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Response Patterns to the Electric Stimulation of Epidural Catheters in Pregnant Women: A Randomized Controlled Trial of Uniport Versus Multiport Catheters

Patel, Ruchira MBBS; Arzola, Cristian MSc, MD; Petrounevitch, Vitali MSc, MD; Balki, Mrinalini MD; Downey, Kristi MSc; Tsui, Ban C. H. MSc, MD; Carvalho, Jose C. A. MD, PhD

doi: 10.1213/ANE.0000000000001236
Obstetric Anesthesiology: Original Clinical Research Report

BACKGROUND: The transcatheter electric stimulation test (Tsui test) can be performed at the bedside to confirm the correct placement of a wire-reinforced epidural catheter within the epidural space. The most commonly observed motor response with a uniport epidural catheter placed in the lumbar area is the unilateral contraction of the lower limbs. Wire-reinforced multiport catheters have recently been introduced into clinical practice; however, the characteristics of the Tsui test with such catheters are unknown. We designed a randomized controlled trial to test the hypothesis that the incidence of a bilateral response to the Tsui test would be higher with a multiport catheter, with all other characteristics of the test remaining unchanged.

METHODS: We recruited laboring women requesting epidural analgesia. The epidural catheter placement was performed in a standard fashion, assisted by ultrasound, aiming at the L3-L4 interspace. Patients were randomly allocated for the placement of either a 19-G uniport or a 19-G multiport wire-reinforced catheter. The Tsui test (frequency 2 Hz; pulse width 0.2 millisecond) was performed immediately after securing the catheter (baseline) and at 5 minutes after a test dose with 3 mL lidocaine 2%. The current output was increased from zero until motor activity was detected up to a maximum of 20 mA. Subsequently, an initial loading dose of 10 mL bupivacaine 0.125% and 50 μg fentanyl was administered. The sensory block level to ice was assessed bilaterally at 20 minutes after injection of the loading dose. The primary outcome was the motor response pattern to the electric stimulation of the epidural catheter, either unilateral or bilateral; secondary outcomes included minimal current intensity needed to elicit a motor response at baseline and 5 minutes after the test dose, sensory block level and incidence of symmetrical sensory block at 20 minutes after injection of the loading dose, pain scores before the test dose and at 20 minutes after the loading dose, and need for catheter replacement within 2 hours of completion of the loading dose.

RESULTS: Sixty-three women were assessed for eligibility and 46 were randomly allocated equally to each group. Three patients were excluded, resulting in 21 subjects in the multiport group and 22 subjects in the uniport group. Patient characteristics in both groups were similar. The incidence of unilateral motor response to the Tsui test was 95.2% (20/21) and 95.5% (21/22) in the multiport and uniport groups, respectively (rate difference 0.22%; 95% confidence interval, for the difference −29.2 to 29.2%; P = 0.99). The minimal current intensity (mean ± SD) required to produce a motor response at baseline was 5.4 ± 3.5 mA and 5.4 ± 4.1 mA in the multiport and uniport groups, respectively (P = 0.98). The sensory block levels to ice on the left and right, as well as pain scores at 20 minutes, were similar in both groups. No epidural catheters were resited.

CONCLUSIONS: The Tsui test produced a high percentage of unilateral motor response in women with both uniport and multiport wire-embedded catheters. A larger study is necessary to confirm that there is no clinically significant difference in the motor response patterns between the 2 catheter types.

Published ahead of print April 22, 2016.

From the *Department of Anesthesia and Pain Management, Mount Sinai Hospital, University of Toronto, Toronto, Ontario, Canada; and Department of Anesthesiology and Pain Medicine, University of Alberta, Edmonton, Alberta, Canada.

Published ahead of print April 22, 2016.

Accepted for publication January 18, 2016.

Funding: Dr. Tsui is supported by a Clinical Scholar Award from the Alberta Heritage Foundation for Medical Research (AHFMR). Dr. Tsui’s research is supported by the Canadian Anesthesia Research Foundation. Dr. Balki is supported by a Merit Award from the Department of Anesthesia, University of Toronto.

The authors declare no conflicts of interest.

