A COMPARISON OF METHODS OF MEASURING THE DIFFUSING CAPACITY OF THE LUNGS FOR CARBON MONOXIDE. INVESTIGATION BY FRACTIONAL ANALYSIS OF THE ALVEOLAR AIR

Abstract

The diffusing capacity of the lungs for CO (DL) was measured in 5 normal subjects and in 11 patients with emphysema by the single breath method and by the steady state method, using end tidal samples as a measure of the mean alveolar CO tension. In 5 of the patients with emphysema, DL was also calculated by a modification of Filley''s method. (J. Clin. Invest. 1954, 33, 530). Fractional samples were taken from a full expiration after breath holding and DL was calculated for each sample by the single breath method. In normal subjects and in patients with emphysema, DL was uniform throughout the expired air. Fractional analysis of the expired air of normal subjects in the steady state breathing CO showed that the concentration of CO in the lungs was uniform. In these subjects end tidal samples approximated the mean alveolar concentration of CO and could be used in the calculation of the steady state DL. Measurement of Dl by the single breath method gave larger values when the breath was held at full inspiration than when held at smaller lung volumes. The single breath and steady state methods gave the same result in normal subjects when allowance was made for the different lung volumes at which the measurements were made. Patients with emphysema did not have uniform concentrations of CO throughout the lung in the steady state; end tidal samples bore no relationship to the mean alveolar CO and could not be used for calculation of DL. A bloodless modification of Filley''s method of calculation of the steady state DL is described. This method gave results which were close to those of the single breath DL measured in the same subjects. The clinical value of the single breath and steady state methods for measurement of DL is discussed.