PMID: 10809328

Issn Print: 0090-3493

Publication Date: 2000/04/01


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The use and interpretation of indirect calorimetry in critically ill patients

Cindy Fitch; Jeffrey Blumer

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Excerpt

Estimating energy requirements is a basic component of medical nutrition therapy for critically ill infants and children. The challenge for clinicians is to provide adequate energy to support optimal tissue synthesis and immune function while avoiding complications associated with overfeeding. Indirect calorimetry with a metabolic cart can provide data regarding energy expenditure during a specific time frame. This issue of Critical Care Medicine contains an article by Dr. Briassoulis and colleagues (1) that compares measured energy expenditure in critically ill infants, children, and adolescents with the energy expenditures predicted by various formulas. Studies such as this make an important contribution to our understanding of the energy requirements of children in an intensive care unit. At the same time, they underscore the need for a careful and thoughtful evaluation of the data available from indirect calorimetry.
Energy requirement is the sum of energy expenditure, energy stored, and energy excreted (2). Indirect calorimetry measures only energy expenditure. To apply the data in a clinically useful manner, it is necessary to understand what is being measured and how the numbers relate to total energy needs. Energy expenditure includes basal metabolic rate (BMR), diet-induced thermogenesis, and energy expended for activity. Various equations have been used to predict BMR in children. Harris and Benedict (3) used indirect calorimetry data from 239 adults and 97 infants to formulate an equation to predict BMR. Their equation was based on a linear regression analysis of weight, height, and age for males and females. Talbot (4) found that these equations were not consistent with his clinical experience and developed standards that were specific for children based on data from 2,200 girls and 800 boys. In 1985, the Food and Agriculture Organization/World Health Organization/United Nations University (FAO/WHO/UNU) committee published new equations for predicting BMR in children (5). These equations were based on data from indirect calorimetry studies of >6,000 people. Schofield (6) used the same data plus additional studies to provide predictive equations that included weight and height.
It is difficult to measure BMR in infants and small children. By convention, it should be measured under standard conditions in which the subject is at rest in a thermoneutral environment after a 10-12 hr fast (7). It is usually measured immediately after awakening in the morning. To apply these standards to infants is not practical. It is more practical and more common to measure resting energy expenditure (REE). REE is measured after a 30-min rest in a nonfasting state (8). Because it includes energy needed for diet-induced thermogenesis it is 5% to 10% higher than BMR (9). It is imperative that clinicians that use either predictive equations or indirect calorimetry to estimate energy needs understand how and under what conditions the studies were done.
Dr. Briassoulis and colleagues (1) compared measured energy expenditure in 37 critically ill, mechanically ventilated infants, children, and adolescents with REE predicted by four different equations. They found that MEE was less than the expected REE on the basis of the Harris-Benedict, Schofield, and FAO/WHO/UNU equations and was similar to the REE expected on the basis of the Talbot equation. These results are consistent with those published by Chwals et al. (10) and Gebara et al. (11). Chwals et al. (10) measured energy expenditure in 20 infants and children who ranged in age from 5 days to 46 months. They compared MEE with a predicted value by using Talbot's equation to estimate BMR and adding a stress factor. MEE was less than expected but it was similar to the REE expected on the basis of the Talbot equation. Gebara et al.
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