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, hyperglycemic, 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 insulin priming on insulin recovery from polyvinyl chloride (PVC) tubing and polyethylene (PE)-lined PVC tubing infused with a standard insulin stock solution.

Methods. Stock insulin solution (0.2 U/mL) was infused through microbore PVC or PE-lined tubing at flow rates of 0.05 and 0.2 mL/h. To determine if saturation of nonspecific binding sites would alter effluent insulin concentration, we compared insulin recovery from tubing previously flushed with the stock solution and tubing primed with 5 U/mL of insulin for 20 minutes. Effluent samples, which were collected at baseline and at six time points during a 24-hour period, were immediately frozen at −20°C. Insulin concentration was measured by IMx immunoassay. Data were analyzed using general 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 ∼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 PVC tubing, respectively. Similar results were obtained from unprimed and insulin-primed PE-lined tubing at 0.2 mL/h flow rate.

Conclusions. Priming of microbore tubing with 5 U/mL of insulin solution for 20 minutes to block nonspecific binding sites enhances delivery of a standard insulin stock at infusion rates typically used to treat hyperglycemic ELBW infants. We conclude that priming the tubing with a higher concentration of insulin before initiation of standard insulin infusion therapy should accelerate achievement of steady-state insulin delivery and correction of hyperglycemia in ELBW infants.