Background: Excessive dynamic airway collapse (EDAC) represents pathologic obstruction, but the flow resistance it creates and the possible impact on pulmonary function tests is unclear. Our aims were to explore the flow resistance in a model, and in patients to relate observed EDAC to pulmonary function tests.
Methods: Model study: A garden hose was compressed from 1 side to simulate the posterior tracheal wall bulging into the lumen for 2 lengths, 3 and 12 cm, in 8 steps. Resistance was measured at each step for airflows 1 L/s through 9 L/s. Digital photos of the luminal area at each obstruction step were analyzed by a computer to estimate the cross-sectional area reduction and the corresponding shortest distance between the bulging (posterior) and the opposite (anterior) wall (AP-distance). Patient study: A total of 104 stable chronic obstructive pulmonary disease patients studied by pulmonary function tests and bronchoscopy. The tracheal obstruction was observed during forced expiration and cough, and the cross-sectional area reduction was estimated from the observed AP-distance using the results from the model study.
Results: Model study: The flow resistance increases significantly when the cross-sectional area is reduced to >70% for an obstruction length of 3 cm. Patient study: EDAC was found in 18 patients during cough, none during forced expiration. There was no correlation between the obstructions found and the results from pulmonary function tests.
Conclusions: EDAC during peak pressure has negligible impact on pulmonary function tests and breathing symptoms. The importance of the severity, length, and duration of the EDAC is discussed.