Reprints will not be available from the authors.

Address correspondence to Jose C. A. Carvalho, MD, PhD, Department of Anesthesia and Pain Management, Mount Sinai, University of Toronto, 600 University Ave, Room 19-103, Toronto, Ontario, M5G 1X5 Canada. Address e-mail to jose.carvalho@uhn.ca.

Lumbar epidural analgesia is widely believed to be the most effective form of analgesia for labor and delivery. However, epidural catheters are typically placed in a blind fashion, and failure rates in obstetric cases can be up to 12%, with up to 6.8% of catheters requiring replacement.1,2 Several methods have been proposed to identify the location of an epidural catheter tip, such as negative aspiration of the catheter for blood and cerebrospinal fluid, a test dose of local anesthetic, assessment of the clinical effect of local anesthetic solution, and confirmation by medical imaging. All these techniques suffer from incomplete reliability, safety, and accuracy.3–5

The transcatheter electric stimulation test (Tsui test) has been used to detect the proper epidural catheter location in pediatric,6 postoperative,7 and laboring obstetric patients.8 Epidural catheter placement is determined to be correct or incorrect depending on the pattern of motor response to the electric stimulation. When used to identify the correct placement of an epidural catheter for labor analgesia in 39 patients, the test showed 100% sensitivity and specificity (38 true positives and 1 true negative).8

In a 2013 observational study, Margarido et al9 performed the Tsui test in women requesting epidural analgesia for labor and found that a wide range of electric current (1–14 mA) was needed to elicit the twitch response. In addition, the most common motor responses were unilateral leg and thigh contraction, whereas bilateral lower limb contraction was seen in only 4% of patients. Similar to all previous studies published on the Tsui test, Margarido et al9 also used a uniport catheter.

Multiport wire-reinforced catheters have been recently introduced in clinical practice. A previous in vitro study of the conductive properties of multiport versus uniport catheters suggested that multiport catheters are also suitable for the Tsui test10; however, the clinical characteristics of the Tsui test performed with multiport catheters remain unknown. In theory, a multiport catheter should allow a more even distribution of conducting fluid, potentially increasing the chance of bilateral motor response.

The aim of this study was to compare the clinical response patterns to the Tsui test performed with uniport and multiport catheters. We hypothesized that the incidence of a bilateral response to the Tsui test would be higher with a multiport catheter with other characteristics of the test, including minimal current intensity required to elicit a motor response before and after a standardized test dose, remaining unchanged.

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METHODS

After receiving institutional Research Ethics Board approval from Mount Sinai Hospital, Toronto, Ontario, Canada (REB 14-0222-A; November 12, 2014), we conducted a randomized double-blind trial on laboring women requesting epidural analgesia. Written informed consent was obtained from all enrolled subjects. The trial was registered at clinicaltrials.gov under number NCT02274467, principal investigator Jose C. A. Carvalho, on October 9, 2014. We followed the CONSORT (Consolidated Standards of Reporting Trials) statement in conducting and reporting our investigation. Inclusion criteria were women older than 18 years requesting epidural analgesia for labor and delivery and who were able to communicate in English. Patients were excluded if they had a known allergy to lidocaine, bupivacaine, or fentanyl; were known to have abnormal spinal anatomy such as scoliosis or previous spine surgery; had coexisting neurological disorders; or had implanted electronic devices.

Women were recruited into the study between November 2014 and January 2015 at Mount Sinai Hospital. Participants were allocated randomly and evenly into 2 groups to receive either a 19-G uniport or a 19-G multiport wire-reinforced epidural catheter (Arrow FlexTip plus; Arrow International Inc, Reading, PA). Randomization was performed by using a computer-generated table in blocks of 6. The patient and research assistant collecting data for the study were blinded to the type of catheter used. The anesthesiologist performing the epidural procedure was not blinded and therefore not involved in data collection for the study. Spinal ultrasound was used to locate the L3-L4 epidural space before performing the epidural procedure. A midline approach for needle insertion was used with the patient in the flexed sitting position. The skin was infiltrated with 2 to 3 mL lidocaine 2%, and the epidural space was identified using loss of resistance to either air or saline technique, depending on the operator’s preference. The epidural catheter was inserted into the epidural space for approximately 5 cm and aspirated for cerebrospinal fluid or blood.

