Indirect calorimetry

Calorimetry refers to the measurement of energy production. Direct calorimetry is the measurement of heat production in a sealed chamber but is impractical for critically ill patients. Indirect calorimetry measures the rate of oxidation of metabolic fuels by detecting the volume of O2 consumed and CO2 produced. The ratio of CO2 production to O2 utilisation (respiratory quotient or RQ) defines which fuels are being utilised (see table). Knowledge of the oxygen utilisation by the various fuels allows the calculation of energy production. Carbohydrate and fat are oxidised to CO2 and water producing 15-17 and 38-39kJ/g respectively. Protein is oxidised to CO2, water and nitrogen (subsequently excreted as urea) producing 15-17kJ/g.

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