The goal of this observational prospective cohort study is to explore the clinical characteristics, treatment strategies, and outcomes of adult patients with acute respiratory distress syndrome (ARDS) admitted to the medical intensive care unit of Asan Medical Center. The main questions it aims to answer are:

• What are the clinical features, current practices, and prognosis of ARDS patients in Korea?
• How do different mechanical ventilation settings affect ARDS patient outcomes?

Participants will:

• Be screened at medical ICU admission for eligibility according to the Berlin definition of ARDS.
• Have demographic, clinical, laboratory, and ventilator data collected from bedside monitoring devices and electronic health records during their ICU stay.
• Complete a quality-of-life questionnaire (EQ-5D) at hospital discharge if they survive.

Inclusion Criteria:

• Age ≥ 18 years.
• Receiving respiratory support with one of the following:
• High-flow nasal oxygen (HFNO) at a flow rate ≥ 30 L/min,
• Noninvasive ventilation (NIV) with positive end-expiratory pressure (PEEP) of ≥ 5 cm H₂O, or
• Invasive mechanical ventilation with PEEP of ≥ 5 cm H₂O.
• Within 48 hours before or after eligibility screening, the following criteria must be met:
• PaO₂/FiO₂ ≤ 300 mmHg under the above respiratory support.
• Bilateral opacities on chest radiograph or computed tomography, not fully explained by effusions, lobar/lung collapse (atelectasis), or pulmonary nodules/masses.
• Respiratory failure not fully explained by cardiac failure or fluid overload; when no clear ARDS risk factor is identified, an objective assessment (e.g., echocardiography) is required to exclude hydrostatic edema.

Exclusion Criteria:

• Refusal of informed consent by the patient or a legally authorized representative.
• Chronic respiratory failure, defined as:
• Patients receiving long-term oxygen therapy or home mechanical ventilation (except for CPAP used solely for obstructive sleep apnea), or
• Outpatients with PaCO₂ > 60 mmHg.
• Diffuse alveolar hemorrhage.
• Interstitial lung disease.
• Moribund patients expected to survive < 24 hours.
• Patients expected to require high-flow nasal oxygen or mechanical ventilation for < 48 hours.
• Patients with a decision to withhold life-sustaining treatment (except those for whom full support is provided other than cardiopulmonary resuscitation).

This study will enroll participants from among patients with acute respiratory failure who are admitted to the medical intensive care unit of Asan Medical Center in Seoul, Korea.