Efficacy and Safety of Sivelestat Sodium and Dexamethasone in the Treatment of ARDS

The goal of this clinical trial is to evaluate the efficacy and safety of Sivelestat sodium and dexamethasone in the treatment of patients with moderate to severe ARDS. The main questions it aims to answer are: * Is Sivelestat sodium more effective in the treatment of patients with moderate to severe ARDS compared with placebo? * Is dexamethasone more effective in the treatment of patients with moderate to severe ARDS compared with placebo? Participants will receive Sivelestat sodium, dexamethasone or placebo. Researchers will compare the efficacy and safety of Sivelestat sodium, dexamethasone and placebo.