- 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
- 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)
- Can be attached to mechanically ventilated or self ventilating patients
- uses indirect calorimetry (e.g. Deltatrac™ II Metabolic Monitor)
METHOD OF USE
- 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)
- 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
- inspired and expired O2 fractions
- inspired and CO2 fractions
- 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
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
- 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
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)
- 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.
Chris is an Intensivist and ECMO specialist at the Alfred ICU in Melbourne. He is also a Clinical Adjunct Associate Professor at Monash University. He is a co-founder of the Australia and New Zealand Clinician Educator Network (ANZCEN) and is the Lead for the ANZCEN Clinician Educator Incubator programme. He is on the Board of Directors for the Intensive Care Foundation and is a First Part Examiner for the College of Intensive Care Medicine. He is an internationally recognised Clinician Educator with a passion for helping clinicians learn and for improving the clinical performance of individuals and collectives.
After finishing his medical degree at the University of Auckland, he continued post-graduate training in New Zealand as well as Australia’s Northern Territory, Perth and Melbourne. He has completed fellowship training in both intensive care medicine and emergency medicine, as well as post-graduate training in biochemistry, clinical toxicology, clinical epidemiology, and health professional education.
He is actively involved in in using translational simulation to improve patient care and the design of processes and systems at Alfred Health. He coordinates the Alfred ICU’s education and simulation programmes and runs the unit’s education website, INTENSIVE. He created the ‘Critically Ill Airway’ course and teaches on numerous courses around the world. He is one of the founders of the FOAM movement (Free Open-Access Medical education) and is co-creator of litfl.com, the RAGE podcast, the Resuscitology course, and the SMACC conference.
His one great achievement is being the father of three amazing children.
On Twitter, he is @precordialthump.