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The Electrical Properties of Epidural Catheters: What Are the Requirements for Nerve Stimulation Guidance?

Section Editor(s): Barker, Steven J.Tamai, Hisayoshi MD; Sawamura, Shigehito MD, PhD; Atarashi, Hidenao BS; Takeda, Kenji MD; Ohe, Kazuhiko MD, PhD; Hanaoka, Kazuo MD, PhD

doi: 10.1213/01.ANE.0000152641.01320.AC
Technology, Computing, and Simulation: Research Report

We designed the present study to investigate the electrical resistance of commercially available epidural catheters and to search for products and procedures suitable for nerve stimulation-guided insertion. Four types of epidural catheters were evaluated: 2 nonwire-reinforced catheters (19-gauge and 20-gauge nylon) and 2 wire-reinforced catheters (19-gauge without stylet and 20-gauge with stylet). The resistance of a catheter was calculated from the voltage level proportional to the fixed resistance in series circuit. In case of physiologic saline, the resistance of nonreinforced catheters was more than 700 kΩ, whereas the wire-reinforced catheter was 14.4 ± 0.20 kΩ without stylet and 10.1 ± 0.42 kΩ with stylet. When the stylet was passed through a 20-gauge nylon catheter, the resistance decreased to 49.2 ± 1.96 kΩ. When catheters were primed with 10% hypertonic saline, the resistance of both nonreinforced catheters decreased by one third compared with physiologic saline. The electrical resistance of the saline-filled epidural catheters significantly differed among products tested. We conclude that epidural catheterization that is guided by electrical stimulation should be performed only with catheters equipped with spiral stainless steel wire reinforcement or with a stainless steel stylet.

IMPLICATIONS: We examined several commercially available epidural catheters for their suitability for nerve stimulation guided insertion and concluded that this procedure requires a catheter with stainless steel wire reinforcement or a stainless steel stylet.

Department of Anesthesiology, The University of Tokyo, Tokyo, Japan

Supported, in part, by the fund of the Department of Anesthesiology, The University of Tokyo.

Accepted for publication November 16, 2004.

Address correspondence and reprint requests to Hisayoshi Tamai, MD, Department of Anesthesiology, The University of Tokyo, 7–3–1, Hongou, Bunkyou-ku, Tokyo, 113–8655, Japan. Address e-mail to

Epidural analgesic methods are widely performed for postoperative pain control and pain therapy. There have been some reports in which the position of the epidural catheter tip was confirmed by monitoring the contraction of skeletal muscle using nerve stimulators (i.e., Tsui test) (1–3). This method is especially useful in pediatric epidural catheterization via the caudal approach and is expected to reduce the risk of neurological damage during catheter insertion under general anesthesia (2,3). However, all previous reports concerning the Tsui test were performed using stainless steel wire-reinforced catheters (1–3). According to the Tsui test, 10 mA direct current should be sufficient to cause muscle contraction. An ordinary electrical nerve stimulator yields a maximal 400 V output in clinical use. Therefore, by definition (i.e., V = IR), the resistance of an epidural catheter must be <40 kΩ to yield a 10-mA current.

Using a simplified electrical model, we have examined the resistances of four types of epidural catheters to determine whether these catheters were suitable for the Tsui test (1).

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Four types of epidural catheters were evaluated: 2 without stainless steel coiling (19-gauge Epineed®; Terumo, Inc. and 20-gauge Perifix®; B. Braun, Inc.) and 2 with stainless steel coiling (19-gauge FlexTip Plus® and 20-gauge FlexTip Plus® with a stylet; Arrow International, Inc.). The 20-gauge Flex Tip Plus with stylet was developed for pediatric caudal placement. The materials and characteristics of each catheter are shown in Table 1.

Table 1

Table 1

The experimental apparatus is shown in Figure 1. The epidural catheter being tested was filled with either physiologic saline or hypertonic saline and the proximal end was connected to the anode of an electrical stimulator (SEM-4201, Nihon Kohden, Japan) with an electrocardiograph (ECG) adapter (Johans ECG adapter, Arrow International, Inc.). The distal end of the catheter was inserted into a U-shaped tube (8.0 mm inner diameter) filled with physiologic saline and the tip was positioned 1 cm from the metal electrode. The metal electrode was connected to a fixed resistor (100 kΩ, Tyco Electronics, Inc., UK) that was connected to the cathode of the electrical stimulator. Figure 2 shows the electrical circuit of the experimental apparatus. R1 indicates the resistance of the epidural catheter and R2 indicates the fixed resistance of 100 kΩ. When the electrical stimulator produced current (I), voltages were produced across the R1 and R2 resistance units (V1 and V2, respectively). The epidural catheter resistance was calculated from the equation R1 = (V1/V2) × R2. For each measurement, direct current (maintained at 100 V) was applied by an electrical stimulator having a 1-ms rectangular wave. V1 and V2 were measured with a two-channel oscilloscope (TekScope THS 710, Tektronix, Inc., Japan).

Figure 1

Figure 1

Figure 2

Figure 2

Initially, the resistance of 4 types of catheters primed with physiologic saline was measured. Then the effect of catheter tip configuration was investigated; 20 mm of the 20-gauge nylon catheter (originally multihole, closed-tip) was removed to make it a single-hole, open-tip catheter, and its resistance was measured. The effect of the stainless steel stylet was then investigated. The stainless steel stylet, from a 20-gauge wire-reinforced catheter, was inserted into a 20-gauge nylon catheter and the resistance was measured. For this purpose, the 20-gauge nylon catheter was cut to approximately 20 cm so that the distance from the tip of the stylet to the tip of the catheter was 25 mm, as in the original styletted catheter. Furthermore, the influence of the distance between catheter tip and stylet tip on resulting resistance was examined.

