Key messages

• Energy expenditure can be measured by direct or indirect calorimetry. Indirect calorimetry, which is more practical and common in critical care, estimates energy expenditure by measuring the amount of oxygen consumed (Vo2).

• Vo2 may be measured by open- or closed-circuit variants of indirect calorimetry, depending on the specific clinical situation.

• Vo2 can also be estimated using the Fick relationship if arterial and mixed venous oxygen contents and cardiac output can be measured.

• Vo2 is a factor in an equation that converts the amount of substrate consumed (oxygen) to the amount of energy expended under 'resting conditions.'

• In the critically ill patient, resting energy expenditure should be measured frequently under standardized conditions to guide appropriate and sufficient nutritional provision.

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