Conflict of interest: J.S.R. Gibbs reports grants and personal fees from Actelion, Bayer, GSK and MSD, personal fees from Arena, Acceleron, Bellephoron, Pfizer and Complexa, and grants from Amco and United Therapeutics, during the conduct of the study. Conflict of interest: A. Torbicki reports personal fees for consultancy services, lectures and investigator honoraria from Bayer Healthcare and Actelion Pharmaceuticals Ltd, personal fees for lectures from AOP and MSD, investigator honoraria from United Therapeutics and Jassen, personal fees for consultancy services from Arena Pharmaceuticals, outside the submitted work; and serves as Chairperson of Pulmonary Hypertension Foundation receiving donations for its statue activities also from medical industry; chairperson is a honorary function and receives neither honoraria nor any other personal benefits.