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Anesth Analg 2004;99:694-696 © 2004 International Anesthesia Research Society doi: 10.1213/01.ANE.0000130617.79600.05
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Abstract |
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We designed this study to determine the threshold current for nerve stimulation of an insulated needle in the epidural space. The intended dermatome was identified using the bony landmarks of the spine. An 18-gauge insulated Tuohy needle was inserted perpendicularly to the skin and advanced until "loss of resistance" was felt. A nerve stimulator was then connected to the insulated needle. Twenty patients were studied using an insulated Tuohy needle and one patient was studied using a noninsulated Tuohy needle. Muscle twitch was elicited with a current of 11.1 ± 3.1 mA (mean ± SD) in all patients in which an insulated needle was used. Muscle twitches were within 2 myotomes of the intended level (based on bony landmarks). Muscle twitch was not elicited with a noninsulated needle. After catheter threading, positive stimulation tests were elicited via epidural catheters in all patients (4.9 ± 2.3 mA). Postoperative radiograph confirmed all catheter placements within 2 myotomes of the muscle twitches. Electrical stimulation may be a useful adjuvant tool to loss of resistance for confirming proper thoracic epidural needle placement. The threshold current criteria for an insulated needle (6–17 mA) would be higher than the original Tsui test criteria described for an epidural catheter (1–10 mA) in the epidural space.
IMPLICATIONS: Electrical stimulation may be a useful adjuvant tool for confirming proper thoracic epidural needle placement. The threshold current for an insulated needle is higher than the original Tsui test criteria described for an epidural catheter in the epidural space.
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Introduction |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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Electrical stimulation (ES) has proven to be an effective technique for confirming epidural catheter and caudal needle placement (1–7). These studies have demonstrated that a low current applied through an insulated needle or a catheter can elicit motor activity only if the needle is within the epidural space. Otherwise, a much higher current will be required to produce a similar motor response. To our knowledge, this is the first study designed to determine the stimulation threshold of an insulated needle in the epidural space.
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Methods |
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After institutional ethics approval and written informed consent from patient’s guardian, 21 pediatric patients (ASA physical status I–III) were enrolled. After induction of anesthesia and endotracheal intubation without muscle relaxant (sevoflurane 6% or IV propofol 3–5 mg/kg combined with remifentanil 2–3 µg/kg), the appropriate dermatome was identified using the bony landmarks of the patient’s spine in a lateral position. After skin puncture with an 18-gauge needle, an 18-gauge insulated Tuohy needle (Pajunk; Dyna Medical Corp., London, Ontario, Canada) was inserted and advanced until loss of resistance (LOR) to saline was detected. The negative lead of the nerve stimulator (Maxistim; Life-Tech Inc., Stafford, TX) was then connected to the needle. The positive lead was attached to a surface electrocardiogram electrode placed remotely on the patient’s trunk. The stimulating current (1 Hz; 0.2 ms) was gradually increased from zero (up to 20 mA) until motor activity was visible at the corresponding myotome. An epidural catheter (Epidural Positioning System using Tsui test with FlexTip Plus Catheter, TS-05430-P; Arrow International, Inc., Reading, PA) was inserted via the needle (Fig. 1). The final catheter position was confirmed with ES and postoperative radiograph. The definition of technical success was the placement of an epidural catheter in a satisfactory location as assessed by the attending anesthesiologist.
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Statistical analysis was done using a paired t-test to compare mean threshold current between insulated needles and epidural catheters. A P level of < 0.05 was considered statistically significant.
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Results |
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Twenty patients, aged 1.6–16.6 yr, were studied using insulated Tuohy needles. We excluded 1 patient, aged 13, in whom a regular noninsulated Tuohy needle was used. This single trial with the noninsulated needle did not result in any muscle twitches. In this patient, a positive motor response was subsequently elicited with ES using a current of 4 mA via the epidural catheter.
Of the 20 patients studied with insulated needles (Fig. 2), the mean threshold current was 11.1 ± 3.1 mA (range, 6–17 mA). Fourteen cases had unilateral, visible, and palpable twitches and six patients had bilateral, visible, and palpable twitches. In one case, the needle was inserted to a depth at which LOR was felt and a stimulated muscle response was obtained at 15 mA. The needle needed to be advanced another 1 mm before the catheter could be passed with ease. After the needle advancement, the threshold current decreased to 12 mA. All muscle twitches were observed within 2 myotomes of the intended level. Positive catheter placement was confirmed in all patients by easy threading into the epidural space and the presence of muscle twitches with mean current of 4.9 ± 2.3 mA (range, 1.5–10 mA) with catheters stimulation. Radiological confirmation of the epidural catheter position was within 2 myotomes of the level demonstrated by stimulation. The technical success rate was 100%.
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Discussion |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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The mean threshold current required to elicit a motor response was significantly higher with insulated needles than with epidural catheters, 11.1 and 4.9 mA, respectively. This may be attributed to the needle tip being placed further away from the nerve roots than the tip of a threaded epidural catheter. With the LOR technique, the Tuohy needle often just punctures through the ligamentum flavum and barely enters the epidural space. In contrast, threaded epidural catheters are totally embedded in the epidural space. Although the range of the threshold currents for insulated needles in the epidural space seems relatively large, there are possible physiological reasons for these observations. Clinical and experimental data on peripheral nerve block demonstrate that the threshold current increases substantially with increasing distance from the nerve (8). The distance between the epidural needle and spinal nerve root can vary depending on patient size, entry site, needle angle, and approach. This accounts for the relatively large range of stimulating currents observed when needles are distant from neurological structures.
