Thoracic epidural catheters are routinely placed in conscious adults but, in contrast, are frequently placed under sedation or general anesthesia in pediatric patients (1,2). Paresthesias and pain on injection are early warning signs of needle encroachment on a nerve root or the spinal cord in awake patients, but these valuable signs cannot be elicited in anesthetized patients (3). There is, therefore, considerable hesitation about performing central neuraxial block in anesthetized patients or in heavily sedated patients because of the danger of neurologic complications. A case report (4) of a child who developed temporary neurological complications after a single-shot thoracic epidural injection performed under general anesthesia reminds us of the serious risks associated with thoracic epidurals performed in anesthetized children (5). Hence, it is desirable to be able to detect needle encroachment on neural structures in unconscious patients. A motor response from electrical stimulation (ES) may provide this early warning sign.
In this pilot study, a porcine model was used to examine the potential application of ES at 5 mA as a continuous method to monitor the response to epidural needle advancement.
After approval by our institution’s Animal Research Committee, 5 farm-bred pigs, weighing approximately 20 kg, were sedated with ketamine (10 mg/kg IM) and anesthetized with isoflurane (1.5%–3%, end-tidal). In each of the 5 pigs, an 18-gauge insulated Tuohy needle (Pajunk, Dyna Medical Corp, Ontario, Canada) connected to a peripheral nerve stimulator (Pajunk, Dyna Medical Corp) set at 5 mA was inserted at 20 different dermatomal levels located between C5 and L5. Because pigs have more individual vertebrae (C7, T14–15, L6–7, and S4), 20 needles could be inserted in each pig (6). With each insertion, the needle was slowly advanced until a muscle twitch was observed. This was accomplished without using the loss-of-resistance (LOR) technique. Once a muscle twitch was observed, LOR with saline was attempted, followed by an attempt to pass a 20-gauge epidural catheter through the needle into the epidural space. Entry of the catheter into the epidural space was supported by the ease of catheter insertion. In addition, fluoroscopy was also used to confirm that, indeed, the catheter was in the epidural space. After confirmation that the needle was in the epidural space and removal of the catheter, the current was gradually reduced from 5 mA until a threshold current (just enough to produce an obvious motor twitch) was observed. This threshold current was recorded. At the end of the experiment an autopsy was performed by a veterinary pathologist, and the spinal cord of each pig was examined for injury.
One-hundred needle insertions were performed in the 5 experimental pigs. The mean threshold current required to elicit a motor response (mean ± sd) was 3.6 ± 0.6 mA. In 59 of the needle insertions, LOR was not obtained at the depth at which a muscle twitch was initially observed. However, after advancing these 59 needles another 1–2 mm, LOR was obtained. There is no difference in the incidence of false-positive results in relationship to the spinal level of needle insertion because 30 of 59 insertions occurred in the lumbar region and 29 of 59 were in the thoracic region. In the other 41 insertions, LOR was observed without any further advancement of the needle. The autopsy revealed that the dura of each pig remained intact, and there was no indication of spinal cord damage.
In this porcine model, we examined the potential application of ES as a method of monitoring epidural needle advancement in a situation in which paresthesias could not be obtained. Using ES through an insulated needle, warning of a needle tip in or approaching the epidural space can be provided.
Ideally, ES could be used to monitor epidural needle advancement in sedated or anesthetized patients. ES is not intended to replace LOR as a confirmation technique but rather to serve as an adjunct to prevent spinal cord or nerve root damage when placing epidural needles in anesthetized patients. By applying sufficient current upon advancing insulated Tuohy needles, one can objectively monitor when the needles are encroaching on a nerve root or the spinal cord, and in some cases, even before LOR is encountered. Thus, it is important to search for an ideal electrical current to be a useful real-time warning of impending neural encroachment while avoiding a false-positive response.
