Balloon pulmonary angioplasty (BPA) treats patients with inoperable chronic thromboembolic pulmonary hypertension (CTEPH). Intra-procedural markers of success to guide the intervention are lacking. The investigators propose to measure right ventricular (RV) pressure volume loops invasively and measure biomarkers at intervals during the course of a course of BPA. These data will be analysed to define load independent indices of RV functional improvement, cross correlated with biomarker data and be used to calibrate non-invasive assessment of ventriculo-arterial coupling by cardiac magnetic resonance imaging (CMR) to better detect responders of BPA and pulmonary endarterectomy (PEA).