Redistribution of pulmonary blood flow in interstitial lung diseases: the chest radiograph as a physiologic tool.

Posteroanterior radiographs of the chest showed enlargement of vessels in the upper lung fields in 18 of 29 patients with interstitial lung diseases, despite normal pulmonary wedge pressures and normal or reduced pulmonary blood volumes. The degree of such redistribution ("diversion") did not correlate either with the severity of pulmonary hypertension observed at cardiac catheterization or with radiologic assessment of predominance of disease at the lung bases. Diversion did correlate with several indices of disease severity: reduction in vital capacity, reduction in diffusing capacity, reduction in pulmonary blood volume and radiographic severity of parenchymal abnormalities. Furthermore, diversion correlated with lung height, a variable which was not statistically related to the other indices of disease severity. Distension of upper lung vessels occurs in interstitial lung diseases as the result of a decreased hydrostatic gradient over which the lung is perfused (decreased lung height), partial obliteration of the vascular bed (decreased pulmonary volume), and, more speculatively, decreased extravascular pressure (increased lung recoil).