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Bicarbonate kinetics and predicted energy expenditure in critically ill children^1,2,3

¹ From the Critical Care Section, Department of Pediatrics, Texas Children's Hospital, Baylor College of Medicine, Houston, TX (JS, AG, WEG, DG, JC-B, LJ, and LC); the Children's Nutrition Research Center, US Department of Agriculture, Houston, TX (JS, WEG, WH, and LC); the Orthopedics Service, Boston Children's Hospital, Boston, MA (DZ); and the Cleveland Clinic Lerner College of Medicine, Cleveland, OH (RMR)

Background: 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 ¹³C 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.

Objective: We conducted a 6-h, primed, continuous tracer infusion of NaH¹³CO₃ 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.

Design: 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).

Results: 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 µmol · 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.

Conclusions: 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.