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Calculation of Oxygen Consumption

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Calculation of oxygen consumption is a relatively simple process that involves subtracting the amount of oxygen exhaled from the amount of oxygen in-haled:

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The volume of O_{2} inspired (I) is computed by multiplying the volume of air inhaled per minute (V̇_{I}) by the fraction (F) of air that is made up of oxygen. Room air is 20.93% O_{2}. Expressed as a fraction, 20.93% becomes .2093 and is symbolized as F_{I}O_{2}. When we exhale, the fraction of O_{2} is lowered (i.e., O_{2} diffuses from the lung to the blood) and the fraction of O_{2} in the expired (E) gas is represented by F_{E}O_{2}. The volume of expired O_{2} is the product of the volume of expired gas (V̇_{E}) and F_{E}O_{2}. Equation (1) can now be symbolized as:

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(2)$V\u02d9O2=(V\u02d92\u22c5F1O2)\u2212(V\u02d9E\u22c5FEO2)$

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The exercise values for F_{I}O_{2}, F_{E}O_{2}, V̇_{I}, and V̇_{E} for a subject are easily measured in most exercise physiology laboratories. In practice, F_{I}O_{2} is not generally measured but is assumed to be a constant value of .2093 if the subject is breathing room air. F_{E}O_{2} will be determined by a gas analyzer, and V̇_{I} and V̇_{E} can be measured by a number of different laboratory devices capable of measuring airflow. Note that it is not necessary to measure both V̇_{I} and V̇_{E}. This is true because if V̇_{I} is measured, V̇_{E} can be calculated (and vice versa). The formula used to calculate V̇_{E} from the measurement of V̇_{I} is called the “Haldane transformation” and is based on the fact that nitrogen (N_{2}) is neither used nor produced in the body. Therefore, the volume of N_{2} inhaled must equal the volume of N_{2} exhaled:

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(3)$[V\u02d91\u22c5F1N2]=[V\u02d9EFEN2]$

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Therefore, V̇_{I} can be computed if V̇_{E}, F_{I}O_{2}, and F_{E}O_{2} are known. For example, to solve for V̇_{I}:

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(4)$V\u02d91=(V\u02d9E\u22c5FEN2)F1N2$

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Likewise, if V̇_{I} was measured, V̇_{E} can be computed as:

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(5)$V\u02d9E=(V\u02d91\u22c5F1N2)FEN2$

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