Effect of ventilation and diffusion nonuniformity on DLCO (exhaled) in a lung model

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Abstract
Lung diffusing capacities measured at multiple intervals during a single exhalation [DLCO(exhaled)] remained constant with lung volume in normal subjects, but decreased with decreasing lung volume in patients who may have had diffusion non-uniformity. The theoretical basis of these results was examined by determining what factors affected DLCO(exhaled) in a computerized lung model in which diffusion in each compartment remained constant with lung volume. DLCO(exhaled) decreased with decreasing lung volume when a small lung region lacked diffusion. Change in DLCO(exhaled) with lung volume was affected by non-uniform ventilation and these effects could not be eliminated by correcting the CO decay for the simultaneous decay of He. DLCO(exhaled) values were influenced by the exhaled flow rate in the presence of non-uniform ventilation and/or non-uniform diffusion. Prolonging the period of breath holding prior to exhalation reduced DLCO(exhaled) values at all lung volumes when non-uniform diffusion was simulated, but did not affect DLCO(exhaled) when non-uniform ventilation only was simulated.