| JOURNAL HOME | CME HOME | THIS MONTH | PAST ISSUES | ETOC | COLLECTIONS |
| AUTHORS | REVIEWERS | EDITORIAL BOARD | FEEDBACK | RSS | HELP |
| ||||||||||||||
| ||||||||||||||
|
|
|
|
||||||||||||
|
||||||||||||||
Department of Anesthesiology and Pain Medicine; University of Alberta Hospital; Edmonton, Canada; btsui{at}ualberta.ca
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.
REFERENCES
This article has been cited by other articles:
|
|
 |
|
  E. de Medicis, R. Martin, and J.-P. Tetrault Epidural Stimulation Test Criteria Anesth. Analg., September 1, 2006; 103(3): 776 - 776. [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
