Energy expended, in the form of heat produced, could be measured by placing the entire patient in an insulated chamber and measuring temperature change. This process (direct calorimetry) is obviously impractical for critical care. Indirect calorimetry, developed and validated using direct calorimetry, is the calculation of energy utilization based on measurement of O2 consumption (Vo2). Vo2 can be measured directly or calculated using the Fick equation. Once Vo2 has been measured, the metabolic rate can be described as either milliliters of O 2 consumed per minute (normal value, 100-120 ml O^min/m2) or kilocalories of foodstuffs metabolized per day (normal value, 25-30 kcal/kg/day). Accurate measurement of Vo2 is the basis for indirect calorimetry. Minor corrections and further calculations can be carried out if VCO2 and urinary nitrogen are also measured.
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Metabolism. There isn’t perhaps a more frequently used word in the weight loss (and weight gain) vocabulary than this. Indeed, it’s not uncommon to overhear people talking about their struggles or triumphs over the holiday bulge or love handles in terms of whether their metabolism is working, or not.