Airways Obstruction From Asbestos Exposure
Workers with clinical asbestosis have been characterized as having “restrictive” disease because of reduced vital capacity (VC) and in some cases reduced total lung capacity (TLC). This is thought to occur despite airway obstruction and increased TLC due to cigarette smoking. The TLC was decreased when functional residual capacities (FRC) were measured by gas dilution using the closed circuit helium method, which underestimates residual volume when there is air trapping. Only a few FRCs in asbestosis were measured by body plethysmography. Two other factors, incomplete expirations (early termination) which undermeasure VC and the use of pneumotachygraphs for spirometry which do not yield accurate volumes at low flows may have underestimated VC, and TLC when VC was added to RV there asthma inhaler. Despite these problems, a 1970 study of asbestos exposure in miners and millers found reduced FEVi, that is, airways obstruction and reduced FVC before there was visible radiographic asbestosis. Subsequently, FEF75-85 and/or FEF25-75 were shown to be reduced before FEVi and FVC were decreased.” This physiologic obstruction has been attributed to peribronchiolar infiltrates and fibrosis as observed originally by Gloyne in 1933 and described subsequently by Wright and Churg.
With damage to more bronchioles, there should be an orderly continuum of advancing airways obstruction. How can airways obstruction, which characterizes early asbestosis, be reconciled with the apparent “restrictive defect” as the disease advances? Does airways obstruction produce a real restrictive defect or only an apparent one as air trapping reduces VC? Is airways obstruction accompanied by alveolar fibrosis, which reduces the size of alveoli or decreases their numbers to “restrict” volume? If this were so, TLC would decrease and be apparent as higher diaphragms and narrower intercostal spaces on chest radiographs. Against this hypothesis, the lung volumes measured from chest radiographs were not decreased in the 85 subjects with advanced asbestosis from a 12,865 worker series. Also, most published radiographs of asbestosis show lungs of normal size, including the 3/3 films of the International Labour Office (ILO) standard set.