Submitted to: American Journal of Clinical Nutrition
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/25/2008
Publication Date: 8/1/2008
Citation: Sy, J., Gourishankar, A., Gordon, W.E., Griffin, D., Zurakowski, D., Roth, R.M., Coss-Bu, J., Jefferson, L., Heird, W., Castillo, L. 2008. Bicarbonate kinetics and predicted energy expenditure in critically ill children. American Journal of Clinical Nutrition. 88(2):340-347. Interpretive Summary: To determine nutrient requirements in healthy and sick children, it is necessary to know the fraction of carbon dioxide produced during nutrient oxidation but not excreted. This fraction has not been described in critically ill children, and it is necessary to determine amino acid oxidation rates. This fraction of carbon dioxide is estimated by infusion of bicarbonate labeled with stable isotopes. Furthermore, by knowing bicarbonate kinetics, and the equivalents of carbon dioxide produced in the diet, the energy requirements of these patients can be estimated. In the current study using non-radioactive, stable isotopes of bicarbonate labeled with 13C carbon, we determine the rates of appearance of bicarbonate, the fraction of bicarbonate that is fixed in metabolic reactions, and the energy requirements of critically ill children. We found that the current energy intake recommendations, based on the World Health Organization (WHO) equations, grossly overestimate energy requirements in critically ill children. Conversely, the Schofield equations underestimate energy requirements of critically ill children. Hence, bicarbonate kinetics can be used to accurately determine energy requirements in critically ill patients, when indirect calorimetry is not possible. These studies describe bicarbonate recovery fraction in critically ill children, to be used in further studies on the determination of amino acid requirements. They also underscore the precarious nutritional support that critically ill children receive, with over- or underestimation of their caloric needs.
Technical Abstract: To determine nutrient requirements by the carbon oxidation techniques, it is necessary to know the fraction of carbon dioxide produced during the oxidative process but not excreted. This fraction has not been described in critically ill children. By measuring the dilution of (13)C infused by metabolically produced carbon dioxide, the rates of carbon dioxide appearance can be estimated. Energy expenditure can be determined by bicarbonate dilution kinetics if the energy equivalents of carbon dioxide (food quotient) from the diet ingested are known. We conducted a 6-h, primed, continuous tracer infusion of NaH(13)CO(3) in critically ill children fed parenterally or enterally or receiving only glucose and electrolytes, to determine bicarbonate fractional recovery, bicarbonate rates of appearance, and energy expenditure. Thirty-one critically ill children aged 1 mo-20 y who were admitted to a pediatric intensive care unit at a tertiary-care center were studied. Patients were stratified by age, BMI, and severity score (PRISM III). Fractional bicarbonate recovery was 0.69, 0.70, and 0.63, respectively, for the parenterally fed, enterally fed, and glucose-electrolytes groups, and it correlated with the severity of disease in the parenteral (P < 0.01) and glucose-electrolytes (P < 0.05) groups. Rates of appearance varied between 0.17 and 0.19 micromol . kg(-1) . h(-1) With these data and estimates of the energy equivalents of carbon dioxide (a surrogate for respiratory quotient), energy expenditure was determined. The 2001 World Health Organization and Schofield predictive equations overestimated and underestimated, respectively, energy requirements compared with those obtained by bicarbonate dilution kinetics. Bicarbonate kinetics allows accurate determination of energy needs in critically ill children.