NPPB and idiopathic pulmonary arterial hypertension: In IPAH patients, PeakVO2%pred correlated significantly with PetCO2 at rest (r = 0.477, P < 0.001), AT (r = 0.609, P < 0.001), peak (r = 0.576, P < 0.001) and NT-proBNP also correlated markedly with above parameters, with a correlation coefficient of − 0.326 to − 0.427 (P < 0.001) (Table 4).