Technology, Computing, and Simulation: Research Report
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).
The following CSA small-bore catheters (each n = 6) were tested: B. Braun Spinocath® 22-gauge, B. Braun Spinocath® 24-gauge (B. Braun Melsungen AG, D-34209 Melsungen, Germany), Pajunk Intralong® 25-gauge, Pajunk Intralong® 27-gauge (Pajunk GmbH, D-78187 Geisingen, Germany), Portex™ Microcatheter system 28-gauge (SIMS Portex Ltd., Hythe, Kent, UK). The outer and inner diameters of CSA catheters were measured with a sliding caliper. Wall thickness was calculated by subtraction of internal diameter from the outer diameter (Table 1). Cross-sectional area was calculated by (ro2 − ri2) π (ro = half outer diameter, ri = half inner diameter).
For tensile testing of CSA small-bore catheters, we used a Shimadzu AGe 100 kN controlled via an EDC 100 control system (Shimadzu Corp., Kyoto, Japan) with testing software (Fa. Messphysik, Fürstenfeld, Austria). Load during testing was detected with a 5-kN load cell. A constant stretching speed of 1 mm/min at room temperature of 21°C was used.
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).
Least square fitting of the straights was done with the Sigma Plot® (SPSS Inc., Chicago, IL) program according to a linear equation. The t-test with Holmes α correction was used for pairwise comparison between different catheters for maximal tensile strength, maximal tensile stress, distensibility, and yield strength. Pearson’s correlation analyses were performed to evaluate the dependence of maximal tensile strength and distensibility on the diameter and the wall thickness of the catheters. P < 0.05 was considered statistically significant.
Maximal tensile strength, maximal tensile stress and distensibility, and yield strength of CSA catheters are shown in Table 2 and in Figure 3.
Maximal tensile strengths were significantly different between the tested catheters (P < 0.05), except for 27- and 28-gauge catheters. Maximal tensile stress (maximal tensile strength per mm2 cross-sectional area) was highest for 28- and 22-gauge compared with all other tested catheters (P < 0.05). Twenty-five-gauge catheters had the least maximal tensile stress. Twenty-four- and 28-gauge showed the least distensibilities compared with all other catheters (P < 0.05). Twenty-five- and 27-gauge (same manufacturer) showed nearly doubled distensibility compared with all other catheters. Yield strength was different among all catheters (P < 0.05). The most yield strength was shown in 22-gauge, the least in 27-gauge followed by 25- and 28-gauge catheters.
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).
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™ 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.
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© 2004 International Anesthesia Research Society
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