Determination of lung volume: a simple constant volume modification of Christie’s method

Abstract

The total lung volume consists of the vital capacity plus the residual air. Since the vital capacity may be measured directly, determination of the total lung volume depends on the measurement of the residual air. Alternatively, the functional residual air may be measured and the figure added to the complemental air (terminology of Christie 1932). As the volume of reserve air expelled from the lungs in untrained subjects is liable to fortuitous variations, calculations of the residual air based on gas dilution at the end of a forced expiration are open to considerable error; for the same reason, determinations of the functional residual air at the end of an ordinary expiration may be inaccurate. Van Slyke and Binger (1923) and Christie (1932) have shown that this source of error may be avoided by taking graphic spirometer records of respiration during the period of mixing. Since the dilution of the lung gases is achieved by ordinary quiet respiration, this method requires no special respiratory efforts and is thus eminently suitable for clinical investigation. Determination of lung volume by Christie’s method, the underlying principle of which is exceedingly simple, involves only the recording of an ordinary spirometer tracing and the analysis of the oxygen and nitrogen content of the spirometer gas at the end of a period of rebreathing. The essence of the method is as follows: