From acute pulmonary embolism to chronic thromboembolic pulmonary hypertension: Pathobiology and pathophysiology

Chronic thromboembolic pulmonary hypertension (CTEPH) represents a unique subtype of pulmonary hypertension characterized by the presence of mechanical obstruction of the major pulmonary vessels caused by venous thromboembolism. CTEPH is a progressive and devastating disease if not treated, and is the only subset of PH potentially curable by a surgical procedure known as pulmonary endarterectomy. The clot burden and pulmonary embolism recurrence may contribute to the development of CTEPH however only few thrombophilic factors have been found to be associated. A current hypothesis is that CTEPH results from the incomplete resolution and organization of thrombus modified by inflammatory, immunologic and genetic mechanisms, leading to the development of fibrotic stenosis and adaptive vascular remodeling of resistance vessels. The causes of thrombus non-resolution have yet to be fully clarified. CTEPH patients often display severe PH that cannot be fully explained by the degree of pulmonary vascular obstruction apparent on imaging studies. In such cases, the small vessel disease and distal obstructive thrombotic lesions beyond the sub-segmental level may contribute for out of proportion elevated PVR. The processes implicated in the development of arteriopathy and micro-vascular changes might explain the progressive nature of PH and gradual clinical deterioration with poor prognosis, as well as lack of correlation between measurable hemodynamic parameters and vascular obstruction even in the absence of recurrent venous thromboembolism. This review summarizes the most relevant up-to-date aspects on pathobiology and pathophysiology of CTEPH.