Pulmonary arterial hypertension (PAH) is a progressive disease characterized by vascular remodelling resulting in elevated pressures in the pulmonary artery (PA). This elevated pressure ultimately leads to fulminant right heart failure. Current therapeutic options are limited and are centred around vasodilatory medications such as phosphodiesterase-5 inhibitors and prostacyclin. While these medications allow for the widening of blood vessels that are narrowed due to remodelling, they have no effect on the remodelling itself. Sotatercept is a novel medication which targets the BMPR2/TGF-β pathway and restore a pro- and anti- proliferative balance to ultimately counteract vascular remodelling. Recent phase 2 and 3 trials showed that treatment with sotatercept led to lower resistance and pressure in the pulmonary vasculature and improved exercise tolerance. However, these results were not coupled with an increase in cardiac output, a change that is seen with other PAH-medications with a primarily vasodilatory effect. These results suggest that cardiac work is reduced and cardiac efficiency is improved in patients being treated with sotatercept, in contrast with conventional PAH therapies. This is a potentially beneficial effect that may result in improved disease control in the long-term. Our study aims to explore the effect of sotatercept on cardiac work and function. We hypothesize that the effects of sotatercept are more beneficial for cardiac function than that of traditional PAH medications. All participants included in the trial will undergo a screening visit in which it will be checked that all inclusion criteria and no exclusion criteria are met. The screening visit involves a physical exam, blood draw, 6-minute walk test, right heart catheterization (RHC) and cardiac magnetic resonance imaging (cMRI) with contrast to assess fibrosis. Upon inclusion, all participants will receive a subcutaneous injection of sotatercept starting at a dose of 0.3 mg/kg. Participants will return to the hospital every three weeks for a blood draw, physical examination and an adverse event review. If the laboratory values (specifically haemoglobin and platelet counts) stay stable after the first dose, the dosage will be escalated to 0.7 mg/kg. The dose will not be escalated beyond 0.7 mg/kg. After 24 weeks of receiving sotatercept, there will be an end of treatment visit including a physical exam, 6-minute walk test, right heart catheterization (RHC) and cardiac magnetic resonance imaging (cMRI) with contrast material.