Myocardial Metabolic Flux in Pulmonary Arterial Hypertension

Inclusion Criteria: \- 1. Diagnosis of Group 1 PAH confirmed by right heart catheterisation under the National Pulmonary Hypertension Service, Royal Brompton Hospital, part of GSTT Foundation Trust 2. Age over 18, less than 85 years 3. Able to give informed consent 4. On a stable dose of PAH-specific therapies (e.g., ERA, PDE5i) for at least 3 months. 5\. Clinically justified prescription of GLP-1 agonist Semaglutide based on following criteria: BMI \> 30 or BMI \> 27 with at least one cardiovascular co-morbidity (systemic hypertension, diabetes, pre-diabetes, COPD, atrial fibrillation, dyslipidaemia, sleep disordered breathing) Exclusion Criteria: * • 1. Pregnancy * 2\. Myocardial infarction within the previous 3 months * 3\. Contraindications to MRI: Pacemakers, metallic implants, or severe claustrophobia. * 4\. Severe renal impairment: eGFR \< 15ml/min/1.73m. * 5\. Current use of SGLT2 inhibitors or GLP-1 agonist therapy (which significantly alter fuel substrate preference) or insulin therapy that cannot be held for the fasting scan