6MWD: 6-minute walking distance; BNP: brain natriuretic peptide; CI: cardiac index; CMR: cardiac magnetic resonance; NT-proBNP: N-terminal pro-brain natriuretic peptide; pred.: predicted; RA: right atrium; RAP: right atrial pressure; SvO₂: mixed venous oxygen saturation; VE/VCO₂: ventilatory equivalents for carbon dioxide; VO₂: oxygen consumption; WHO: World Health Organization. ^aMost of the proposed variables and cut-off values are based on expert opinion. They may provide prognostic information and may be used to guide therapeutic decisions, but application to individual patients must be done carefully. One must also note that most of these variables have been validated mostly for IPAH and the cut-off levels used above may not necessarily apply to other forms of PAH. Furthermore, the use of approved therapies and their influence on the variables should be considered in the evaluation of the risk. ^bOccasional syncope during brisk or heavy exercise, or occasional orthostatic syncope in an otherwise stable patient. ^cRepeated episodes of syncope, even with little or regular physical activity.