Indirect calorimetry is a means of assessing energy requirements of the individual on the basis of the relationship between energy expended, proportion of carbon in energy source burnt, and oxygen consumed. Hence, the respiratory quotient varies according to the fuel:
Energy source Respiratory quotient
Using urinary urea excretion, it is theoretically possible to assess the contribution of protein oxidation and work back to determine the relative contributions of carbohydrate and fat.
Practically, the patient needs to be connected to an apparatus for measuring inspired and expired oxygen and carbon dioxide. Devices include the ungainly Douglas bag and sealed head canopies.
Pragmatically, in the majority of patients, indirect calorimetry is too difficult to carry out to be of daily use. Instead, approximate assessments of energy requirements are made on the basis of clinical assessment and / or anthropometric measures.
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