Blood glucose increments as a measure of body physiology

In this issue of Critical Care the Japanese research group headed by Ishihara [1] highlight the issues involved in measuring blood glucose. Their approach does not involve attempting to reduce mortality by controlling the glucose level; rather, they use its response to a glucose bolus as an index of physiological parameters such as plasma volume and cardiac output. Their work is admirable, and more effort should be given to simplifying the measurement of parameters that are useful in intensive care. We should not overlook the potential utility of glucose measurement, as pioneered by Ishihara over the past decade.Ishihara and coworkers introduced the concept of the initial distribution volume of glucose (IDVG), which is determined by measuring the arterial blood glucose level just before and repeatedly for about 10 min after a bolus infusion of 5 g glucose is administered. The data are analyzed by computer according to a one-compartment kinetic model, in which the concentration–time profile is represented as a function of glucose clearance and the volume of distribution of administered glucose. It is crucial that measurements are taken from an arterial line because glucose is rapidly distributed over 65% of the total extracellular fluid volume as soon as the blood passes through the capillary bed [2]. When measured in this manner the glucose concentration extrapolated to time zero is diluted in proportion to cardiac output. This principle is identical to that of thermodilution. The glucose clearance then becomes a hybrid of the distribution of exogenous glucose between plasma and the interstitial fluid and the glucose uptake into cells.This approach was first reported in 1993, and Ishihara and coworkers [3,4] concluded that only two samples are needed to estimate IDVG: one before the bolus infusion and another 3 min later. Such a simplification is justified because variations in glucose clearance and endogenous glucose production are of little importance over short t