Dynamic effects of obstructed airways mechanics on the forced expiratory curve

Abstract

Spirometry is the most popular test of lung function. Its status arises from the effort-independence of the maximal expiratory flow-volume (MEFV) curve, its reproducibility for a given subject and simultaneous sensitivity to respiratory disorders. Previous trials have shown that the morphology-based, quasi-static models cannot reproduce characteristic swings in the MEFV curve, sometimes visible in the case of obstructive diseases. The aim of this work was to test the hypothesis that the aforementioned details in the MEFV curve are caused by dynamic phenomena occurring during forced expiration, and that they manifest particularly in obstructive diseases. To this end, the computational model for forced expiration including the fundamental physical phenomena in quasi-static conditions was further developed by including the dynamic phenomena: additional flows from narrowing airways, and gas compressibility and inertia. The MEFV curves simulated using the quasi-dynamic and quasi-static models were then compared for a variety of respiratory system states. For most simulated cases, the differences between forced expiratory curves computed with the models were negligible. Only implementing some specific conditions causing flow limitation in small airways yielded a visible alteration and the characteristic swing after the peak expiratory flow (PEF). Concluding, the dynamic effects of airway narrowing and gas compressibility and inertia modify the MEFV curve near the PEF slightly and only in specific cases. This finding justifies the general use of the quasi-static models as an adequate tool for forced expiration simulations.