Approximately 3.7 million diabetic patients are currently treated with insulin in the U.S. Frequent glucose monitoring is essential to adjust daily insulin doses. Although intensive insulin treatment has been shown to delay the onset and progression of long-term diabetic complications (Diabetes Control and Complications Trial) it increases the risk of hypoglycemic events, particularly when based on a few periodic glucose measurements per day. Continuous glucose sensors can potentially improve glucose control and provide reliable alarms for both hypo-and hyper-glycemic conditions. Subcutaneous glucose sensors are convenient and safe for the patient, however, they measure glucose in the interstitial fluid (ISF) surrounding the sensor rather than blood glucose. The extent to which ISF glucose levels differ from blood levels is not fully known. If blood vessels represent a diffusion barrier to glucose the ISF glucose kinetics are expected to be delayed with respect to changes in plasma glucose and to have a steady state gradient. In the present study, an enzyme based (glucose oxidase/hydrogen peroxide) glucose sensor developed by MiniMed Inc. is used to investigate the relationship between subcutaneous glucose and blood glucose. Glucose kinetics in whole blood, plasma,interstitial fluid, and lymph are followed to determine in a plasma: ISF gradient exists and if the ISF glucose signal is delayed relative to plasma. The effect of insulin to alter the relationship between plasma and ISF glucose is also investigated as is the dependence of subcutaneous glucose dynamics on tissue type (muscle versus adipose tissue) and tissue sensitivity (insulin sensitive versus insulin resistant). Methods for correcting any gradient or delay are proposed. Obtaining a basic understanding of the physiology of ISF glucose dynamics will aid in the development of all subcutaneous glucose sensing systems including those using non-invasive approaches.
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