Fig. 4

CSA changes in two patients without COPD are shown in the four graphs, where the horizontal axis is the respiratory phase, the vertical axis in images (a and b) is CSA (mm²), and the vertical axis in images (c and d) is CSA/S₀, which is the direct-current component of the DFT components. Case 1 had low residual air volume (%RV 91.8, RV/TLC 0.366); case 2 had high residual air volume (%RV 116.5, RV/TLC 0.428). In the latter case with high residual air volume, the decrease in CSA during exhalation was more pronounced than during inhalation. COPD, Chronic obstructive pulmonary disease; CSA, Cross-sectional area; RV, Residual lung volume; TLC, Total lung capacity