Impaired diffusion at submaximal lung inflation in asthma and COPD patients.

<bold>Introduction:</bold> Dissolved-phase ¹²⁹Xe MRI metrics suggest that gas diffusion may be more compromised at submaximal lung inflation compared to maximal inflation. We hypothesized that this diffusion deficit could be detected by comparing the carbon monoxide transfer coefficient (Kco) at submaximal lung inflation to that measured routinely at total lung capacity (TLC). <bold>Methods:</bold> Asthma and COPD patients performed carbon monoxide diffusion tests, first at maximal lung inflation for routine Kco and alveolar volume VA and then, at a 30% reduced inflation (redux; obtaining Kcoredux and VAredux). At both inflations mixing efficiency was determined as VA/TLC and VAredux/TLCredux to examine a potential effect on Kcoredux/Kco behavior. <bold>Results:</bold> In normal subjects (n=36), median Kcoredux/Kco amounted to 130 [IQR:122-136]% as expected for normal Kco recruitment response. However, 60% of asthma patients (49/83) and 80% of COPD patients (44/55) showed reduced Kco recruitment at submaximal inflation (Kcoredux/Kco<122%). In the asthma group, Kcoredux/Kco was significantly correlated with RV/TLC ratio (r=-0.53;P<0.001), but not with VA/TLC. In COPD patients, all with abnormal routine Kco, abnormal Kcoredux/Kco response occurred in those patients with lower FEV₁, higher RV/TLC and lower VA/TLC (P<0.01 for all). <bold>Conclusion:</bold> Sixty per cent of our asthmatic patients showed a lack of normal recruitment of Kco at lung volumes closer to physiological breathing volumes, despite normal Kco at full lung inflation. The association with RV/TLC suggests that hyperinflation at lung volumes less than TLC affects the carbon monoxide diffusion rate constant Kco by distorting pulmonary capillaries and alveolar–capillary membranes.