To examine the effect of hyperinflation on the volume displaced by diaphrag
m motion (Delta Vdi), we compared nine subjects with emphysema and severe h
yperinflation [residual volume (RV)/ total lung capacity (TLC) 0.65 +/- 0.0
8; 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 (L
ap), cross-sectional area of the pulmonary ribcage, DVdi, and volume beneat
h the lung-apposed dome of the diaphragm. Emphysema subjects, relative to c
ontrols, 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 ca
ge cross-sectional area (emphysema/controls 1.22 +/- 0.03, P< 0.001 at func
tional residual capacity), and <Delta>Vdi/Delta Lap (0.25 vs. 0.14 liters/c
m, P< 0.05). During a vital capacity inspiration, relative to controls, <De
lta>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 c
onclude that DVdi can be normal in emphysema because 1) hyperinflation is s
hared between ribcage and diaphragm, preserving Lap, and 2) the diaphragm r
emains flat during inspiration.