After securing the catheter, the Tsui test was performed using monopolar stimulation. The nerve stimulator (Stimpod NMS450; Xavant Technology (Pty) Ltd; Pretoria, South Africa) was connected to the epidural catheter through an adaptor (Johans ECG Adapter; Arrow International Inc). The epidural catheter and the adaptor were primed with a standard volume of 3 mL normal saline. The frequency of the nerve stimulator was set at 2 Hz with a pulse width of 0.2 milliseconds. During the performance of the Tsui test, the current output was increased from zero to a maximum of 20 mA until motor activity was observed by the practitioner performing the Tsui test. After this baseline Tsui test, an epidural test dose of 3 mL lidocaine 2% was administered through the epidural catheter. The Tsui test was repeated 5 minutes after the epidural test dose to determine the change in current intensity required to elicit a similar motor response. After the second epidural stimulation test, an initial loading dose of 10 mL bupivacaine 0.125% with 50 μg fentanyl was administered, followed by initiation of a patient-controlled epidural analgesia with bupivacaine 0.0625% plus fentanyl 2 μg/mL (10 mL/h continuous infusion; bolus dose 5 mL; lockout 10 minutes; maximal volume 20 mL/h). Sensory block to ice was determined 20 minutes after injection of the loading dose, bilaterally on the midclavicular line, starting from the sacral dermatomes and moving cephalad until the patient reported the first sensation of cold. The need for catheter replacement within 2 hours was documented.

The primary outcome was the motor response pattern of the lower limbs to the baseline electric stimulation of the epidural catheter, either unilateral or bilateral. Secondary outcomes were the minimal current intensity (mA) needed to elicit a motor response at baseline and 5 minutes after the test dose; the sensory level to ice bilaterally at the midclavicular line at 20 minutes after injection of the loading dose (bupivacaine and fentanyl); the incidence of symmetrical sensory block at 20 minutes after the loading dose (defined as a difference of <2 dermatomes between the 2 sides); the need for catheter replacement within 2 hours of completion of the loading dose; and the numeric rating pain score (from 0 to 10; 0 = no pain; 10 = worst pain imaginable) measured before the lidocaine test dose and 20 minutes after the epidural loading dose.

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Sample Size Calculation and Statistical Analysis

According to Margarido et al,9 the Tsui test elicited a unilateral response of the lower limbs in 91% of patients. In contrast, Charghi et al11 showed a unilateral response in only 52% of patients when using epidural stimulation of a Theracath catheter (Arrow International Inc), which features an exposed metallic distal tip. Assuming that a multiport catheter allows even distribution of current with a conducting pattern resembling that achieved by the Theracath catheter, we hypothesized that the incidence of unilateral motor responses using a multiport catheter would be reduced from 90% to approximately 50%. With a power of 0.8 and a type I error of 0.05, we required 20 subjects in each group to show a statistically significant difference. To compensate for exclusions and loss of data, we chose to recruit 23 subjects in each group.

Statistical analyses of categorical and continuous data were performed with the Fisher exact test and Student t test, respectively. A P value <0.05 was considered statistically significant. For continuous parametric variables, mean (SD) was reported and the difference between groups was assessed with the Student t test. For continuous nonparametric variables, median (interquartile range) was reported, and the groups were compared using the Wilcoxon rank test. For categorical variables, count (percentage) was reported and the groups were compared using the Pearson χ2 test or Fisher exact test between 2 groups. The difference in incidence rate of the primary outcome between the groups was calculated, and the confidence interval was estimated by the exact unconditional risk difference method (Appendix 1). A P value <0.05 was considered statistically significant. All analysis was conducted by SAS 9.2 (SAS Institute Inc, Cary, NC).

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RESULTS

A convenience sample of 63 women was assessed for eligibility and 46 were randomly allocated equally to each group. Three patients were excluded, resulting in 22 subjects in the uniport group and 21 in the multiport group. The patient flow diagram is summarized in Figure 1. The 2 study groups were similar in terms of demographic data (Table 1).

