Comparison of Changes in Tensile Strength in Three Different Flexible Epidural Catheters Under Various Conditions.

BACKGROUND: Rarely, epidural catheters may fracture upon removal or insertion. Understanding some of the mechanical properties of epidural catheters, such as their tensile strength and how external factors (including temperature) can influence their strength, will aid physicians in making decisions if faced with an entrapped catheter. In the present study, we evaluated the impact in tensile strength when catheters are exposed to 37 ± 1°C, after the removal of the inner metal coil and after the injection of sterile saline through the catheter. METHODS: We analyzed the tensile strength of a total of 120 catheters (19-gauge) from 3 different brands and materials. The reinforced epidural catheters were affixed to opposing, specially designed tensile test fixtures and then installed in an Applied Test System tensile test apparatus. We evaluated the strength of 10 catheters from different brands and materials for each of the following variables: at room temperature (control group), after the removal of the inner wire present in all the flexible catheters tested, injection of normal saline, and at 37 ± 1°C. RESULTS: When compared with their control groups, the Arrow catheter (2.85 kg) was shown to be superior to B-Braun (2.17 kg; P < 0.0001) and Smith catheters (2.33 kg; P < 0.0005). No statistical difference was noted between the Smith and B-Braun catheters (P = 0.39). When comparing catheters after wire removal against their respective control group, no statistical difference was noted. A decrease in tensile strength was noted in the B-Braun catheters (1.53 kg) when tested at 37°C (P ≤ 0.0001). In contrast, the Smith and the Arrow catheters did not show a statistically significant change when tested at 37°C (P = 1.0 and P = 0.063, respectively). After the injection of normal saline, the Arrow (2.33 kg) and the B-Braun (1.58 kg) catheters showed a decrease in tensile strength (P = 0.0010 and P = 0.0001, respectively). CONCLUSIONS: The current recommendation of injecting saline through an entrapped catheter resulted in a decrease in tensile strength of the Arrow and B-Braun catheters. A decrease in tensile strength also was noted in the B-Braun catheters when tested at 37°C. There is no benefit, at least in terms of tensile strength, in removing the wire or inner coil from any of the tested brands. The Smith catheter was the most resilient, showing no decrease in tensile strength at 37°C and after the injection of normal saline when compared with control.

Semiparametric simultaneous confidence bands for the difference of survival functions.

In the analysis of censored survival data, simultaneous confidence bands are useful devices to help determine the efficacy of a treatment over a control. Semiparametric confidence bands are developed for the difference of two survival curves using empirical likelihood and compared with the nonparametric counterpart. Simulation studies are presented to show that the proposed semiparametric approach is superior, with the new confidence bands giving empirical coverage closer to the nominal level. Further comparisons reveal that the semiparametric confidence bands are tighter and, hence, more informative. For censoring rates between 10 and 40 %, the semiparametric confidence bands provide a relative reduction in enclosed area amounting to between 2 and 10 % over their nonparametric bands, with increased reduction attained for higher censoring rates. The methods are illustrated using an University of Massachusetts AIDS data set.