Finally, the resistance of catheters primed with 10% hypertonic saline was investigated for two types of nonwire-reinforced catheters (19-gauge and 20-gauge nylon).

In each experiment, 5 catheters were examined and results were expressed as mean ± sd. Data between the two groups were analyzed by Student’s t-test and P < 0.05 was considered statistically significant.

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Calculated resistance are shown in Table 1. Two types of nonwire-reinforced catheters showed very high resistance: the 19-gauge nylon catheter had a resistance of 754.9 ± 14.5 kΩ and the 20-gauge nylon catheter had a resistance of 974.7 ± 58.3 kΩ. Conversely, catheters with stainless steel coiling showed considerably less electrical resistance: the 19-gauge wire-reinforced catheter had a resistance of 14.4 ± 0.20 kΩ. Furthermore, the wire-reinforced catheter with stainless stylet yielded a smaller resistance (10.1 ± 0.42 kΩ). However, when the stylet was removed from this catheter, catheter resistance increased to almost the same level as for the 19-gauge wire-reinforced catheter.

When catheter tip configuration was changed from multihole and closed end to single open end, the catheter resistance decreased significantly from 974.7 ± 58.3 kΩ to 865.4 ± 70.7 kΩ.

A stainless steel stylet was examined in the 20-gauge nonwire-reinforced catheter. When the 20-gauge nylon catheter was adjusted to fit the stylet length, the resistance measured 782.8 ± 19.5 kΩ. When the stylet was passed through the adjusted catheter, resistance decreased to 49.2 ± 1.96 kΩ. There was a high correlation between catheter-stylet tip distance and catheter resistance (r = 0.989), as the resistance decreased proportionally as the distance decreased (Fig. 3).

Figure 3

Figure 3

When physiologic saline was exchanged for hypertonic saline during the priming step, the resistance of the two types of nonwire-reinforced catheters, i.e., 19-gauge and 20-gauge nylon, were significantly decreased to 226.6 ± 14.0 kΩ and 326.1 ± 11.8 kΩ, respectively.

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In general, it is difficult to measure pulse waved direct current immediately in milliamperage, and thus catheter resistance cannot be calculated using the direct current value applied by nerve stimulators. Although our experimental method shown in Figures 1 and 2 is simple, catheter resistance can be calculated using stable electric current, and voltage levels can be easily and accurately measured with oscilloscopes.

Once primed with physiologic saline, catheter resistance actually includes a 1-cm saline portion, as shown in Figure 1. When electric stimulation of a nerve is applied in a clinical setting the tip of the catheter must complete an electrical circuit with the nerve. In the Tsui test, saline is injected through the epidural catheter before electrical stimulation. Our method has a 1-cm saline portion, shown in Figure 1, to account for the resistance between the catheter and the nerve in situations where the two do not touch.

The two wire-reinforced epidural catheters, 19-gauge without stylet and 20-gauge with stylet, showed very low resistances, 14.4 ± 0.20 kΩ and 10.1 ± 0.42 kΩ, respectively, compared to the nonreinforced catheters. This shows that the stainless steel coiling in the catheter is in electrical contact with the physiologic saline in the lumen, thus completing the electrical circuit.

When electric stimulation is performed in a patient, additional resistance must be considered, that is, the resistance of the human body, estimated between 1–6 kΩ (assume 6 kΩ for future reference) (4). If we apply the Tsui test with the 19-gauge wire-reinforced catheter (14.4 kΩ) in a patient, the stimulator needs a 204 V output [(14.4 + 6) × 10] to obtain a 10-mA current. Nerve stimulators can achieve this output and previous clinical studies have been accomplished using these wire-reinforced catheters. However, 2 types of nonreinforced catheters, i.e., 19-gauge and 20-gauge nylon, showed extremely high resistances: 754.9 ± 14.4 kΩ and 974.7 ± 58.3 kΩ, respectively. If the output of a stimulator is maximally set at 400V it cannot yield even a 1.0-mA current with a saline filled nonreinforced catheter, and the Tsui test requires currents of 1 mA to 10 mA. When the tip portion of the 20-gauge nylon catheter is modified to have a single open end, its resistance decreases significantly (from 974.7 ± 58.3 kΩ to 865.4 ± 70.7 kΩ), but this still has no clinical utility.

Introducing a stainless steel stylet through a 20-gauge nylon catheter causes the resistance to decrease to 49.9 ± 1.96 kΩ. For clinical use, a stimulator would need a 550-V output to yield a 10-mA current. However, cutting the catheter such that the distance between the tip of the stylet and the tip of the catheter is 17.4 mm reduces the resistance further to 20 kΩ, making it suitable for clinical use (Fig. 3). Because resistance is proportional to tip distance and inversely proportional to the cross-sectional area (5), the saline connection through the narrow catheter causes very high resistance.

When we used 10% hypertonic saline for the priming step in the 20-gauge nylon catheter, resistance was reduced from 974 ± 58.3 kΩ to 326 ± 11.8 kΩ. For the 19-gauge nylon catheter, the resistance also decreased approximately one third but was too high to use clinically.

In summary, we have examined several commercially available epidural catheters with respect to their suitability in nerve stimulation-guided insertion and have concluded that this procedure requires either a catheter with stainless steel coiling or a stainless steel stylet.

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