A novel mechanism for pulmonary arterial hypertension?

Abstract

The pathophysiology of primary pulmonary hypertension (PPH) involves alterations in vascular reactivity, vascular structure, and interactions of the vessel wall with circulating blood elements.1 An imbalance of vasodilator and vasoconstrictor influences is likely to be an early derangement. Progressive intimal and medial thickening, due to proliferation and migration of vascular smooth muscle cells and fibroblasts, reduces the cross-sectional area of the pulmonary microvasculature, causing fixed alterations in pulmonary resistance. Contributing to the progressive increase in pulmonary resistance is thrombosis of the small pulmonary vessels, which explains the benefit of anticoagulation in these patients. In advanced disease, “plexiform arteriopathy” of the small pulmonary vessels is observed. These lesions proliferate into the lumen, creating high-resistance, convoluted endoluminal channels. There is some controversy regarding the nature of the cells constituting these lesions, one group suggesting they are of endothelial origin, whereas more recent evidence indicates that they are myofibroblasts.2,3 See p 1493 The normal pulmonary endothelium maintains a low vascular resistance, suppresses vascular smooth muscle growth, inhibits platelet adherence and aggregation, and stems inflammation. In patients with PPH, the endothelium has lost these vasoprotective functions.1 The endothelium of the PPH patient is characterized by the increased elaboration of vasoconstrictors, mitogens, and prothrombotic and proinflammatory mediators (such as thromboxane, endothelin, plasminogen activator inhibitor, and 5-lipooxygenase). These endothelial alterations promote the pathophysiology of PPH. Furthermore, there is less influence of the countervailing factors prostacyclin and NO. Endothelium-derived NO plays a critical role in pulmonary …