Is pulmonary embolism a common cause of chronic pulmonary hypertension? Limitations of the embolic hypothesis

The hypothesis that chronic thromboembolic pulmonary hypertension results from unresolved pulmonary embolism has strongly influenced the diagnosis and management of this disease since the 1960s. However, it is nearly impossible to induce chronic pulmonary hypertension in any animal species by means of repeated embolization of thrombotic material. The haemodynamic effects of thrombotic pulmonary embolism of different degrees of magnitude have also been studied in humans and there is little to suggest that chronic pulmonary hypertension is a likely long term outcome. Furthermore many conditions which predispose to venous thromboembolism do not appear to cause thromboembolic pulmonary hypertension. Other arteriopathic and atherosclerotic risk factors, are found in patients with chronic thromboembolic pulmonary hypertension, but not in those with venous thrombosis, suggesting that these may be unrelated conditions. Thrombosis in situ of the pulmonary arteries is common in severe pulmonary hypertension of any cause. Such thrombosis cannot usually be distinguished from pulmonary embolism. It is hypothesized that in situ thrombosis and pulmonary arteriopathy are common causes of vascular occlusion which is usually diagnosed as "chronic thromboembolic pulmonary hypertension" and that venous thromboembolism is unlikely to be a common cause of chronic pulmonary hypertension. It is further hypothesized that pulmonary embolism is seldom the sole cause of "chronic thromboembolic pulmonary hypertension".