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REGIONAL ANESTHESIA

The Mechanical Properties of Continuous Spinal Small-Bore Catheters

Engelbert Deusch, MD^*, Justus Benrath, MD^*, Lukas Weigl, PhD^*, Konrad Neumann, PhD, and Sibylle A. Kozek-Langenecker, MD^*

*Department of General Anesthesiology and Intensive Care-B, Vienna Medical University, General Hospital Vienna, Vienna, Austria; and Charité-University Medicine Berlin, Campus Benjamin Franklin, Department for Medical Computer Science, Biometry and Epidemiology, Berlin, Germany

Address correspondence to Engelbert Deusch, MD, Department of Anesthesiology and Intensive Care-B, Vienna Medical University, General Hospital Vienna, Waehringer Guertel 18-20, A-1090 Vienna, Austria, E.C. Address e-mail to engelbert.deusch{at}meduniwien.ac.at

	Abstract

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Continuous spinal anesthesia (CSA) has a nearly 100-yr history. In situations of difficult removal of a CSA small-bore catheter, mechanical properties of the different catheters might be important, because breakage could occur. We compared 5 different CSA small-bore catheters, 22- to 28-gauge from 3 manufacturers, for tensile strength, tensile stress, distension, and yield strength. Maximal tensile strength is the force applied before breakage of the catheter. The material characteristics of different CSA small-bore catheters for maximal tensile strength were: 22-gauge = 29.56 ± 1.56 (mean ± SD) Newton (N), 24-gauge = 16.77 ± 1.61 N, 25-gauge = 9.20 ± 0.48 N, 27-gauge = 4.61 ± 0.25 N, 28-gauge = 5.07 ± 0.59 N at room temperature. A strong correlation between maximal tensile strength and the outer diameter (r = 0.957, P < 0.001) and maximal tensile strength and the wall thickness (r = 0.9, P < 0.001) was observed. Although extrapolation from experimental studies to clinical routine should be made with care, our data suggest that catheters with higher-strength characteristics may reduce the risk of catheter breakage in patients, although clinical correlations are lacking.

IMPLICATIONS: Material characteristics of continuous spinal anesthesia small-bore catheters are very different. Although extrapolation from experimental studies to clinical routine should be made with care, our data suggest that catheters with higher strength characteristics may reduce the risk of catheter breakage in patients.

	Introduction

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Continuous spinal anesthesia (CSA) was first described by Dean (1) in 1906 using a continuous needle technique and was improved by Tuohy (2) in 1941 by inserting a number 4 ureteral catheter into the subarachnoid space. Known advantages of CSA are hemodynamic stability (3–5) and an option to deliver intrathecal anesthetics and analgetics postoperatively, which may provide more effective pain control than standard therapy (6). Because of possible postdural puncture headache, manufacturers try to keep diameters as small as possible. This may have an impact on tensile strength and durability of the intrathecal catheter, which has a smaller diameter than an epidural catheter. Small-bore catheters in CSA with 5% lidocaine or 1% tetracaine were associated with cauda equina syndrome in 3 of 4 cases reported by Rigler et al. (7) in 1991. In 1992, the Food and Drug Administration withdrew approval for small-bore catheters smaller than 24-gauge for CSA in the United States (8). As a consequence of a nearly ruptured CSA small-bore catheter during attempted removal in our institution (Fig. 1), we looked for comparative data describing material strengths of various CSA small-bore catheters. Only one published study investigated the tensile strength of CSA small-bore catheters (9). Therefore, we compared five different CSA small-bore catheters from three manufacturers for maximal tensile strength (force applied before breakage of the catheter), maximal tensile stress (maximal tensile strength derived by cross-sectional area, for comparison of catheter material properties independent of their diameters), distension, and yield strength (force at which the catheter starts to deform).

View larger version (87K): [in this window] [in a new window]   Figure 1. A deformed continuous spinal anesthesia catheter after removal. The deformation of the catheter indicates that the threshold between elastic strain and plastic strain has been exceeded.

	Methods

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Maximal tensile strength is the force applied before breakage of the catheter and the maximal tensile stress is strength derived by cross-sectional area.

Distensibility (%) = (l – l₀) * l₀^–1 describes the relative elongation at a certain force (l = length at tensile strength, l₀ = length at beginning).

Yield strength is the calculated force at which the material loses its elastic properties and is deformed (plastic range). Yield strength was determined by the point of intersection between two straight lines through the elastic and plastic range of the catheter characteristic line in the force distensibility diagram (Fig. 2).

View larger version (16K): [in this window] [in a new window]   Figure 2. Schematic force-distensibility diagram of a catheter. Defined yield strength (Newton, N) was determined by the point of intersection between two straights through the elastic and plastic range of the catheter characteristic line in the force distensibility diagram. Yield strength is the calculated force at which the material loses its elastic properties and is deformed (plastic range).

	Results

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Maximal tensile strength, maximal tensile stress and distensibility, and yield strength of CSA catheters are shown in Table 2 and in Figure 3.

