Macitentan to Prevent PRVO

About half of patients who suffer of a pulmonary embolism (blood clots in the lungs), do not clear completely the clots overtime, increasing the risk of having a new blood clot in the lungs and elevated blood pressure in the pulmonary artery; therefore, developing fatigue, shortness of breath, and heart failure. The objective of this clinical trial is to compare 3 months of daily Macitentan (a medicine to increase the diameter of the pulmonary artery) in addition to blood thinners vs. blood thinners alone in patients with acute blood clots in the pulmonary artery: Question 1: Whether the use of Macitentan in addition to blood thinners enhances the process of cleaning the blood clot in the pulmonary artery compared to blood thinners alone. Question 2: Whether the use of Macitentan in addition to blood thinners is associated to improve the fatigue and shortness of breath in patients after a pulmonary embolism compared to blood thinners alone. Participants will be asked to take a capsule for 3 months in addition to blood thinners. The capsule could contain either Macitentan (medicine) or a placebo (not medicine). In 3 months, participants will have a new image of the pulmonary artery to document the cleaning of the clots. Also, blood and urine samples will be collected, as well as a questionnaire will be filled out. In 6 months, patients will have repeated images of the heart as well as repeated questionnaires.