Risk assessment and treat-to-target approach for pulmonary arterial hypertension. Risk assessment and treat-to-target approach for pulmonary arterial hypertension. WHO-FC: World Health Organization-functional class; 6MWD: 6-min walking distance; CPET: cardiopulmonary exercise testing; Echo: echocardiography; BNP: brain natriuretic peptide; NT-proBNP: N-terminal-pro-brain natriuretic peptide. Adapted from [15] with permission from the publisher. Lewis J. Rubin et al. Eur Respir Rev 2012;21:234-238 ©2012 by European Respiratory Society