Air Trapping: A Cause of Heterogeneous Attenuation

Purpose: To investigate important factors affecting the frequency of air trapping through observation of normal lung attenuation, and propose a cause of heterogeneous attenuation. Materials and Methods: In this ethical committee-approved study, a total of 109 patients (30 males, 79 females; mean age, 58.2 years; range, 27 81 years) were included. All patients had undergone inspiratory and expiratory chest thin-section computed tomography (CT) examinations and pulmonary function tests. Air trapping on CT images was graded subjectively. Hounsfield units (HU) lung attenuation value and lung volume were measured on CT images. All variables (age, sex, indices of pulmonary function test results, air trapping score, HU value, and rate of change in lung volume) were compared by diagnoses and air trapping findings cohorts. The correlation between lung function test results and expiratory HU attenuation were analyzed. Results: Interstitial pneumonia showed higher and bronchiolitis obliterans showed lower HU attenuation at normal and air trapping regions. The variables affecting air trapping findings were age, a ratio of forced expiratory volume in 1 second to forced vital capacity (FEV1.0/FVC), maximal expiratory flow at 50% of forced vital capacity (MEF50), HU attenuation at normal regions, and rate of change in lung volume. Compared with expiratory HU attenuation, significant positive correlation was shown to FEV1.0/FVC and negative correlation to single-breath diffusion capacity for carbon monoxide, which was confirmed by a simple regression analysis. Conclusion: It can be suggested that lung attenuation can increase when fibrosis is advanced, and this is exaggerated when lungs are compressed.