Expired gas analysis indirect calorimetry can be performed in an open circuit system with a metabolic cart or in a closed circuit system with a respiration chamber. Selecting the indirect calorimetry system depends mainly on the purpose of measurement and user requirements.

Figure 1: Simplified principle of metabolic processes and calorimetry techniques

Open-circuit system

In the open-circuit system, the subject inspires room air or air supplied by a mechanical ventilator through an apparatus (e.g. a canopy, mask, or mouthpiece and noseclip) and expires into a gas sampling system, which eventually vents the expired air back into the room. The sampled air is corrected for standard conditions and the volume, oxygen and carbon dioxide content is analysed. Volume of oxygen consumed (VO2) and carbon dioxide produced (VCO2) are derived from these measurements. For instance with the Omnical developed by Maastricht Instruments BV, a mechanical ventilator supplies in-flow of room air to the subject through a mask or hood with no breathing restrictions and directs out-flow into the system. Air flow and respiratory gas samples are continuously obtained and accurately analysed by gas analysers within the Omnical.


  • real-time monitoring of metabolic parameters and energy expenditure
  • portability
  • affordability
  • user-friendliness

Open-circuit systems are fairly common in metabolic research units within academia, medical clinics, human performance labs and sports expertise centres. These devices are designed for shorter measurement durations up to a few hours. For the determination of VO2 and VCO2 during a longer time interval (up to several days), a closed circuit respiration chamber may be the method of choice.

Closed-circuit system

In closed-circuit systems, the subject stays in an airtight room through which a stream of fresh air is directed, often with furniture, washbasin and deep-freeze toilet. Composition and volume of the inlet and outlet airstream are measured. The respiration chambers developed by Maastricht Instruments BV gives the impression of normal furnished rooms and feature a double set of gas analyzers with continuous automated calibration and automated data collection. The chamber itself contains a sophisticated ventilation and climate control system. In addition, physical activity can be performed using a cycle ergometer or a treadmill.


  • real-time monitoring of metabolic parameters and energy expenditure
  • feasibility of long periods of measurement (e.g. detailed 24 hour energy profiles)
  • high accuracy
  • no movement restrictions as subjects are not tethered to any equipment

These easy-to-use chambers have been operational for over 30 years and form part of the Metabolic Research Unit Maastricht (MRUM). Two such chambers have been set up in established research facilities in the University of Warwick, UK and also in the Singapore Institute of Clinical Science (SICS).

If you are interested in using indirect calorimetry systems, please contact us for more information.