With regards to disease severity, in COPD with PAH, Pearson correlation analysis showed a good relationship between ET-1 in EBC and pulmonary-artery systolic pressure (PaPs) (r = 0.621; p = 0.031) (Figure 4); A positive correlation was also observed between arterial blood levels of ET-1 and PaPs (r = 0.648; p = 0.022) (Figure 5), while the arterial blood levels of ET-1 inversely correlated to the FEV1% of the group 1 (r = -0.59, p = 0.043) (Figure 6). Here, EDN1 is linked to pulmonary arterial hypertension.