Calculation of Oxygen Consumption
Calculation of oxygen consumption is a relatively simple process that involves subtracting the amount of oxygen exhaled from the amount of oxygen in-haled:
The volume of O2 inspired (I) is computed by multiplying the volume of air inhaled per minute (V̇I) by the fraction (F) of air that is made up of oxygen. Room air is 20.93% O2. Expressed as a fraction, 20.93% becomes .2093 and is symbolized as FIO2. When we exhale, the fraction of O2 is lowered (i.e., O2 diffuses from the lung to the blood) and the fraction of O2 in the expired (E) gas is represented by FEO2. The volume of expired O2 is the product of the volume of expired gas (V̇E) and FEO2. Equation (1) can now be symbolized as:
The exercise values for FIO2, FEO2, V̇I, and V̇E for a subject are easily measured in most exercise physiology laboratories. In practice, FIO2 is not generally measured but is assumed to be a constant value of .2093 if the subject is breathing room air. FEO2 will be determined by a gas analyzer, and V̇I and V̇E can be measured by a number of different laboratory devices capable of measuring airflow. Note that it is not necessary to measure both V̇I and V̇E. This is true because if V̇I is measured, V̇E can be calculated (and vice versa). The formula used to calculate V̇E from the measurement of V̇I is called the “Haldane transformation” and is based on the fact that nitrogen (N2) is neither used nor produced in the body. Therefore, the volume of N2 inhaled must equal the volume of N2 exhaled:
Therefore, V̇I can be computed if V̇E, FIO2, and FEO2 are known. For example, to solve for V̇I:
Likewise, if V̇I was measured, V̇E can be computed as:
The values for FIN2 and FEN2 are obtained in the following manner. If the subject is breathing room air, FIN2 is considered to be a constant .7904. The final remaining piece to the puzzle is FEN2. Recall that the three principal gases in air are N2, O2, and CO2 and the sum of their fractions must add up to 1.0 (i.e., FECO2 + FEO2 + FEN2 = 1.0). Therefore, FEN2 can be computed by subtracting the sum of FECO2 and FEO2 from 1 (i.e., FEN2 = 1 − (FECO2 + FEO2)). Because the expired fractions of O2 and CO2 will be determined by gas analyzers, FEN2 can then be calculated.
Calculation of Carbon Dioxide Production
The volume of carbon dioxide produced (V̇CO2) can be calculated in a manner similar to . That is, the volume of CO2 produced is equal to:
The steps in performing this calculation are the same as in the computation of . That is, V̇E and V̇I must be measured (or calculated) and the fraction of expired carbon dioxide (FECO2) must be determined by a gas analyzer. Similar to FIO2, the fraction of inspired carbon dioxide (FICO2) is considered to be a constant value of .0003.
Standardization of Gas Volumes
By convention, or V̇CO2 are expressed in liters · min−1 and standardized to a reference condition called “STPD.” STPD is an acronym for “standard temperature pressure dry.” In a similar manner, pulmonary ventilation is expressed in liters · min−1 and standardized to a reference standard called BTPS, an acronym for “body temperature pressure saturated.” The purpose of these reference standards is to allow comparison of gas volumes measured in laboratories throughout the world, which may vary in ambient temperature and barometric pressures. Standardization of gas volumes to a specified temperature and pressure is necessary because gas volume is dependent upon both temperature and pressure. For instance, a given number of gas molecules will occupy a greater volume at a higher temperature and lower pressure than at a lower temperature and higher pressure. This means that a fixed number of gas molecules would change volume as a function of the ambient temperature and barometric pressure. This poses a serious problem to researchers trying to make comparisons of respiratory gas exchange, because temperature and pressures vary day by day and vary from one laboratory to another. By standardizing temperature and pressure conditions for gases, the scientist or technician knows that two equal volumes of gas contain the same number of molecules. For these reasons, respiratory gases must be corrected to a reference temperature and volume.