BJA Advance Access originally published online on April 28, 2008
British Journal of Anaesthesia 2008 100(6):846-849; doi:10.1093/bja/aen086
Withdrawal forces of lumbar spinal catheters: no dependence on body position
1 Department of Special Anaesthesia and Pain Control, Vienna Medical University, General Hospital Vienna, Waehringer Guertel 18–20, 1090 Vienna, Austria
2 Institute for Clinical Epidemiology, Charité–University of Medicine Berlin, Berlin, Germany
* Corresponding author. E-mail: engelbert.deusch{at}meduniwien.ac.at
Accepted for publication March 12, 2008.
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Abstract |
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Background: Spinal catheters, because of their smaller diameter, have lower tensile strength than epidural catheters. This study was designed to measure the withdrawal forces needed to remove lumbar spinal catheters and to determine whether patient position affects withdrawal forces.
Methods: Eighty-two patients with a 24-gauge spinal catheter placed midline at the lumbar L3/4 or L4/5 level were randomly assigned to catheter removal either in flexed lateral or sitting position. Withdrawal forces were measured using a tension spring balance.
Results: Mean withdrawal force was 0.91 N (95% CI: 0.73, 1.09) with extremes up to 5 N. Withdrawal force in the flexed lateral position was 1.04 N (95% CI: 0.73, 1.34) or in the sitting position was 0.78 N (95% CI: 0.59, 0.97). The 95% CI for the difference of the means was –0.62 N, 0.10 N. Thus, the absolute mean difference between the positions can be assumed to be smaller than 0.62 N. Neither the length of the spinal catheter under the skin or in the subarachnoid space, nor BMI influenced withdrawal force.
Conclusion: Withdrawal force of spinal catheters is not influenced by body position during catheter removal, length of catheter under skin, or BMI.
Keywords: equipment, catheters subarachnoid; safety
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Introduction |
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Breakage of a spinal catheter as a result of difficult removal is a rare complication. Exact data of incidence are missing.1 2 Breakage will occur if the force needed for catheter removal exceeds the mechanical properties of the spinal catheters. It might be speculated that factors influencing withdrawal forces of spinal catheters are analogue to those of epidural catheters.3 4 There are, however, no data on withdrawal forces of spinal catheters or influence of body position during spinal catheter removal on these forces. Therefore, we performed a prospective, randomized study to measure withdrawal forces of lumbar spinal catheters in either flexed lateral position or sitting body position. We also studied a possible influence of other factors such as height, BMI, age, sex, or length of spinal catheter under the skin and in the subarachnoid space on catheter withdrawal forces.
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Methods |
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The study was performed at the Department of Special Anaesthesia and Pain Control, Medical University of Vienna, Austria, between September 2004 and July 2007. After approval by the Institutional Ethics Committee at the University of Vienna and obtaining written informed consent, 82 patients (males and females, ASA II–III, aged 37–92 yr), undergoing elective interstitial radiation therapy of urological, gynaecological, or perineal malignancies were included in this prospective, randomized study. Exclusion criteria were allergy to local anaesthetics, psychiatric or neurological diseases, coagulation disorders, and infection at the site of the injection. Lumbar puncture through the anaesthetized skin (plain lidocaine 2%, 2 ml) was performed under aseptic conditions at the L3/4 or L4/5 interspace by a midline approach with the patient in the sitting position. A 24-gauge spinal catheter (Spinocath®, B. Braun, Melsungen, Germany) was introduced into the subarachnoid space.
After completion of the interstitial radiation therapy, the spinal catheter was removed 4–25 h after insertion. For removal of the spinal catheter, patients were randomly assigned either to the flexed lateral or sitting position. Withdrawal forces of the spinal catheter were measured by a tension spring balance with a drag indicator calibrated to Newton (Fig. 1) (Schuhmann GmbH, Sillerup, Switzerland). Calibration of the tension spring balance scale was performed by certified weights over a range of 100–900 g by triplicate measurements. The regression line between Newton indicated by the scale and the calibrated weights yielded Y(N)=0.0098 x weight (pounds)+0.0132.
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Statistical analysis
Statistical analysis was performed using SPSS 14.0.1 (SPSS Inc., Chicago, IL, USA). The dependence of the withdrawal force from height, BMI, age, sex, length of spinal catheters under the skin and in the subarachnoid space, and position during catheter removal were examined by a general linear model. In order to achieve normality, withdrawal forces were log-transformed before analysis. Age, height, weight, BMI, length of catheters under skin and in the subarachnoid space were compared by t-tests and sex, diagnosis by Pearsons
2 test. In order to compare body positions, the 95% CI was calculated for the difference of the mean withdrawal forces. A two-tailed P-value of <0.05 was considered statistically significant. |
Results |
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Patient characteristics were similar between groups (Table 1) except for weight (P=0.047) with heavier patients in the sitting position group and catheter length in the subarachnoid space, which was 0.5 cm longer in the sitting position group (P=0.045). However, the linear model of dependence demonstrated no influence of these factors on withdrawal forces (Table 2).
