Chronic primary and secondary thromboembolic pulmonary hypertension

The term 'thromboembolic pulmonary hypertension' usually gives rise to the image of large thromboemboli obstructing the more proximal pulmonary arteries. However, the term is also applied to a subset of patients who are diagnosed as having primary pulmonary hypertension. In these patients, thromboemboli are only present in the small pulmonary arteries. It is not clear whether the obstruction is caused by microemboli from an unidentified source, by thrombosis in situ, or even by the deposition of fibrin and other material (as occurs in hepatic veno-occlusive disease). This paper briefly mentions some of the recent work on pulmonary hypertension caused by large, proximal thromboemboli and then discusses the pathogenesis of the microthromboembolic form of primary pulmonary hypertension.