The role of thrombosis in severe pulmonary hypertension.

Considering the important surface in pulmonary circulation where blood can interact with the endothelium, the maintenance of blood fluidity through the lung, by antithrombotic pathways and products of the endothelium, is essential. This function appears to be ineffective in primary pulmonary hypertension and in severe secondary pulmonary hypertension. Thrombotic lesions are frequently found in pulmonary arteries in these diseases. Thrombin activity appears to be increased in severe pulmonary hypertension. Antithrombotic pathway disorders may account for this abnormality, particularly in chronic thromboembolic pulmonary hypertension and primary pulmonary hypertension. Injured endothelium, a constant feature in severe pulmonary hypertension, either primary or secondary, enhances thrombus formation in pulmonary vessels. This is probably related to thrombomodulin and tissue factor imbalance, impairment of prostacyclin and nitric oxide release, as well as inefficiency of fibrinolysis. Moreover, platelets appear to be activated in the pulmonary circulation of these patients. They release several mediators acting on vascular tone and as mitogenic agents, and may also contribute to thrombin and clot generation. Long-term oral anticoagulant and continuous infusion of prostacyclin, treatments which impede thrombosis, are known to improve the survival rate in patients with primary pulmonary hypertension. These are the strongest arguments, so far, in favour of the role of thrombosis in severe pulmonary hypertension. However, we do not know whether these abnormalities result from a previous vascular injury or represent the primary disturbance.