When stimulation techniques are used to facilitate the performance of peripheral nerve blocks, one usually begins stimulating with currents in the range of 1–2 mA and gradually decrease the current to 0.5 mA, which is the minimal acceptable current required to ensure that the needle is in close proximity to a neural structure (7). Because the intended current (1–10 mA) used to confirm epidural catheter placement is much greater than 0.5 mA (8–11), we hypothesized that a motor response evoked by a low current (<1 mA) may provide an early warning sign that the stimulating needle is either approaching a nerve root or actually within the subarachnoid space. Our group (8,9) demonstrated that the minimum current required to elicit a motor response via an insulated needle after thoracic epidural and intrathecal placement in pediatric patients was 11.1 ± 3.1 mA and 0.6 ± 0.3 mA, respectively. These observations support the potential application of ES as an alternative method to monitor needle advancement in anesthetized patients. In another study using the same porcine model, we (12) also demonstrated that a mean current of 3.45 ± 0.73 mA is required to elicit a motor response in the epidural space using an insulated needle. In contrast, the current required to elicit a motor response after the needle was advanced into the intrathecal space was significantly lower at 0.38 ± 0.19 mA (12). Thus, the mean threshold current in the epidural space (3.6 ± 0.6 mA) in our current investigation is in keeping with this published work.
In this study, we selected 5 mA as the continuous current for 2 reasons. First, based upon our previous porcine studies, the mean current of an insulated needle in the epidural space was 3.45 ± 0.73 mA. Assuming a normal distribution, the 95% confidence limit for this threshold current (mean ± 2 sd) would be between 2.0 and 4.9 mA. We therefore anticipated that a current >4.9 mA (the upper confidence level) should induce a motor response once the needle has penetrated the epidural space. Second, in another study (13) that used ES to locate thoracic pedicle screw placement in a porcine model, a threshold current of <4 mA indicated entry of the screws into the epidural space. Although pedicle screw placement differs significantly from epidural anesthesia, the concept of using ES to objectively assess screw (or in our study, needle) location is very similar.
In this pilot study using ES, supramaximal currents of 5 mA consistently induced a motor twitch without any evidence of encroachment on the dura or spinal cord. However, for 59 of the needle insertions, ES induced a motor response before LOR was evident. This may indicate that the needle tip just barely entered the epidural space, and perhaps the lumen was still covered by the ligamentum flavum. Nevertheless, this high supramaximal current of 5 mA has a 0% false-negative predictive value but an unacceptable high 59% false-positive predictive value.
With such a high false-positive predictive value using 5 mA, the ideal stimulation current obviously needs to be investigated further. Based upon the results of this study, a constant current of 2.0 mA (the lower limit of the 95% confidence interval for the epidural space) may be more appropriate than the 5 mA when advancing an insulated Tuohy needle using the LOR technique with saline. By using the lower limit of the 95% confidence interval (2.0 mA), one might avoid false-positive responses at 5 mA, yet this current would still be sufficient to elicit a motor response when the needle is in proximity to nervous structures. It should be stressed that by setting the current to 2 mA, the technique should only be used to prevent intraneural or intrathecal injections and not be used as a warning to the anesthesiologist about the location of the epidural space. Thus, entry into the epidural space would still primarily be indicated by LOR. Any motor response observed when using this current (2.0 mA) would indicate that the needle has entered the epidural space (with a 95% confidence limit). This should alert the anesthesiologist to carefully check for LOR and to reevaluate or withdraw the needle altogether to avoid possible complications. Under these circumstances, further advancement of the needle should be performed with extreme caution. If, at any time, a motor response occurs at a current <1 mA, proximity to a nervous structure should be suspected, and further advancement of the needle would be ill advised. Obviously, future studies will be required to confirm the utility of this technique using the lower 95% confident threshold current.
A limitation of this study is that the experiments were performed in pigs, and the actual current range may not be directly applicable to humans. Nevertheless, the results from this investigation provide preliminary information that the upper limit confidence current is unreliable as a continuous monitoring current because of its unacceptably high false-positive predictive value. The information could be used to design further animal experiments and future clinical studies to determine the ideal ES current for monitoring the position of epidural needles during insertion.
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