Exercise Science
Section 2
The composition of air (and hence inspired air) is considered to be constant (CO2 = 0.03%, O2 = 20.93%, N = 79.04%). It is therefore possible to calculate the volume of air inspired. During respiration the number of O2 molecules removed does not necessarily equal the number of CO2 molecules produced. As a result there is a change in the proportion of nitrogen in the samples taken and in the composition of air. Using the Haldane Transformation, calculate VISTPD, the volume of air inspired per minute.
Using the Haldane Transformation to calculate VISTPD
Because nitrogen gas is not metabolized:
VI and VE are VISTPD and VESTPD, respectively,
and variables of the type FIN2% represent concentrations of the gas specified (as a percentage) in the inspired (I) or expired (E) air.
Mass inspired N2 = Mass expired N2
Mass = Concentration x Volume. Hence this equates to:FIN2% x VI = FEN2% x VE
where:VI and VE are VISTPD and VESTPD, respectively,
and variables of the type FIN2% represent concentrations of the gas specified (as a percentage) in the inspired (I) or expired (E) air.
The equation above rearranges to the Haldane Transformation:
VI = VE x FEN2% / FIN2%
VE is measured, FIN2% is constant at 79.04%, and FIN2% can be calculated from the meaured concentrations of expired oxygen and carbon dioxide:FEN2% = 100 – FEO2% – FECO2%
Worked Example
O2 in expired air in the Douglas bag: 16.8%
CO2 in expired air in the Douglas bag: 4.1%
VESTPD: 8.36 litres/min
VI = VE x FEN2% / FIN2%
FEN2% = 100 – 16.8 – 4.1 = 79.1
Hence VI = 8.36 x 79.1/79.04
i.e. VI = 8.37 litres/min
VISTPD: litres/min (2 DP)
or
David P. Leader