NPPB and pulmonary arterial hypertension: Fischer’s ratio (branched-chain amino acids/aromatic amino acids) and circulating amino acid profile (aminogram): Over time, clinical studies have shown that the plasma concentrations of amino acids are significantly increased in patients with PAH, and the Fischer index was negatively correlated with NYHA, NT-proBNP, PVR, and 6MWT in a group of 160 patients with PH [113].