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Departement d’Anesthesiologie Centre Hospitalier Universitaire de Sherbrooke, Sherbrooke, Quebec, Canada
Address correspondence and reprint requests to Etienne de Medicis, MD, MSc, Departement d’Anesthesiologie Centre Hospitalier Universitaire de Sherbrooke 3001, 12 E. Avenue Nord Sherbrooke, Quebec, Canada J1H 5N4. Address e-mail to estria1{at}globetrotter.net.
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Abstract |
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Introduction |
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The confirmation of the appropriate localization of epidural catheters by epidural nerve stimulation (5) and epidural pressure waveform (6) has been described. These tests are attractive because they can be performed within 2 min in an OR without the use of extensive or cumbersome equipment. We prospectively compared both methods in the surgical population.
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Methods |
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After the patient entered the OR, before surgery, the epidural catheter was placed by the staff anesthesiologist in charge of the case or this anesthesiologist’s resident. Sedation, the site of epidural catheterization and the epidural space approach (median or paramedian) were left to the discretion of the attending anesthesiologist. The epidural space was localized by using a loss of resistance technique. All catheters used were the Arrow Flex Tip Plus® with the Arrow Johans electrocardiogram (ECG) Adapter®. After the catheter was inserted 5 cm into the epidural space, 5 mL of 0.9% saline was injected in the catheter and both the epidural nerve stimulation (EST) and epidural pressure waveform analysis (EPWA) were performed randomly according to a previously determined random order. After the 2 tests were computed, 3 mL of 2% lidocaine + epinephrine 1/200,000 was injected as a test dose. If the test dose was negative, an additional 2 mL of the solution was injected for thoracic epidural and 5 mL for lumbar epidural. Five minutes later, sensation for pinprick in the dermatome of the epidural insertion level was tested for confirmation of the correct placement of the epidural catheter. If the attending anesthesiologist, unblinded to results of the tests, felt confident about the epidural catheter placement, the case would start and the patient would be evaluated the next day by our acute pain team to confirm appropriate postoperative analgesia. If the attending anesthesiologist believed that the catheter was not in the epidural space, then either the epidural catheter was replaced or an alternative analgesic technique was initiated, at which point the placement of the epidural would be considered negative (not in the epidural space) and the study would be finished. These patients were included in the final analysis.
The EST was performed as previously described (7). After epidural catheter priming, the cathode was connected to the metal hub of the adapter, and the anode was connected to the patient’s skin. The stimulator was set at a frequency of 1 Hz with a pulse width of 0.2 ms. The current output was increased until a segmental motor response was visualized in a dermatome congruent with the epidural insertion level. A segmental unilateral motor response <1 mA or a segmental response (unilateral or bilateral) 1–10 mA was considered positive for adequate epidural space catheterization. Current of up to 15 mA was used to evaluate the response.
The EPWA was performed by connecting a transducer apparatus to the epidural catheter, leveled to the insertion site of the catheter. The test was considered positive for epidural catheter placement in the epidural space when positive pressure waveform deflections, suggestive of arterial pulsation transmitted by the cerebrospinal fluid pushing the dura mater, were seen on the monitor screen in synchrony with cardiac contractions (either by ECG or pulse oximetry). No minimal value was set for the positive waveform deflections.
The primary data were the results of EST and EPWA compared with the clinical response of the patient to the epidural catheter. Block randomization was done by an independent person using a random number table. Assuming that the EST would have a sensitivity of 90%, 214 patients were required to show that both tests are equivalent within 10% of each other with an 
coefficient of 0.05 and a ß coefficient of 0.20. An interim analysis after 107 patients, supervised by an independent statistician, led to an adjustment of the total number of patients to 218. Additional secondary data considered were the patient’s sex, age, body mass index, level of epidural site (thoracic versus lumbar), and approach used (median versus paramedian). Each method was analyzed for secondary data by 
2 or Fisher’s exact probability test. Paresthesia or other complaints were also noted if they occurred as part of any test. Sensitivity, specificity, and positive and negative predictive values (PPV and NPV) were calculated for each technique.
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Results |
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There were no significant differences detected between the 2 techniques (Table 1). There were no false positive tests; hence the PPV was 100%. There were a significant number of false negatives in both methods, hence the NPV were <20%. Both methods had sensitivities of around 80%.
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Sixteen patients (7.35%) with segmental motor response at a current more than 10 mA noted sensory response at a current <10 mA in the appropriate dermatome before the apparition of the motor response. Including those patients as positive and adding them to the patients who met the usual EST criteria improved the sensitivity to 87% (P < 0.05); the NPV remained low, at 23% (modified EST [MEST]). Combining both methods (i.e., a positive result in either EST or EPWA) gave a 97% sensitivity and a 57% NPV (EST + EPWA) (P < 0.001) (Table 2).
