 The main area of research focus in this study will be improving understanding of the natural history of chronic thromboembolic pulmonary hypertension. The purpose of this research protocol is to conduct a retrospective review of patients with CTEPH diagnosed at the Sheffield Centre between 2001 and 2014 to understand more about the natural history of disease including prognostic indicators. The Sheffield Service has one of the largest cohorts of CTEPH in the world 650 diagnosed cases since 2001. Approximately half of these patients undergo curative surgery with pulmonary endarterectomy. However, a significant proportion of patients with operable disease decline surgery and a significant proportion of patients have disease that is too distal for surgery, so called distal CTEPH. There is a relative paucity of data on i the natural history of this disease, ii an understanding of the relative contribution of obstruction and distal vasculopathy to elevations in pulmonary vascular resistance iii the effects of distal CTEPH on right ventricular function and iv how distal CTEPH differs from idiopathic pulmonary arterial hypertension IPAH. 