 Chronic thromboembolic pulmonary hypertension causes progressive right heart hypertrophy, dilatation and dysfunction. Surgical treatment is pulmonary endarterectomy, which although only carried out in a single UK centre, provides an excellent model for assessing right ventricular function. Right heart function is most commonly assessed using echocardiography, either transthoracic pre- and post-operatively, or transoesophageal intra-operatively. Measurement of tricuspid annular plane systolic excursion is the best validated and most commonly performed measurement for right heart function, however it may be inaccurate after sternotomy and pericardial opening, making accurate assessment difficult immediately after surgery. Therefore, we aim to compare established methods of assessing right heart function with 3-dimensional echocardiographic reconstruction of the ventricle, using a novel reconstruction mechanism. Right ventricular function will be assessed in 51 patients who undergo pulmonary endarterectomy surgery at baseline, after the pericardium has been opened, following the surgical procedure, using transoesophageal echocardiography, and at six-month outpatient followup using transthoracic echocardiography, as 3D-reconstruction is valid using both modalities. This comparison should allow the investigators to determine whether such a method could replace current measurement parameters for assessment of right ventricular function, which is important for clinical management of patients in a variety of settings. 