Table 1

Table 1

Figure 1

Figure 1

The incidence rate of unilateral motor response to the Tsui test was 95.2% (20/21) and 95.5% (21/22) (rate difference, 0.22%; 95% confidence interval for rate difference, −29.2 to 29.2%) in the multiport and uniport groups, respectively (P = 0.99).

There were no differences between groups in the current required to produce a motor response, sensory block levels, symmetry of sensory block at 20 minutes or pain scores at 20 minutes (Table 2).

Table 2

Table 2

No epidural catheter had to be replaced within 2 hours of the administration of the loading dose. All patients tolerated the Tsui test well, and no patient experienced any discomfort or harm as a result of the test.

In 1 obese patient (body mass index 43.4 kg/m2), we observed an initial positive motor response to the Tsui test at 8 mA, and subsequently at 9 mA after the lidocaine test dose. However, the patient did not develop a sensory block after the test dose and the first loading dose. Only after an additional dose of 7 mL lidocaine 2% did we observe a bilateral block at T4. Less than 2 hours after the study start, a cesarean delivery was performed because of persistent fetal bradycardia. Surgery was performed under successful epidural anesthesia after an additional injection of 15 mL lidocaine 2% with epinephrine 1:200,000 and fentanyl 50 μg.

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DISCUSSION

To our knowledge, this is the first clinical study to examine the motor response patterns to the electric stimulation of uniport and multiport wire-reinforced epidural catheters after priming with normal saline. The Tsui test showed a high percentage of unilateral motor response in women with both uniport and multiport wire-embedded catheters, and such results suggest that previously reported interpretations of the Tsui test when using uniport catheters appear applicable when using multiport catheters for the same purpose. However, a larger study is necessary to confirm that there is no clinically significant difference in the motor response patterns between the 2 catheter types.

Previous in vitro results have shown that metal-reinforced multiport catheters primed with saline have significantly lower electric impedance than uniport catheters.10 The underlying mechanism of this response is likely that the additional holes in the multiport catheter allow multiple metal contact points for the conducting saline, thereby decreasing electric impedance compared with catheters with a single end hole. According to our results, however, these differences between multiport and uniport catheters do not appear to be associated with differences in the results of the Tsui test, although a larger study is necessary to confirm such results. Furthermore, although the multiport catheter used in our study is suitable for the Tsui test, it differs substantially from the one used by Charghi et al11 and may explain the different findings in each study.

Our study showed that the incidence of a unilateral response to the Tsui test was 95.5% and 95.2% in the uniport and multiport groups, respectively. These results contrast to those obtained by Charghi et al11 that showed a 52% incidence of unilateral lower limb motor response when using the Tsui test for thoracic epidural placement, which is considerably lower than that in the current study or previous observations from our group.9 Although the reasons underlying this difference are not fully understood, we suspect that it may be in part because of the type of catheter used in each study. Charghi et al used a catheter containing a removable metal stylet with 1 cm of exposed metal surface tip, which may have affected electric conduction by allowing the current to be dispersed with 360° coverage. Furthermore, the catheter used by Charghi et al may have displayed different impedance compared with the one used in our study, which may have resulted in differences in electric conduction. Only catheters with metal components show low electric impedance, such that they are capable of conducting the current required for a successful Tsui test. In contrast, conventional epidural catheters (nylon, polyurethane) that lack metal elements, regardless of the number of distal end holes, are unsuitable for the stimulation test.

Another factor that could account for the difference in incidence between the 2 studies is pulse width: in the study by Charghi et al, a pulse width of 1.0 millisecond was applied, whereas we used a pulse width of 0.2 milliseconds. Indeed, it has previously been shown that increasing pulse width can potentially stimulate a nerve some distance away from the stimulating electrode.12 This may account for the higher incidence of bilateral stimulation, and further studies may be warranted to address this factor. A final difference is that lumbar epidural catheters were placed in our study versus thoracic catheters in the Charghi et al study. Thoracic epidural catheters are commonly placed using paramedian approaches, which in theory may lead to more unilateral catheter placement and stimulation compared with lumbar epidural catheters placed via a midline approach. However, the result of this study did not support this notion. In contrast, although the underlying reason was unknown, it seemed that unilateral stimulation occurred more often with lumbar epidural catheters than previously reported with thoracic catheters.

