Can non-atmospheric gas concentrations be given to a subject when using the LabChart Metabolic Module?

The LabChart Metabolic Module for Windows and the LabChart 7 Metabolic Module for Mac are configured to only be used with subjects who are breathing atmospheric gas concentrations.  Use of any other gas concentration mixtures will result in incorrect calculated values. Below are the details of the calculations performed by the Metabolic Module, which illustrate how the module relies on the subject to be breathing atmospheric gas concentrations.

The rates of oxygen consumption, VO2, and carbon dioxide production, VCO2, are calculated as the differences between the expired and inspired minute volumes:

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,

where FI is an inspired fraction, FE is an expired fraction, and VI and VE are the minute volumes of inspired and expired air.

The application of these equations would be straightforward if you were measuring both VI and VE. It is standard practice, however, to measure only one of these, and to calculate the other from it. This is done via the Haldane transformation, which assumes that at nitrogen equilibrium:

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,

and the components of dried air are such that

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Composition of Dry Air

Gas

Fraction

Nitrogen and Argon (inert)

0.7904

Oxygen

0.2093

Carbon Dioxide

0.0003

Important Terms

·ATPS - Ambient Temperature and Pressure, Saturated with water vapor.
·BTPS - Body Temperature (36.6 oC), Ambient Pressure, Saturated with water vapor.
·STPD - Standard Temperature and Pressure in Dry conditions: 0 oC (273.15 K) and 760 mmHg, zero humidity.

Volume Conversions

The Metabolic Module always displays a minute volume of air as VE at BTPS, regardless of recording settings. When you are measuring VE directly, the Metabolic Module uses the averaged values of flow as measured. Alternatively, you may specify (in the Environment Tab of the Metabolic Settings dialog) that you are recording inspired air flow. In this case VI is recorded, and the module converts this to a value that reflects what VE would have been had it been measured. This requires a conversion from ATPS to BTPS using the gas law equation:

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which now becomes:

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where Psat as a function of T is:

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Expired volumes are assumed to be at a temperature Tbreath. This can either be measured by a thermistor placed in the stream of expired air close to the Flow Head, or entered as a constant value (in the Environment tab of the Metabalic Settings dialog). In either case, the expired volume is assumed to be saturated with water vapor at a pressure Psat.

Patm is the atmospheric pressure in mmHg, VI (ATPS) is the measured inspired flow, and Troom is room temperature in oC. The water vapor is given by:

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where Hrel is % relative humidity.

Calculating VO2

By convention, VO2 is expressed at STPD. The calculation differs according to whether VI or VE is measured.

If VI is measured, then the equation above becomes:

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where

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If VE is measured, then the equation above becomes:

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where

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Calculating VCO2

VCO2 is calculated using equations that correspond to those given above for VO2, but with the obvious substitutions.

Respiratory Exchange Ratio

The Respiratory Exchange Ratio (RER) is calculated as:

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Resting Energy Expenditure

The Resting Energy Expenditure (REE; kJ/min) is calculated as:

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