Indirect Calorimetry and Metabolic Cart

OVERVIEW

• Indirect calorimetry is a technique that measures inspired and expired gas flows, volumes and concentrations of O2 and CO2
• allows measurement of oxygen consumption and carbon dioxide production
• non-invasive and accurate
• the equipment used is also known as a metabolic cart

USES

• determine energy requirements and response to nutrition over time
• calculation of energy expenditure allows determination of nutritional requirements
• helps to determine the proportion of substrates used for energy production
• measures O2 consumption in shock
• typically used in difficult patient groups (e.g. obese, burns)

DESCRIPTION

Metabolic cart

• Can be attached to mechanically ventilated or self ventilating patients
• uses indirect calorimetry (e.g. Deltatrac™ II Metabolic Monitor)

METHOD OF USE

Overview

• continuous measurement of O2 consumption (VO2) and CO2 production (VCO2)
• used to determine caloric needs and estimates substrate oxidation
• calculates Respiratory Quotient (CO2 production/O2 consumption) and Resting Energy Expenditure (REE)

Principles

• production of chemical energy is proportional to gas exchange
• Weir equation used to calculate energy expenditure:
  • Energy expenditure = (3.9VO2 – 1.1VCO2) – 2.17 (urinary nitrogen)
  • Abbreviated Weir Equation = used to calculate REE: REE = (3.94 x VO2) + (1.1 x VCO2)

Methods of measurement

• Mixing chamber
• Breath-by-Breath method
• Dilution method

Measured values

• inspired and expired O2 fractions
• inspired and CO2 fractions
• MV

Derived values

• energy expenditure (VO2 and VCO2)
• respiratory quotient is the ratio of CO2 produced : O2 consumed (1.0 when fuel carbohydrates, 0.8 when protein, 0.7 when fat); normal is 0.8

COMPLICATIONS

Sources of error

• Inaccuracies can arise from air leaks (e.g. circuit or pneumothorax)
• errors in recording FiO2 and MV are multiplied leading to increased error
• most O2 sensors are inaccurate at higher FiO2s
• gas concentrations samples often done via long thin tubing which can block easily
• PEEP alters ventilator circuit compressibility causing volume changes and measurement errors

Other issues

• difficult to use in children (e.g. uncuffed tubes, HFOV)
• VO2 from PAC will be less than VO2 from indirect calorimetry as PAC does not measure lung O2 consumption
• over or underfeeding may occur based on results
• no strong evidence of clinical benefit
• actually measures consumption, not needs
• single snap-shots worse than prolonged, averaged studies

OTHER INFORMATION

Theoretically a metabolic cart could be used to calculate CO using the Direct Fick equation (see pulmonary artery catheter for this as well as an alternative method of calculating REE)

• VO2 can be directly measured
• arterial oxygen concentration determined from an arterial blood gas
• mixed venous gas inferred from a central venous line specimen (e.g. PreSepTM catheter)

EVIDENCE

• no benefit in terms of mortality or other clinical outcome
• in burns patients it has been shown that a decline in energy expenditure predicted mortality

References and Links

• Lev S, Cohen J, Singer P. Indirect calorimetry measurements in the ventilated critically ill patient: facts and controversies–the heat is on. Crit Care Clin. 2010 Oct;26(4):e1-9. PMID: 20970041.
• McClave SA, Martindale RG, Kiraly L. The use of indirect calorimetry in the intensive care unit. Curr Opin Clin Nutr Metab Care. 2013 Mar;16(2):202-8. PMID: 23340008.