A Study of Macitentan in Children Below 2 Years of Age

Inclusion Criteria: * Pulmonary arterial hypertension (PAH): 1) including participants with Down syndrome. Diagnosis must have been confirmed by (historical, any time before screening) right heart catheterization mean pulmonary arterial pressure (mPAP) greater than or equal to (\>=) 25 millimeter of mercury (mmHg), pulmonary arterial wedge pressure (PAWP) less than or equal to (=\<)15 mmHg, pulmonary vascular resistance index greater than (\>) 3 Wood units \* meter square (m\^2) where in the absence of pulmonary vein obstruction and/or significant lung disease PAWP can be replaced left atrium pressure or left ventricular end diastolic pressure (in the absence of mitral stenosis) assessed by heart catheterization. a) Idiopathic PAH, or b) Heritable PAH, or c) PAH associated with congenital heart disease: i) Eisenmenger syndrome (Qp/Qs less than (\<) 1.5 and saturation of peripheral oxygen ≤ 90 percent (%) measured by pulse oximetry at room air), or ii) Inoperable open left-to-right shunts (with a Pulmonary vascular resistance \[PVR\] \> 8 WU and Qp/Qs \<2), or iii) Co-incidental shunt (that is, not explaining hemodynamically the presence of PAH), or iv) Post-operative PAH (persisting/recurring/developing ≥ 6 months after repair of shunt), or d) Drug or toxin induced PAH, or e) PAH associated with Human immunodeficiency viruses (HIV) * World Health Organization Functional Class (WHO FC) I, II, or III * PAH-specific treatment-naive participants or participants on PAH specific monotherapy or combination of 2 therapies. Use of macitentan before or during screening is allowed * Body weight of greater than or equal to (\>=) 3.5 kilogram (kg) * Parent(s) (preferably both if available or as per local requirements) or participant's legally designated representative must sign an informed consent form (ICF) indicating that they understand the purpose of, and procedures required for, the study and is/are willing to allow the child to participate in the study Exclusion Criteria: * PAH due to portal hypertension, schistosomiasis, pulmonary veno-occlusive disease and/or pulmonary capillary hemangiomatosis * Persistent pulmonary hypertension of the newborn * The following congenital cardiac abnormalities: a) Cyanotic congenital cardiac lesions such as transposition of the great arteries, truncus arteriosus, pulmonary atresia with ventricular septal defect, unless operatively repaired and with no residual shunt. b) Univentricular heart and/or participants with Fontan-palliation * Pulmonary hypertension due to lung disease * Known diagnosis of bronchopulmonary dysplasia