183 – VO2 and RER Measurement

Precise real-time Oxygen Consumption (VO₂) and Respiratory Exchange Ratio (RER) measurements can be performed with the MP150/100 System and the CO2100, O2100 and DA100 modules with some airflow accessories. When performing tests of this kind, many factors exist to confound the measurement. For example, it may be reasonable to think that the volume of expired air is always the same as inspired air, but this is only true when the volume of carbon dioxide expired is equal to the volume of oxygen consumed. Namely, the inspired and expired volumes are equal only when the RER equals one.

The Respiratory Exchange Ratio (RER) is defined as VCO₂ produced divided by VO₂ consumed (where V is volume). Accordingly, when performing these measurements, precise determination of inspired and expired volumes plus accurate gas concentration level recording is required. This application note focuses on the details of the setup and calculations to obtain precise VO₂ and RER measurements from resting or exercising subjects.

The subjects breathe through a non-rebreathing “T” valve to inspire fresh air through a pneumotach (TSD107B) and expire to a mixing chamber for gas concentration (O₂ and CO₂) monitoring. When using a mixing chamber to average CO₂ and O₂ concentrations over several breaths, there is no performance degradation (due to the module gas concentration response time) when measuring these changes for arbitrarily high breathing rates. Accordingly, a mixing chamber is typically recommended for quick, accurate and easy metabolic analysis.

Additional accessories required, but not shown, include a calibration syringe (AFT6A) and calibrated gas mixture. The calibration syringe is used to calibrate the TSD107B and to flush out the mixing chamber for ambient gas concentration level calibration. The calibrated gas mixture is used to introduce known concentrations of CO₂ and O₂ to calibrate the upper and lower ranges of the gas measurement modules respectively.