Direct Calorimetry (Whole-Room Calorimeter)

• Direct Measurement of the Heat Generated by the Body to Determine Energy Expenditure
• The Gold Standard for Measuring Metabolic Rate
• Does Not Provide Any Information About the Nature of Substrates Being Oxidizes to Generate Energy
• Requires Placing the Whole Body into a Large Chamber for Measurement
• Not Practical in the Hospital Setting – Large, Expensive, and Complex Engineering

Indirect Calorimetry (Metabolic Cart)

• Measures Oxygen Consumed and Carbon Dioxide Produced to Estimate Energy Expenditure
• Gold Standard for Determining In-Hospital Energy Requirements
• Generally Performed by Attaching a Monitor in Line with a Ventilator for Mechanically Ventilated Patients
  • Can Be Performed in Extubated Patients but Less Common in the ICU
• Measurement Provides a Respiratory Quotient (RQ) for Interpretation
• Often Used to Guide Nutrition in Difficult Patient Populations (Burns, Obese, etc.)
• Use Has No Proven Benefit to Morbidity or Mortality

Respiratory Quotient (RQ)

• Respiratory Quotient (RQ) = CO2 Produced / O2 Consumed
• Measure of Energy Expenditure
• Values of Pure Utilization:
  • Carbohydrate/Glucose = 1.0
  • Protein = 0.8
  • Fat = 0.7
• Interpretation:
  • RQ > 1.0: Overfeeding (Lipogenesis)
    • Increased CO2 from Overfeeding Can Increase Ventilator Dependence
    • Management: Decrease Carbohydrates and Calorie Intake
  • RQ < 0.7: Starving (Ketosis)
    • Management: Increase Carbohydrates and Calorie Intake