–6.
11. Tsui BC, Wagner A, Finucane B. The threshold current in the intrathecal space to elicit motor response is lower and does not overlap that in the epidural space: a porcine model. Can J Anaesth 2004;51:690–5.
12. Tsui BC, Wagner A, Cunningham K, et al. Threshold current of electrical stimulation in the intrathecal space using insulated needles in pediatric patients. Anesth Analg. 2005;100:662–5.