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1 Department of Pulmonary Physiology, Sir Charles Gairdner Hospital, and 2 Department of Physiology, University of Western Australia, Nedlands, Western Australia 6009, Australia
To examine
the effect of hyperinflation on the volume displaced by diaphragm
motion (
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,
Vdi, 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/
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
Vdi can be normal in
emphysema because 1) hyperinflation is shared between
ribcage and diaphragm, preserving Lap, and 2) the diaphragm remains flat during inspiration.
hyperinflation; subphrenum; dome; zone of apposition; rib cage
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