in this issue of critical care the japanese research group headed by ishihara   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 . when measured in this manner the glucose concentration extrapolated to time zero is diluted in proportion to cardiac output . 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   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 time increments , at least when it is applied in a fairly coherent group of patients . in subsequent studies , however , sampling was usually extended to 7 min . idvg correlates reasonably well with plasma volume in bled dogs   and with the incidence of hypovolaemic hypotension after surgery for oesophageal cancer , although these findings may merely reflect the effect of hypovolaemia on cardiac output . in later studies , ishihara and coworkers   suggested that idvg is a measure of the ' central ' extracellular fluid volume as well as of cardiac preload . their views became more complex when indocyanine green was introduced as a direct measure of plasma volume . a deviation in the ratio of indocyanine green to idvg is said to indicate capillary leakage of proteins after cardiac surgery , because this marker binds to proteins . as for glucose clearance , however , < 10 min is a very short period for ongoing capillary leakage to make a difference . furthermore , any such leakage would cancel out if a concentration  time curve of dye is extrapolated back to time zero . alternative explanations should not be overlooked because control methods designed to measure capillary leak ( such as use of radioactive albumin ) were not applied , and a similar increase in the indocyanine / idvg ratio also occurs early after induction of intravenous anaesthesia   and after administration of histamine . all of these ideas about the physiological meaning of idvg may be difficult to differentiate for the intensivist . although idvg perhaps most directly reflects a fraction of the extracellular fluid volume , several of ishihara and coworkers ' later studies actually strongly support their first interpretation , namely that idvg indicates cardiac output . this correlation between idvg and cardiac output is logical and renders idvg highly useful because it both predicts a key parameter in body physiology and can be measured quickly and with little effort and cost . unfortunately , this view is somewhat countered by the fact that idvg is said to be undisturbed by vasoactive drugs , although most of these agents actually change cardiac output . what the limits are and the errors that may be hidden in the determination of idvg are the most recent issues . idvg is apparently accurate if it is repeated after 30 min in clinical patients , although the study presented in this issue   holds that blood glucose remains elevated for another 30 min , which is also the case in healthy individuals ( fig . ishihara and coworkers , in addition to repeating their previous finding that a 3-min postinfusion sample is sufficient to predict idvg , also compare calculations of idvg based on plasma glucose and blood glucose . the difference is quite small and , logically , should be accounted for by displacement of glucose from the parts of the red blood cells that consist of haemoglobin . blood difference in idvg and the haematocrit , but i believe that haemoglobin would be a more reasonable comparitor because haemoglobin reflects differences in the water content of blood . nevertheless , the authors conclude that measurements can be based on both plasma and blood samples , although these are not interchangeable . it appears that the idvg will be ready for limited implementation once it has been tested by additional independent groups . the method is safe and easy to apply , but intensivists must reach a consensus about its place in the clinic and what it really shows . i foresee that idvg may serve as a screening test to separate patients with a hyperkinetic circulation from those with a hypokinetic one . relative stability of the volume of distribution of glucose when 5 g glucose is given by intravenous infusion over 10 min on three occasions immediately following each other in one healthy volunteer . ishihara and coworkers infuse glucose even faster and use the arterial plasma glucose level measured between 3 and 7 min to calculate the volume of distribution .
the initial distribution volume of glucose ( idvg ) can be calculated from the arterial plasma glucose level between 3 and 7 min after a bolus intravenous infusion of 5 g glucose . 
 ishihara and colleagues have investigated the value of idvg over the past decade . 
 although idvg is simple and cheap to measure , there have been several very different proposals regarding what it should be used for . 
 the most interesting and logical correlate is that between idvg and cardiac output . 
 a recent study showed that it does not matter much whether the calculation of idvg is based on blood or plasma samples .