The rationale for this study is that GLP-1 agonist treatment is likely to influence myocardial substrate utilisation, changing the predominant source of metabolic energy within the heart to a more energetically efficient form. This is represented by a surrogate for improved mitochondrial efficiency with reduction in myocardial lactate levels (produced by inefficient myocardial glycolysis, prevalent in the ventricles of patients with pulmonary hypertension), which can be measured by 31P-magnetic resonance spectroscopy (31P-MRS).