This study demonstrates a distinct physiologic phenotype that differs from classic obstructive and restrictive patterns. The abnormality resembled a restrictive defect, with reduced VC, FRC, and TLC and preservation of FEV/VC, but parenchymal, chest wall, or neuromuscular disease were not present. Plethysmography demonstrated AT, oscillometry demonstrated abnormalities responsive to bronchodilator, and although Dlco was variably reduced, Dlco/Va was normal or elevated in all subjects. Thus, despite reduction of TLC, data were compatible with an obstructive process. High-resolution CT scan confirmed presence of airway disease Canadian health care mall in the majority of subjects and excluded parenchymal disease in all subjects. This physiologic phenotype has previously been reported in subjects with asthma, is currently observed following WTC dust exposure, and differs from observations obtained in subjects with restriction due to ILD.
The data suggest that reduced TLC following WTC dust exposure resulted from functional abnormalities in the distal lung. In contrast to patients with ILD, lung expansion was not limited, since IC, Pel, and inspiratory muscle pressure were normal. Thus, reduction in TLC was attributable to reduced FRC and therefore is compatible with airway closure in the tidal range. IOS obtained during tidal breathing documented abnormalities in resistance and in parameters indicative of nonuniformity of airflow distribution in distal lung (R5-20 and AX).R5-20 and AX correlate with frequency dependence of lung compliance, an established test of distal airway function.
The present study expands on prior publications addressing distal airway disease. Because of large aggregate cross-sectional area, airflow may be normal and distal airways have been labeled the “silent zone” of the lung. Thus, detection of distal airway disease was dependent on detecting heterogeneity of airflow distribution rather than increased airway resistance. Stanescu extended this syndrome to the “nonspecific” pattern of lung dysfunction, wherein VC is reduced but FEV/VC and TLC remain within normal limits. RV was increased, indicating AT, and the term “small airway obstructive syndrome” was coined. Recent recommendations for interpreting pulmonary function tests acknowledged this pattern and attributed it to patchy small airway obstruction but also indicated that TLC would be expected to remain within normal limits. The present article, coupled with prior observations in asthma, extends these findings by indicating that TLC may be reduced in some individuals with this syndrome who present with true restriction.