Importance of adjusting carbon monoxide diffusing capacity (DLCO) and carbon monoxide transfer coefficient (KCO) for alveolar volume

The volume dependence of single breath carbon monoxide diffusing capacity ( DLCO) and carbon monoxide transfer coefficient (KCO) was determined in 24 h ealthy subjects. The change in DLCO [fraction of DLCO measured at total lun g capacity (TLC)] to change in alveolar volume [fraction of alveolar volume (V-A) at TLC] closely fitted a simple linear regression and matched a theo retical model. As V-A decreased, DLCO fell linearly and KCO increased as ex pected from the relation of DLCO to V-A. The equations for adjustment of pr edicted DLCO and KCO for alveolar volume are: DLCO/D-L COtlc = 0.58 +/- 0.42VA/VAtlc KCO/KCOtlc = 0.42 + 0.58/(VA/VAtlc) DLCO and KCO were evaluated in 2313 patients. Subgroups of patients with as thma, emphysema, extrapulmonary lung disease, interstitial lung disease and lung resection were identified. Unadjusted DLCO and KCO percent predicted values showed large differences and much variability, so can be misleading As expected, KCO and DLCO percent predicted values adjusted for alveolar vo lume were nearly identical. Subgroups have characteristic patterns of V-A a nd unadjusted and adjusted DLCO and KCO. Changes in DLCO and KCO with alveo lar volume are relevant for accurate interpretation of diffusion in patient s with low lung volumes. Adjusting predicted DLCO and KCO for alveolar volu me provides a better assessment of lung function.