Inspiratory and mixed expiratory gases must be sampled. O2 concentration may be measured by a fuel cell sensor or a fast response, paramagnetic sensor. CO2 is usually measured by infrared absorption. Sensors may be calibrated with reference to known concentrations of standard gas or by burning a pure fuel with a predictable O2 consumption. Measurements are usually made at ambient temperature, pressure and humidity prior to conversion to standard temperature, pressure and humidity. In order to calculate metabolic rate (energy expenditure) inspired and expired minute volumes are required. It is common for one minute volume to be measured and the other derived from a Haldane transformation:
The nitrogen concentrations are assumed to be the concentration of gas which is not O2 or CO2. Calculation of energy expenditure utilises a modification of the de Weir formula Energy expenditure = (3.94 VO2 + 1.11 VCO2) x 1.44
Although it is possible to calculate the rate of protein metabolism by reference to the urinary urea concentration, and therefore to separate non-protein from protein energy expenditure, the resulting modification of the above formula is not usually clinically significant.
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