Effect of flow rate and insulin priming on the recovery of insulin from microbore infusion tubing

Background. A retrospective medical record review of 13 consecutive, hyperg lycemic, extremely low birth weight (ELBW) infants treated with continuous insulin infusions revealed a 14- to 24-hour delay (mean, 19 hours) in blood glucose normalization despite stepwise increases in insulin infusion rates . Objective. This in vitro study examined the effects of flow rate and insuli n priming on insulin recovery from polyvinyl chloride (PVC) tubing and poly ethylene (PE)lined PVC tubing infused with a standard insulin stock solutio n. Methods. Stock insulin solution (0.2 U/mL) was infused through microbore PV C or PE-lined tubing at flow rates of 0.05 and 0.2 mL/h. To determine if sa turation of nonspecific binding sites would alter effluent insulin concentr ation, we compared insulin recovery from tubing previously flushed with the stock solution and tubing primed with 5 U/mL of insulin for 20 minutes. Ef fluent samples, which were collected at baseline and at six time points dur ing a 24-hour period, were immediately frozen at -20 degrees C. Insulin con centration was measured by IMx immunoassay. Data were analyzed using genera l linear modeling with repeated measures. Results. At 0.05 mL/h flow rate, insulin recovery from unprimed PVC tubing at 1, 2, 4, and 8 hours was 17%, 11%, 27%, and 55%, respectively, with 100% recovery at 24 hours. From insulin-primed tubing, insulin recovery was sim ilar to 70% at 1, 2 and 4 hours, and close to 100% at 8 hours. At a faster flow rate of 0.2 mL/h, insulin recovery at 1, 2, 4, and 8 hours was 22%, 38 %, 67%, and 75% vs 42%, 85%, 91% and 95% from unprimed and insulin-primed P VC tubing, respectively. Similar results were obtained from unprimed and in sulin-primed PE-lined tubing at 0.2 mL/h flow rate. Conclusions. Priming of microbore tubing with 5 U/mL of insulin solution fo r 20 minutes tot block nonspecific binding sites enhances delivery of a sta ndard insulin stock at infusion rates typically used to treat hyperglycemic ELBW infants. We conclude that priming the tubing with a higher concentrat ion of insulin before initiation of standard insulin infusion therapy shoul d accelerate achievement of steady-state insulin delivery and correction of hyperglycemia in FLEW infants.