NPPB and chronic obstructive pulmonary disease: The results of the present study showed that the serum Apelin in patients with COPD is significantly lower than that in the control group, and the serum Apelin in the COPD/PH+ group is lower than that in the COPD/PH− group, indicating that, with the worsening of COPD in patients, the expression of Apelin in the serum gradually decreases, which is consistent with the findings by Andersen et al. In addition, the present study analyzed the correlation between BNP gene SNPs and serum Apelin expression levels.