View larger version (15K): [in this window] [in a new window]   Figure 3. Force (N)-distensibility (%) diagrams (representative tracings) of 5 different continuous spinal anesthesia small-bore catheters from 3 manufacturers: B. Braun Spinocath® 22-gauge, B. Braun Spinocath® 24-gauge, Pajunk Intralong® 25-gauge, Pajunk Intralong® 27-gauge, and PortexTM Microcatheter system 28-gauge.

A strong correlation of maximal tensile strength and the outer diameter (r = 0.957, P < 0.001) and of maximal tensile strength and the wall thickness (r = 0.9, P < 0.001) was observed. Furthermore, the distensibility also depended on outer diameter (r = 0.457, P = 0.011) and the wall thickness (r = 0.387, P = 0.035).

	Discussion

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Material characteristics of the tested CSA small-bore catheters for maximal tensile strength are very different ranging from 4.61 ± 0.25 to 29.56 ± 1.56 Newton (N) at room temperature before rupture occurs. Similar maximal tensile strength values were reported by Ley and Jones (9) who determined break strength at a stretching speed of 25 cm/min for 6 continuous spinal catheters measuring 20- to 32-gauge from 3 manufacturers. One of these catheters, the Burron 24-gauge, was available to us, because Burron is a subsidiary of B. Braun Melsungen AG of Germany. The authors reported for the Burron 24-gauge continuous spinal catheter a break strength of 15.79 N, which is very close to our value of 16.77 N at a stretching speed of 1 mm/min. See below.

There is a positive correlation between outer diameter or wall thickness of the CSA catheters and tensile strength and distensibility. Distension and rupture of these catheters may occur during removal depending on the force required. Although we did not experience a definite rupture of a CSA small-bore catheter, we have noted three incidents in which a catheter distended during removal and the catheter started to deform. This means that, under certain circumstances, retraction forces for CSA small-bore catheters may exceed the yield stress values of the catheters. The required force for removal of a 16-gauge epidural catheter was 1.57–3.78 N (130–390 g) with a maximal withdrawal force of 11.47 N (1170 g) (the force developed by 102 g equals approximately 1 N) (10); greater force is required to remove the catheter with the patient in the sitting position than in the lateral position (11). Maximal tensile strength for epidural catheters was approximately 20 N or more. If a catheter is traumatized, mechanical properties can be halved (12).

Maximal distensibility before rupture varied between different CSA small-bore catheter products from 195.7% ± 20.5% to 567.0% ± 69.9%. Both tested small-bore catheters 25- and 27-gauge Pajunk Intralong® showed the highest distensibility (%) values in conjunction with low values of yield strength. This means that these CSA small-bore catheters are extended nearly four- to sixfold the initial length at room temperature before rupture. By examining the diagrams of the different tested CSA small-bore catheters, marked differences, especially after transition from elastic strain to plastic strain, are obvious (Fig. 3). All tested catheters from 25- to 28-gauge showed flat slopes of the curves after transition from elastic strain to plastic strain. Accordingly, only small incremental amounts of retraction force will lead to catheter breakage once the transition from elastic to plastic strain (yield strength) has been reached. The maximal tensile strength of the tested catheters from 27- and 28-gauge was approximately 5 N (approximately 500 g). Interestingly, the mechanical properties of the smaller Portex^TM Microcatheter system 28-gauge were better or at least equivalent to the bigger Pajunk Intralong® 27-gauge (Table 2, Fig. 3).

Several treatment options have been advocated in the case of difficult removal of CSA small-bore catheters such as changing the position of the patient, delaying removal for 30–60 min until the patient is moved, and injection of saline (13,14). However, one should not try to extract a trapped CSA small-bore catheter forcefully because materials could break easily. Differences in maximal tensile stress (Table 2) probably reflect differences in material composition and production of the catheters.

All tested CSA small-bore catheters are made of polyamides, as nylon is a trade name of a special polyamide developed by Du Pont in 1939. The exact composition and production of these catheters, however, are not disclosed by the manufacturers.

One limitation of the present study is that mechanical properties of CSA catheters were tested in vitro at a stretching speed of 1 mm/min at room temperature of 21°C, which does not completely reflect clinical conditions at 37°C body temperature and a faster removal speed of a CSA catheter which may be achieved in clinical practice. However, this experimental study design permits reproducible comparison of the data. Although extrapolation from experimental studies to clinical routine should be made with care, our data suggest that catheters with higher strength characteristics, i.e., higher maximal tensile strength and higher yield strength may reduce the risk of catheter breakage with all possible consequences, although clinical correlations are lacking.

	Acknowledgments

 
This work was supported by the Department of Anesthesiology and Intensive Care-B, Vienna Medical University, General Hospital Vienna, and Österreichische Nationalbank Jubiläumsfond 9583.

Measurements were performed by M. Payer at the Technical University Graz, Institut für Materialprüfung und Baustofftechnologie mit angeschlossener Technischer Versuchs-und Forschungsanstalt für Festigkeits-und Materialprüfung, Graz, Austria. We thank B. Braun Melsungen AG and SIMS Portex Ltd. for providing the continuous spinal anesthesia small-bore catheters.

	References

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