A previous study of caudal needle placement demonstrated that an average of 3.7 mA was required to elicit a motor response in the caudal space (4). This is significantly less than the mean current found to be required by insulated needles in the thoracic region (11.1 mA), but not significantly different from the mean current required by the epidural catheters (4.9 mA). This discrepancy may be explained on the basis of the depth of needle insertion into the epidural space. The extra caution taken when placing thoracic epidural needles using LOR may make it more likely for the thoracic epidural needle to have just barely entered into the epidural space. In contrast, caudal needles after passing through the sacrococcygeal ligament are usually advanced several millimeters into the caudal canal at an angle parallel to the sacrum. Thus, caudal needles are placed more deeply within the epidural space and thus require a lower current to elicit a motor response.
ES may help alleviate concerns when one decides to try advancing a needle further after initial placement by LOR in order to facilitate the passage of a "difficult to thread" epidural catheter. In the case observed in this study, the needle insertion ceased after initial LOR and a stimulated muscle response was obtained at 15 mA. At this distance, the catheter would not pass through the distal end of the Tuohy needle. When this happens, it is suspected that the orifice of the curved tip of the epidural needle is straddling the ligamentum flavum (9). Clinically, it is not uncommon practice for an anesthesiologist to advance a Tuohy needle an additional 1 mm or less under these circumstances. In this patient, the high threshold current provided additional reassurance that the needle was likely to be well removed from any nerve structures. Therefore, the needle was advanced another 1 mm and the catheter was subsequently passed with ease. After needle advancement, the threshold current diminished to 12 mA. Although ES may serve as an additional objective sign, there is no substitute for practical experience coupled with a sound knowledge of the anatomy of the epidural space when making the clinical judgment to further advance an epidural needle after LOR.
Of the 20 patients studied with insulated needles, all muscle twitches were observed within 2 myotomes of the intended level. Two studies have shown that the estimated intended level as determined by surface landmarks can differ from the actual spinal level by one or two spaces (10,11). Therefore, it is not uncommon for the intended spinal level as determined by an anesthesiologist using surface anatomy to be inaccurate. Assuming the stimulation occurs at the nerve root immediately adjacent to the needle, the discrepancy observed in this study can be explained by the inherent inaccuracy of using surface landmarks to determine spinal levels.
In this study, one patient was excluded when a regular noninsulated Tuohy needle was used. Trial stimulation through the noninsulated needle did not result in any muscle twitches but the catheter was successfully threaded into the epidural space and produced a catheter-transmitted muscle twitch threshold of 4 mA. It is well documented that noninsulated needles require more current to stimulate nerves than do insulated needles. This may explain why no motor response was observed in this case with the noninsulated needle.
In conclusion, this study establishes the threshold current Tsui test criteria for an insulated needle in the epidural space (6–17 mA). This is higher than our established criteria for an epidural catheter (1–10 mA) in the epidural space (7). Readers are cautioned that, because of the small number of patients studied, the milliamperage current settings for insulated needles are intended as guidelines and may require adjustment as experience increases.
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Acknowledgments |
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Supported in part by Education and Research Fund, Department of Anesthesiology and Pain Medicine, University of Alberta Hospitals, Edmonton, Canada, and Clinical Investigatorship Award, Alberta Heritage Foundation for Medical Research, Alberta, Canada.
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Footnotes |
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Presented in part at the annual meeting of the Canadian Society of Anesthesiologists, Ottawa, Ontario, June 20–24, 2003.
BCHT has a patent license agreement (US patent #6190370) with Arrow International Inc. (Reading, PA) for the epidural kit described in the article.
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References |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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- Tsui BCH, Seal R, Koller J, et al. Thoracic epidural analgesia via the caudal approach using nerve stimulation in pediatric patients undergoing fundoplication. Anesth Analg 2001; 93: 1152–5.
[Abstract/Free Full Text] - Tsui BCH, Gupta S, Emery D, Finucane B. Detection of subdural placement of epidural catheter using nerve stimulation. Can J Anaesth 2000; 47: 471–3.[Web of Science][Medline]
- Tsui BCH, Guenther C, Emery D, Finucane B. Determination epidural catheter location using nerve stimulation with radiological confirmation. Reg Anesth Pain Med 2000; 25: 306–9.[Web of Science][Medline]
- Tsui BCH, Tarkkila P, Gupta S, Kearney R. Confirmation of caudal needle using nerve stimulation. Anesthesiology 1999; 91: 374–8.[Web of Science][Medline]
- Tsui BCH, Gupta S, Finucane B. Detection of subarachnoid and intravascular epidural catheter placement. Can J Anaesth 1999; 46: 7: 675–8.[Web of Science][Medline]
- Tsui BCH, Gupta S, Finucane B. Determination of epidural catheter placement using nerve stimulation in obstetric patients. Reg Anesth Pain Med 1999; 24: 17–23.[Web of Science][Medline]
- Tsui BCH, Gupta S, Finucane B. Confirmation of epidural catheter placement using nerve stimulation. Can J Anaesth 1998; 45: 640–4.[Web of Science][Medline]
- Raj PP, Rosenblatt R, Montgomery SJ. Use of the nerve stimulator for peripheral blocks. Reg Anesth 1980; 5: 14–21.
- Cousins MJ, Veering BT. Epidural neural blockade. In: Cousins MJ, Brienbaugh PO, eds. Neural blockade in clinical anesthesia and management of pain. Philadelphia: Lippincott-Raven, 1998: 243–320.
- Broadbent CR, Maxwell WB, Ferrie R, et al. Ability of anaesthetists to identify a marked lumbar interspace. Anaesthesia 2000; 55: 1122–6.[Web of Science][Medline]
- Render CA. The reproducibility of the iliac crest as a marker of lumbar spine level. Anaesthesia 1996; 51: 1070–1.[Web of Science][Medline]
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