FiO2

Room air FiO2 is 0.21 (21%); each L/min of nasal cannula increases FiO2 by ~3-4%, while a non-rebreather can deliver up to ~0.90 and mechanical ventilation/Venturi masks allow precise titration. The alveolar gas equation uses FiO2: PAO2 = FiO2(Patm - PH2O) - PaCO2/R. The PaO2/FiO2 (P/F) ratio defines ARDS severity per the Berlin criteria: mild ≤300, moderate ≤200, severe ≤100 (with PEEP ≥5). Prolonged exposure to high FiO2 (>0.6 for >24-48 hours) causes oxygen toxicity (free-radical injury, absorption atelectasis), so clinicians target the lowest FiO2 that keeps SpO2 88-95% (or 92-96% non-COPD). In COPD patients, excessive O2 can blunt hypoxic drive and worsen V/Q mismatch (Haldane effect).