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The Role of Small Airways in Connective Tissue Disorders

Taivans Immanuels, Strazda Gunta

The role of small airways in the pathogenesis of interstitial lung diseases is not well understood because this lung area is overlooked in conventional lung function testing methods. Only in recent years new approaches has been developed allowing us to investigate in more detail this silent lung zone. Small airways are membranous bronchi with diameter less than 2 mm which are incorporated into elastic lung network and which patency changes depending of respiratory phase. Pathologic changes in lung parenchyma influence patency of small bronchi and vice versa-the closure of small bronchi induces ventilation/perfusion mismatch in lungs that results in hypoxemia. High Resolution Computed Tomography (HRCT) can visualize small airways, detect wall thickness and presence of pathologic processes in and around airways walls but cannot evaluate the impact of the lesion into overall lung function. At the present time two methods are recognized to be able to detect obstruction in small airways-Forced Oscillation Technique (FOT) and Nitrogen Washout Test (NWT). FOT is based on the introduction of air waves of different frequencies into the airways during quiet breathing. The penetration of airwaves depends on their frequencies and allows selecting for measurement selected level of bronchial tree for measurement of resistance and reactance. NWT is based on equilibration of nitrogen concentration among lung regions after inhalation of pure oxygen. Sloping of alveolar Plato indicates on uneven gas distribution among lung units that may be caused by closure of part of small airways.