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).
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.
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.
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
input group exp tot;
1 1 22
2 1 21
proc freq data=rd;weight count;
table group*test/ riskdiff(cl=exact);
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.
The authors thank Junmin Yang, MSc, for the statistical analysis.
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