M Cassart, PA Gevenois and M Estenne
Department of Radiology and the Chest Service, Erasme University Hospital, Brussels, Belgium.

Using computed tomography we assessed rib cage dimensions at the levels of Th4, Th6, Th8, and Th10 in seven supine patients with COPD who were severely obstructed (FEV1, 25 +/- 7% of predicted) and hyperinflated (FRC, 234 +/- 34% of predicted) and seven matched normal control subjects. The midsagittal anteroposterior (AP) diameter, the maximal AP diameter of the right and left hemithorax, and the maximal transverse (T) diameter were measured on scans obtained at relaxed TLC, FRC, and RV. At each volume, AP diameters were invariably 2 to 3 cm greater in patients with COPD than in normal subjects, but no significant differences in T diameters were found. When compared at a given absolute lung volume, T diameters at all thoracic levels and AP diameters at Th4-6 were smaller in patients with COPD than in normal subjects; in contrast, at Th8-10, AP diameters were similar in the two groups. We conclude that the marked hyperinflation of patients with severe COPD produces complex changes in rib cage dimensions: (1) there is an increase in AP but not in T diameters such that the rib cage adopts a more circular shape; (2) at a given absolute lung volume, AP diameters are smaller in patients with COPD than in normal subjects in the upper but not in the lower portion of the rib cage. These differences in rib cage dimensions may have implications regarding respiratory muscle length and function in patients with COPD.

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