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Comparing both methods for level of epidural (thoracic versus lumbar), approached used (median versus paramedian), sex, age and body mass index yielded no significant result except for the EPWA versus age. The sensitivity increased from 63% for patients aged 20–39 to 73% for patients 40–59, to 85% patients aged 60–79, and to 94% for patients older than 79 yr (P = 0.03).
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Discussion |
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We reported a sensitivity of 80% in this prospective cohort of 218 patients using Tsui et al.’s (9) motor response to electrical stimulation criteria. We injected 5 mL of 0.9% NaCl, contrary to only 0.2–1.0 mL, as was used in Tsui et al.’s studies. This could have had an effect of current dispersion and decreased the sensitivity of the test (9).
We used standard neurostimulators (Digistim 3PLUS or III) that were different in different ORs. We do not know if Tsui et al. used the same neurostimulator for every patient in each study.
The sensitivity of the EST was improved to 87% using the modified criteria: the same motor response as the previously published sensory response in the appropriate dermatome with a current output <10 mA if the motor response was seen at a current more than 10 mA.
The EPWA technique has not been evaluated in a prospective study. It has been published in a single retrospective study (the result of analgesia already known) comparing EPWA and computed tomography (CT) scan contrast cathetergram (6). It showed a perfect correlation between EPWA and the cathetergram: 5 patients had a functioning epidural catheter with positive EPWA and confirmation of the localization of the catheter in the epidural space on the CT scan study, 13 patients with failed epidural analgesia had negative EPWA and localization of their catheter outside the epidural space, with contrast in the paraspinous muscles on the CT scan study. The test relies on the catheter conducting the pressure wave generated by the pulsating dura mater. We used the same amount of 0.9% saline as in the first report. More studies are needed to define the optimal amount of fluid to be injected.
Sensitivity of EPWA increases with age. This may be a result of the reduction of the epidural space and closure of the intervertebral foramina occurring with older age causing a decreased compliance of the epidural space and a better pulse wave transmission (10).
Using both methods increases sensitivity significantly to 97%. However, 71 (32%) of the pairs tested did not match. Calculating Cohen’s Kappa (an index of inter-rater reliability for categorical/qualitative variables), we found a value of 0.07, showing a very poor correlation between tests (generally, Kappa > 0.70 is considered satisfactory). We do not have an explanation concerning the lack of concordance between tests.
Both techniques had very poor NPV, approximately 16%. This is in the context of the loss of resistance technique for localizing the epidural space. Our success rate was 96.2% with this technique, on par with previously published studies (2,3). Using both methods, the NPV increased to 57% (8/14). All false negatives were found by the 5 minute lidocaine test dose test.
A major limitation of this study is that it was not blinded. The organization of our OR made the availability of three assessors plus the attending anesthesiologist impossible to respect blinding. Tsui et al.’s studies were not blinded. Their criteria are objective. We always looked for the minimum current for motor stimulation. Different body habitus and level of epidural insertion (abdominal versus thoracic dermatome) may influence the results of EST, as the motor response may not be as visible from one patient to another. We did not find any correlation between EST and level of epidural insertion or body mass index. For EPWA, there is no published minimum waveform deflection value, so there may be some subjectivity in the assessment (present or not). Obviously, the scale of the monitoring screen can have an impact on the visualization of the waves, as most of the epidural pressures are measured around 20–30 mm Hg. Depending on the patient, determination of sensory blockade may be unclear. Also, because we do not have an area where blocks and epidurals can be performed ahead of time, we needed a practical and rapid protocol to avoid unduly delaying procedures. This is why we chose a 5-minute interval between the lidocaine test dose injection and pinprick testing. A 20-minute time interval may have been more appropriate.
This study was designed to compare the ability of EST and EPWA to predict adequate epidural space catheterization. Adequate epidural postoperative analgesia is not only dependent on adequate catheter location but also on the epidural drug regimen used (11).
In summary, we prospectively compared EST and EPWA in 218 surgical patients. There were no false positive results, but the tests were comparable, with sensitivities of around 80%. The sensitivity of EST could be increased to 87% by including sensory stimulation with a current output <10 mA in patients with negative EST. We suggest the inclusion of sensory stimulation in the appropriate dermatome at current <10 mA as a criterion for adequate epidural catheter localization for EST testing. Combining both tests increased the sensitivity to 97%. The sensitivity of EPWA increases with the age of the patient.
We thank Theophile Niyon Senga, PhD, for his interim analysis.
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Footnotes |
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Accepted for publication June 28, 2005.
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This article has been cited by other articles:
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  J. G. Forster, T. T. Niemi, M. T. Salmenpera, S. Ikonen, and P. H. Rosenberg An Evaluation of the Epidural Catheter Position by Epidural Nerve Stimulation in Conjunction with Continuous Epidural Analgesia in Adult Surgical Patients Anesth. Analg., January 1, 2009; 108(1): 351 - 358. [Abstract] [Full Text] [PDF]
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B. C. H. Tsui Epidural stimulation test criteria. Anesth. Analg., September 1, 2006; 103(3): 775 - 776. [Full Text] [PDF]
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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]
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