Lack of agreement between arterial and central venous blood glucose measurement in critically ill children

Abstract

Dear Sir, We were interested to read the article by Critchell et al. [1] highlighting concerns about the accuracy of bedside capillary blood glucose measurements in critically ill patients. We would agree that the importance of sampling site for blood glucose measurement has been under-emphasized in the published studies on hyperglycaemia and tight glycaemic control. The majority of studies linking hyperglycaemia with adverse clinical outcome have not provided data on the site of sampling. We would like to share preliminary data regarding the use of central venous blood sampling for glucose measurement in critically ill children. The objective of this preliminary study was to compare blood glucose concentrations in simultaneously drawn arterial and central venous blood glucose samples taken for clinical purposes. Paired arterial and central venous blood gas samples were taken to obtain information about oxygen extraction and the results were prospectively documented. Blood glucose was measured on a blood gas analyzer (Bayer Rapidlab; electrochemical biosensor), which is subject to strict quality control by the Clinical Chemistry department. No glucose containing solution was infused into either the arterial or the central venous sampling lines. A total of 245 paired arterial and central venous blood samples were obtained from 71 children, age range 5 days–14 years, between January and October 2006. The majority of cases were children post cardiac surgery, reflecting a group of patients in whom regular assessment of oxygen extraction is undertaken. The median number of samples per patient was 2 (range 1–22). The samples included a wide range of arterial blood glucose values, from 2.4 to 23 mmol/l. There was poor agreement between arterial and central venous blood glucose (Fig. 1). The central venous blood glucose was frequently higher than the arterial blood glucose (median 0.4 mmol/l, CI 0.3–0.5, P \ 0.001 Wilcoxon Signed Rank test). On occasions, the difference exceeded 5 mmol/l. The implications of these findings for titration of insulin therapy are obvious. Inappropriate adjustments to insulin therapy could be made and, importantly, hypoglycaemia could go undetected if central venous sampling is used to monitor blood glucose in this population of critically ill children. It is common for a child to be managed without an arterial line once their cardio-respiratory condition has stabilized, and for bloods to be taken through a central line, if present, to avoid the need for venepuncture. In contrast with our findings, a recent study by Evron et al. [2] found no difference between arterial and central venous blood glucose concentration in adult patients undergoing major orthopaedic or colon surgery though these patients were all studied in the context of general anaesthesia and surgery and were not critically ill. It is possible that glucose containing fluids infused into a vein peripheral to the central venous line or infused via one of the other central venous line lumens could have contaminated the central venous sample that was taken. As this was an opportunistic study performed under uncontrolled conditions information about glucose infusions was not collected, other than confirmation that no glucose containing fluid was infused into either the arterial or central venous sampling lines. However, alternative explanations are possible in that glucose may be generated in peripheral tissues, most notably in muscle, during critical illness and released into the venous system [3, 4]. 0 5 10 15 20 25