Within the study group, MPO activity (↓67.65%, p = 0.0497) and the concentration of peroxynitrite (↓68%, p = 0.049) were significantly lower in HF participants with NS compared to HF patients with HS, while NO concentration (↑64.59%, p = 0.0344) was considerably higher. The gene discussed is MPO; the disease is hydrops fetalis.