Fluctuation in insulin delivery due to pump movement may increase risk of complications in diabetics

Researchers at Sansum Diabetes Research Institute and University of California, Santa Barbara have concluded that changing the height of a conventional insulin pump in relation to its tubing and infusion set can significantly impact expected insulin delivery rates. Such changes can occur during routine daily activities like dressing, sleeping or showering. The study, "Siphon Effects of Continuous Subcutaneous Insulin Infusion Pump Delivery Performance," evaluated the siphon or hydrostatic pressure action effects on continuous subcutaneous insulin infusion and was published in the January issue of Journal of Diabetes Science and Technology.

"In this study we found a pronounced siphon effect in conventional insulin pumps, which caused significant fluctuations in the accuracy of insulin delivery rates when the pump position was moved higher or lower relative to its tubing and infusion site," said lead investigator Howard Zisser, MD, Director of Clinical Research and Diabetes Technology at the Sansum Diabetes Research Institute in Santa Barbara, CA. "Insulin pump therapy allows for precise control of insulin delivery for patients with type 1 diabetes. The unintended fluctuation in insulin delivery, which may arise from pump movement during normal daily use, can increase blood glucose variability, a risk factor for the progression of complications of diabetes. The effect of hydrostatic pressure was most significant at low basal rates and therefore these findings may be particularly important for pediatric diabetes patients, who often use insulin pumps at low basal rates."

The investigators reached this finding by quantifying the effect of hydrostatic pressure (the pressure exerted on a portion of a column of fluid as a result of the weight of the fluid above it) on insulin delivery during bolus dosages, basal rates and static changes in insulin pumps. They tested conventional insulin pumps from Medtronic Diabetes (MiniMed 512 & 515, which uses 110 cm tubing) and Smiths Medical (Deltec Cozmo 1700, which uses 80 cm tubing), and also compared them to the tubing-free pump from Insulet Corporation (the OmniPod System).

The researchers found that raising or lowering a conventional insulin pump, to the full extent of its tubing, can significantly affect the accuracy of insulin delivery, especially at low basal infusion rates. The most pronounced differences were seen during basal delivery in the Cozmo and MiniMed pumps. For the 1U/hr rate, differences ranged from 74.5% of the expected delivery when the pumps were below the pipettes and pumping upward to 123.3% when the pumps were above the pipettes and pumping downward. For the 1.5U/hr rate, differences ranged from 86.7% to 117.0% when the pumps were below or above the pipettes, respectively.

In contrast, the OmniPod, which has no external tubing, was the least affected by pumping orientation and direction. With the OmniPod System, the 1U/hr rate differences only ranged from 98.3% when its delivery cannula was in a level pumping position to 101.3% when the cannula was in an upward pumping position. For the 1.5U/hr rate, its differences only ranged from 96.0% in a level pumping position to 102.5% in an upward pumping position.

The researchers measured the change in the fluid level in an in-line graduated glass pipette when the Cozmo and MiniMed pumps were moved either up or down to its maximum length in relation to the end of the tubing/pipette. For the OmniPod, the unit was held in a clamp vertically with the delivery cannula up for "upward" pumping position and the delivery cannula down for the "downward" pumping position. The OmniPod was horizontal for the level pumping position.