Variable adsorption of insulin at catheter materials used in intensive care units: polyethylene vs polyurethane – possible cause for hypoglycemia during intensive insulin treatment?

Introduction

Intensive insulin therapy reduces morbidity and mortality in postoperative critical care; however, this treatment also increases the risk of hypoglycemia. A possible cause for unstable blood glucose (BG) levels could be a variable adsorption of insulin at plastic material of infusion tubings. We evaluated in vitro and in vivo the adsorption of insulin at standard tubing materials (polyethylene (PE) and polyurethane (PU)) and the effects of this adsorption process on blood glucose levels.

Methods

• (1) In vitro, a standard perfusor syringe (Perfusor^®; BBraun, Germany) was filled with 50 IE normal insulin (Actrapid^®; NovoNordisk, Germany) dissolved in 50 ml saline 0.9%. The syringe was connected to PE or PU tubings (BBraun) and, at an infusion rate of 1 ml/hour, the insulin concentration in the syringe and at the end of the tubings was measured at hourly intervals for 5 hours and again after 24 hours by the Bradford protein assay. Insulin concentrations were compared using the Student t test. (2) In a prospective, double-blinded, cross-over study, approved by the ethics committee, 10 patients on the surgical ICU received insulin via PE or PU tubing each for 24 hours in random sequence. All blood BG values, total infused insulin solution volume, and critical care scores were documented and statistically analysed by the Wilcoxon test.

Results

• (1) The insulin concentration in all syringes was always >97% of the estimated value. The initial concentrations of insulin at the end of PE and PU tubings were lower than expected (23 ± 4% of anticipated concentration in the first 6 min). In PE, the concentration rose to 37 ± 2% and in PU to 78 ± 4% after 24 hours (P < 0.0001). (2) In vivo the mean BG values did not differ between PE and PU (PE 141 ± 17 mg/dl; PU 132 ± 23 mg/dl (not significant)). Severity of illness was not different between the groups: TISS 37 ± 5 (PE) vs 39 ± 5 (PU), SAPS 43 ± 13 (PE) vs 41 ± 15 (PU) on both days; neither were catecholamine doses and 24-hour fluid balance. However, significantly more insulin solution was infused in PE (66 ± 18 ml/24 hours) compared with PU (44 ± 15 ml/24 hours) (P = 0.0015).

Conclusion

Infusion of insulin using PE and PU tubings leads to a relevant adsorption of the drug in both materials. Adsorption to PE is significantly higher compared with PU. Thus, a large variation of insulin application to the patient is possible if different tubing materials are used. Furthermore, variability of adsorption, a competitive adsorption with other drugs if insulin is not infused via a single line as well as changes of effective insulin application following routine change of tubings, may be one cause of unexpected hypoglycemia that can be deleterious to the patient.