Structural and pathologic changes in the lung vasculature in chronic liver disease.

The hepatopulmonary syndrome results from erythrocytes bypassing the lung without oxygenation. In addition to ventilation-perfusion mismatching, the hypoxemia may result from portapulmonary shunting, passage around alveoli through pleural and hilar blood vessels, and intrapulmonary vascular dilatations. Dilated vascular channels between arteries and veins on the pleural surface are seen more often than dilated intrapulmonary capillaries in chronic liver disease. These anastomoses appear grossly as vascular "spider nevi" on the pleura. Portal vein-to-pulmonary vein anastomoses could produce arterial hypoxemia because the deoxygenated portal venous blood can mix with oxygenated pulmonary venous blood. There is an association of esophageal varices with the hepatopulmonary syndrome and anastomoses between the portal veins and pulmonary veins have been found in both animals and humans. As portal pressures increase, the mediastinal veins enlarge, enhancing the chance that they may penetrate the pleura and drain into pulmonary veins. Direct splenic injections in patients, however, suggest that this shunt pathway is uncommon and small. Pulmonary artery injection studies have demonstrated dilated intrapulmonary vascular segments in humans and animals. Dilation of capillaries may allow a more rapid flow through the lung and the greater distance between the erythrocyte and alveolar wall may make it more difficult to oxygenate rapidly passing erythrocytes. Pulmonary capillary dilation can explain the abnormalities of the perfusion lung scan and contrast echocardiogram that portapulmonary shunting cannot. Pulmonary hypertension may occur in chronic liver disease even without arterial hypoxemia, but it is rare. The prevalence of hypertensive pulmonary vascular disease in patients with cirrhosis of the liver is less than 1%, although a higher percentage (2%) has been found when patients with portal hypertension were studied by cardiac catheterization. The hypertensive pulmonary vascular disease (pulmonary arteriopathy with plexiform lesions) that occurs in patients with liver disease appears identical to that encountered in patients with congenital cardiac shunts and unexplained (primary) pulmonary hypertension.