Determination of Distribution of Diffusing Capacity in Relation to Blood Flow in the Human Lung*

Abstract

A method for appraising the distribution of diffusing capacity of the lungs (D_L) in relationship to pulmonary capillary blood flow ([unk]Q_C) in normal human subjects was derived from measurements of oxygen diffusing capacity (D_{LO2 ) and carbon monoxide diffusing capacity (DLCO) performed during breath holding. This method utilizes the fact that the observed DLO2 is considerably reduced in value if uneven distribution of DL with respect to [unk]QC (uneven DL/[unk]QC) is present. In contrast, DLCO is barely affected by uneven DL/[unk]QC, and from its measured value one can calculate the value DLO2 would have if no uneven DL/[unk]QC were present (true DLO2 ). Once observed DLO2 and true DLO2 are known, the degree of uneven DL/[unk]QC in the lung can be calculated. In five normal, resting, sitting subjects average values for true DLO2 were 57 ml per (minute × mm Hg), and the directly measured DLO2 was 33 ml per (minute × mm Hg). These values could be explained if one-half of total [unk]QC were distributed to approximately 15% of total DL. These measurements did not permit the determination of the alveolar to end capillary O2 gradient, but calculations demonstrate that an important factor in determining its size may be the pattern of uneven DL/[unk]QC present in the lungs. Estimations of the alveolar-end capillary O2 gradient from measurements of DLCO or DLO2 that do not take into account uneven DL/[unk]QC may underestimate its size.}