WHO: World Health Organization; 6MWD: 6-min walking distance; NT: N-terminal; BNP: brain natriuretic peptide; CMR: cardiac magnetic resonance; V′_O2: oxygen consumption; V′_E: minute ventilation; V′_CO2: carbon dioxide production; RA: right atrial; CI: cardiac index; S_vO2: mixed venous oxygen saturation. ^#: most 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 idiopathic pulmonary arterial hypertension and the cut-off levels used above may not necessarily apply to other forms of pulmonary arterial hypertension. Furthermore, the use of approved therapies and their influence on the variables should be considered in the evaluation of the risk. ^¶: occasional syncope during brisk or heavy exercise, or occasional orthostatic syncope in an otherwise stable patient. ⁺: repeated episodes of syncope, even with little or regular physical activity. Reproduced from [1].