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Mean withdrawal force required for removal of lumbar 24-gauge spinal catheters was 0.91 (SD 0.83) N; extreme outliers needed forces up to 5 N. No clinically relevant difference of withdrawal forces was found between patients in the flexed lateral 95% CI (0.73, 1.34) and sitting position 95% CI (0.59, 0.97) during spinal catheter removal (Fig. 2). The 95% CI for the difference of the means were –0.62 N, 0.10 N. Thus, the absolute mean difference between the positions can be assumed to be smaller than 0.62 N. Neither length of spinal catheter under the skin nor in the subarachnoid space influenced withdrawal forces of spinal catheters (Table 2). Also, patient factors such as height, BMI, age, or sex did not influence withdrawal forces (P=0.496) (Table 2). There was no breakage of spinal catheters during removal.
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Discussion |
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We could show for the first time that forces needed for withdrawal of lumbar and midline spinal catheters are not dependent on patient position during catheter removal, length of catheter under the skin, and in the subarachnoid space or patient height, BMI (hence also weight), sex, and age (Table 2). According to the 95% CI for the difference of the means between the body positions during spinal catheter removal, the absolute mean difference between the positions can be assumed to be smaller than 0.62 N, which is a clinically irrelevant difference between the body positions.
Our results are in contrast to studies investigating the withdrawal forces of epidural catheters. Boey and Carrie3 found that the force to remove a lumbar epidural catheter was increased by 2.5 times when the sitting instead of the lateral position was adopted during catheter removal. The approach of the epidural catheter, whether midline or paramedian, did not influence withdrawal forces. Morris recommended the same patient position for insertion and withdrawal of epidural catheters, as this resulted in lower withdrawal forces.4
Mean withdrawal forces of spinal catheters in our study were small with a mean of 0.91 N. Extremes up to 5 N occurred, which might be of clinical importance, as spinal catheters with smaller diameters, for example 28-gauge, although not tested in this study, might break at 5 N. However, outer diameter of catheter might influence withdrawal force needed for removal and we assume that thiner catheters might need less force, but exact data are missing and were not aimed in this study. Spinal catheters used in this study showed higher maximal tensile strengths with 16.77 (1.61) N, and no breakage of spinal catheters during removal occurred in this study.6
Maximal tensile strength, the force when a catheter breaks, is dependent on outer diameter, wall thickness, and intrinsic material properties. At room temperature maximal tensile strength of spinal catheters ranges between 4.61 (0.25) N and 29.56 (1.56) N depending on the catheter and its material properties.5 6 No data are available for 37°C and moisture conditions that would be present when the catheter is in situ in a patient. Thus, the maximal tensile strength of spinal catheters is approximately one-third to 50% lower than that of epidural catheters.5 Mechanical damage to spinal catheters further reduces their maximal tensile strength.7 Damage may occur during insertion or removal of a spinal catheter. Possible causes, especially in older patients are ligamentous calcifications or bony spurs between reduced intervertebral spaces.2 Case reports on difficulties during removal of spinal catheters are sparse and exact data on the incidence of spinal catheter breakage are lacking.2 However, there seems to be a real risk at least with small bore catheters, as Hurley and Lambert1 report two instances of breakage of 32-gauge spinal catheters in a series of 58 withdrawals.
In conclusion, patient position during removal does not influence withdrawal force of lumbar 24-gauge spinal catheters.
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References |
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1 Hurley RJ, Lambert DH. Continuous spinal anesthesia with a microcatheter technique: preliminary experience. Anesth Analg (1990) 70:97–102.
2 Sprung J, Tabak J, Markowski BM, Thomas P, Bourke DL. Resistance to injection may predict spinal catheter breakage. Anesthesiology (1994) 81:1296–8.[CrossRef][Web of Science][Medline]
3 Boey SK, Carrie LES. Withdrawal forces during removal of extradural catheters. Br J Anaesth (1994) 73:833–5.
4 Morris GN, Warren BB, Hanson EW, Mazzeo FJ, DiBenedetto DJ. Influence of patient position on withdrawal forces during removal of lumbar extradural catheters. Br J Anaesth (1996) 77:419–20.
5 Ley SJ, Jones BR. Strength of continuous spinal catheters. Anesth Analg (1991) 73:394–6.
6 Deusch E, Benrath J, Weigl L, Neumann K, Kozek-Langenecker SA. The mechanical properties of continuous spinal small-bore catheters. Anesth Analg (2004) 99:1844–7.
7 Atec Y, Yücesoy CA, Ünlü A, Saygin B, Akka
N. The mechanical properties of intact and traumatized epidural catheters. Anesth Analg (2000) 90:393–9.
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