Critical illness can significantly affect metabolism, so an accurate measurement of the resting energy expenditure (REE) can help determine the energy requirements in ICU patients. REE (usually 70% of the total energy expenditure) can increase after burns, sepsis, trauma, and surgery. A precise calculation of energy expenditure may prevent overfeeding or underfeeding.
REE can be measured with indirect calorimetry using a metabolic cart, which is used to measure the oxygen consumption (VO2) and carbon dioxide production (VCO2). Every liter of oxygen consumed is equivalent to the energy cost of 5 kcal. A metabolic cart can also be used to assess the energy requirements during exercise and to determine work capacity.
In 1949, Weir introduced an equation to facilitate the calculation of REE, as follows:
REE = [VO2 (3.941) + VCO2 (1.11)] 1440 min/day
Metabolic cart is indicated for the following:
To guide appropriate nutritional support
To determine the oxygen cost of work of breathing and to help select appropriate ventilator mode and settings 
To determine the causes of increased ventilatory requirements (high glucose intake can increase carbon dioxide production, stimulating ventilation and complicating weaning)
To measure cardiac output 
In general, metabolic cart has no contraindications unless transient disconnection from mechanical ventilation cannot be tolerated.
The following are relative contraindications to indirect calorimetry:
Leaks around endotracheal or tracheostomy tube, including cuffless tubes
Chest tube to suction and leaks around the chest tube
Subcutaneous emphysema and communicating tracheal esophageal fistula
Ventilatory modes that use bias flow or leak compensation
Although not representing a contraindication, measurements may be inaccurate in patients who require high levels of oxygen (FiO2 ≥60%), high PEEP (>10 cm H2 O), air leaks, peritoneal or hemodialysis up to 4 hours after (the latter due to elimination of carbon dioxide across the artificial dialysis membrane).
The key to indirect calorimetry is that all inspired and expired air must be collected; any potential leaks in the closed ventilatory system leads to errors in the readings or uninterpretable results.
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