In theory, multiport epidural catheters provide an advantage over uniport catheters because additional ports may allow for enhanced distribution of the local anesthetic solution. This has been shown to result in a lower incidence of inadequate analgesia, including unilateral sensory blockade and missed sensory segments.13–18 In all studies13–16,18 but the one by Jaime et al17 (where a wire-reinforced uniport open-ended catheter was used), nonwire-reinforced catheters were used. As wire-reinforced multiport catheters become available for clinical use, it is important that these characteristics are investigated and confirmed for accuracy. Although our study has a relatively small sample size, we observed no significant difference in clinical outcomes between the uniport and the multiport wire-reinforced catheters. In support, a previous study by Spiegel et al19 found that epidural analgesia in actively laboring women was similar, regardless of whether a flexible wire-reinforced multiport or uniport catheter was used.

In clinical practice, an additional “top-up” bolus of local anesthetic is occasionally administered via epidural catheter in an attempt to establish solid and rapid anesthesia for cesarean delivery before one has sufficient time to establish and confirm epidural analgesia. In this study, we encountered such a situation in one specific obese patient within 2 hours of commencing the epidural. The unusual aspect of this isolated case was that this patient did not develop typical clinical block characteristics after injection of the standard loading dose. Normally, we would be hesitant to inject further boluses of local anesthetic because of concerns about the placement of the catheter, and these situations would often lead to abandoning the epidural catheter. Fortunately, we observed a positive Tsui test in this patient, which from our experience and from reports in the literature suggesting high positive predictive value of the test,6–8,11,20 reassured us that further loading of local anesthetic was warranted. As anticipated, the epidural catheter was later proven to be functioning, and we were able to convert to surgical anesthesia without any difficulty. In retrospect, we believe that, without the positive Tsui test, we would have had to repeat epidural catheter placement or abandon it and switch to general anesthesia.

In summary, the results of this study suggest that the Tsui test can be applied effectively and successfully by using either uniport or multiport wire-reinforced epidural catheters. However, a larger study is necessary to confirm that there is no clinically significant difference in the motor response patterns between the 2 catheter types. At the authors’ institutions, staff members use the Tsui test routinely after placement of an epidural catheter. We hope that the information from this study will facilitate more widespread use of the Tsui test because it is often limited by equipment and knowledge of those performing it.

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APPENDIX 1

The following SAS command was used to calculate the 95% confidence interval of the difference in incidence of motor response between the 2 types of catheter

data rd;

 input group exp tot;

 test=‘Y’;count=exp;output;

 test=‘N’;count=tot-exp;output;

 datalines;

 1 1 22

 2 1 21

;

proc freq data=rd;weight count;

 table group*test/ riskdiff(cl=exact);

 exact riskdiff;

run;

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DISCLOSURES

Name: Ruchira Patel, MBBS.

Contribution: This author contributed to data collection, data analysis, and manuscript writing.

Name: Cristian Arzola, MSc, MD.

Contribution: This author contributed to study design, data collection, data analysis, and manuscript writing.

Name: Vitali Petrounevitch, MSc, MD.

Contribution: This author contributed to study design and data collection.

Name: Mrinalini Balki, MD.

Contribution: This author contributed to study design, data collection, data analysis, and manuscript writing.

Name: Kristi Downey, MSc.

Contribution: This author contributed to study design, data collection, and data analysis.

Name: Ban C. H. Tsui, MSc, MD.

Contribution: This author contributed to study design, data analysis, and manuscript writing.

Name: Jose C. A. Carvalho, MD, PhD.

Contribution: This author contributed to study design, data collection, data analysis, and manuscript writing and is the archival author.

This manuscript was handled by: Cynthia A. Wong, MD.

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ACKNOWLEDGMENTS

The authors thank Junmin Yang, MSc, for the statistical analysis.

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