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ABSTRACT
Indirect calorimetry measures oxygen consumption and carbon dioxide production to calculate resting energy expenditure and respiratory quotient. The respiratory quotient can be determined from indirect calorimetry to determine substrate utilization and used to alter the patient's nutrition support regimen. All but one indirect calorimeter manufactured in the United States are open-circuit rather than closed-circuit systems. JAm Diet Assoc. 19974 97(suppl 2):S154-S160.
Energy expenditure (EE) is defined as the ability of a person to do internal and external work. Energy exists in many forms that are interchangeable and related. Thus, the chemical energy of 1 g glucose can be converted to heat, so that if completely oxidized to carbon dioxide and water, it yields 3.74 kcal. Nutrients are converted from chemical energy to various energy forms that can be used by the body. Chemical energy is changed to mechanical energy for muscle contractions, thermal energy for regulation of body temperature, and electrical energy for brain and nerve activity.
EE can be estimated by various formulas. There are more than 190 published formulas in the literature to estimate EE (1). Many of these equations provide the clinician with a reasonable estimate of energy need, but there are occasions (eg, intensive care unit patients) when it may be necessary to measure EE directly.
One can measure EE directly by placing a person in a calorimeter and measuring the amount of heat given off by the body mass. This equipment is expensive and has limited clinical potential. EE also can be measured indirectly with an indirect calorimeter (metabolic cart). Indirect calorimetry (IDC) measures oxygen consumption (Vo2) and carbon dioxide production (Vco) to calculate resting energy expenditure (REE) and respiratory quotient (RQ). One liter of oxygen consumed generates 3.9 kcal; 1 L carbon dioxide produced generates 1.l kcal. The Weir equation then is used to calculate EE (Figure 1) (2). If a total 24-hour urine nitrogen is collected on the same day as the IDC measurement, the complete Weir equation is used to adjust the calculation of REE for the incomplete oxidation of protein. However, because the difference between the results of the complete and abbreviated formulas is less than 2% (2), many clinicians use the abbreviated Weir equation because of the difficulties associated with a 24-hour urine...