At what FiO2 is the accuracy of indirect calorimetry of mechanically-ventilated patients decreased?

• A. FiO2 < 30
• B. FiO2 40-50
• C. FiO2 ≥ 60
• D. FiO2 20

Answer: (C)

Indirect calorimetry relies on precise measurement of inspired and expired O2 and CO2 to calculate O2 consumption and CO2 production. When FiO2 is very high, the gas readings become more prone to sensor drift, nonlinearity, and sampling line issues (such as condensation or leaks in humidified circuits). This makes the small differences used to derive VO2 and VCO2 less reliable, so energy expenditure estimates lose accuracy. Most equipment is validated up to FiO2 around 60%; at or above this level, accuracy decreases. Therefore, the threshold where accuracy drops is FiO2 ≥ 60%.