Gravitational gradients in expiratory computed tomography examinations of patients with small airways disease: effect of body position on extent of air trapping.

Purpose: First, to test the hypothesis that air trapping in diseased patients follows a gravitational gradient and is more extensive in dependent than in nondependent lung regions. Second, to test the hypothesis that the dependent lung regions on combined supine and prone expiratory computed tomography (CT) examinations will show more air trapping than would a supine expiratory CT examination alone.

Materials And Methods: For this ethics committee-approved study, supine and prone multidetector-row CT (4×1 mm collimation, 0.5 s rotation time, 140 kVp, and effective 80 mAs) was performed at full end-expiration on 47 lung transplant recipients (mean age 41±12 y; 18 without bronchiolitis, 18 with potential bronchiolitis, and 11 with bronchiolitis). The extent of air trapping was visually quantified in the supine and prone positions, and in dependent and nondependent lung regions. Individual air trapping scores from these regions were thus available and could be combined for later analysis. Differences in the extent of air trapping between the positions and regions were tested with a Wilcoxon signed-rank test.

Results: Air trapping was significantly more extensive in the combined dependent lung regions than in the combined nondependent lung regions (15.00% vs. 5.77%; P<0.001). Air trapping was also significantly more extensive in the combined dependent regions than in the supine body position (15.00% vs. 7.50%; P<0.001). No statistically significant difference in the extent of air trapping was found between the supine and the prone positions (7.50% vs. 12.14%; P=0.735).

Conclusions: In patients with suspected or overt small airways disease, air trapping follows a gravitational gradient. A change from the supine to the prone position can make air trapping visible in formerly nondependent lung regions. The combined readings from supine and prone CT examinations in dependent lung regions show more air trapping than a standard supine CT examination alone.