Monitoring blood glucose with microdialysis of interstitial fluid in critically ill children.

To the Editor: Hyperglycemia, a common feature in critically ill patients, was traditionally perceived as an adequate stress response reflecting the severity of the disease state and was treated only if glycemia exceeded 11–13.5 mmol/L. In recent studies in intensive care patients, we showed that tight glycemic control (TGC) with intensive insulin therapy (IIT) reduced the risk of organ failure and death (1 ). In critically ill children, peak blood glucose (BG) and duration of hyperglycemia are associated with risk of mortality (2 ). Implementing TGC and avoiding hypoglycemia with intensive insulin therapy requires frequent BG sampling. Microdialysis of interstitial fluid (ISF) is a promising approach to reduce diagnostic blood loss. Continuously sampling dialyzed ISF and converting the ISF glucose concentration (IFG) to a BG value is a promising new method for glucose monitoring in diabetes patients. We conducted a prospective clinical trial in critically ill children to evaluate the feasibility of prolonged subcutaneous microdialysis and the correlation between BG and IFG. The study was approved by the Institutional Ethical Review Board. Twenty children were enrolled after written informed consent was obtained from the parents. A CMA 60 microdialysis catheter (CMA Microdialysis) was inserted subcutaneously. This catheter has a dialyzing membrane with a molecular cutoff of 20 kDa and was continuously perfused with a 5% mannitol solution at a flow rate of 1 L/min. BG was determined on an ABL 715 blood gas analyzer (Radiometer) and dialysate glucose (DG) values with a Cobas Mira Analyzer (Roche). Both techniques use the enzyme glucose dehydrogenase. Because of the used flow rate of 1 L/min, the concentration in the dialysate reaches only partial equilibration and thus does not reflect the absolute concentration in the extracellular fluid. Therefore, IFG was calculated using the ionic reference technique (3 ). This technique is based on the simultaneous measurement of glucose and ions in the samples. The ionic recovery can be calculated as the ratio of sodium in the sample to the sodium concentration in plasma, using an ion-free perfusate. Assuming that recovery rates of glucose and sodium were the same, we calculated the glucose concentration of the ISF as IFG dialysate glucose plasma sodium/dialysate sodium. Thus, the relative recovery of a particular substance is the concentration of this substance in the dialysate expressed as percentage of the concentration of this substance in the surrounding tissues. Mean age and body weight were 3.4 years and 14.5 kg. No complications with the technique occurred. Median microdialysis recovery rate was 89% (79–94%).