The determination of the percentage of CO2 in the alveolar air, by the method of Haldane and Priestley, is sufficiently accurate for clinical purposes when the individual is at rest.
It is evident, however, that an error may arise in the determination of the percentage of CO2 in the alveolar air, due to the time during the forced expiration, short of the extreme limit, at which the sample may be unconsciously taken. This error can only be overcome by taking several samples.
In individuals at rest having a tidal air below 425 cubic centimeters, the method of Douglas and Haldane for the calculation of the dead space gives approximately accurate results; with a tidal air above 425 cubic centimeters the results obtained may be wholly inaccurate.
In patients with marked pulmonary tuberculosis so slight an effort as that involved in collecting the expired air tends to lower the percentage of CO2 in the alveolar air.
In collecting the expired air by the rubber bag method, it is absolutely essential that the bag be washed out with ordinary atmospheric air after every observation.