iNO300 Therapy in Critically Ill Patients With Pneumonia

The goal of this clinical trial is to learn the formation and recovery rate of methemoglobin (MetHb) in severely sick patients with pneumonia who receive high doses of inhaled nitric oxide (iNO) therapy at 250 parts per million (ppm), not exceeding 300 ppm. Meanwhile, the benefits of the therapy to treat severely sick patients with pneumonia will be explored. Patients who are 18 years or older, newly diagnosed with pneumonia, and severely sick with requirement of a breathing machine could be included. The main questions it aims to answer are: How does methemoglobin change through the iNO treatment? Does iNO therapy increase the number of patients recovering from pneumonia? Researchers will compare iNO treatment to placebo, which means using the same device as the treatment group without delivering the study drug. Participants will: * Receive iNO treatment starting at 250 ppm, not exceeding 300 ppm, 40 min, every 6 hours, from day 1 to day 5 * Be followed up for 60 days