The volume displaced by diaphragm motion in emphysema.

To examine the effect of hyperinflation on the volume displaced by diaphragm motion (Delta Vdi), we compared nine subjects with emphysema and severe hyperinflation [residual volume (RV)/ total lung capacity (TLC) 0.65 +/- 0.08; mean +/- SD] with 10 healthy controls. Posteroanterior and lateral chest X rays at RV, functional residual capacity, one-half inspiratory capacity, and TLC were used to measure the length of diaphragm apposed to ribcage (Lap), cross-sectional area of the pulmonary ribcage, DVdi, and volume beneath the lung-apposed dome of the diaphragm. Emphysema subjects, relative to controls, had increased Lap at comparable lung volumes (4.3 vs. 1.0 cm near predicted TLC, 95% confidence interval 3.4-5.2 vs. 0-2.1), pulmonary rib cage cross-sectional area (emphysema/controls 1.22 +/- 0.03, P<0.001 at functional residual capacity), and Vdi/Delta Lap (0.25 vs. 0.14 liters/cm, P<0.05). During a vital capacity inspiration, relative to controls, Vdi was normal in five (1.94 +/- 0.51 liters) and decreased in four (0.51 +/- 0.40 liters) emphysema subjects, and volume beneath the dome did not increase in emphysema (0 +/- 0.36 vs. 0.82 +/- 0.80 liters, P<0.05). We conclude that DVdi can be normal in emphysema because 1) hyperinflation is shared between ribcage and diaphragm, preserving Lap, and 2) the diaphragm remains flat during inspiration.