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Assess oxygenation and ARDS severity using the Berlin Definition
Normal: 80-100 mmHg on room air
Room air = 21%
ARDS requires PEEP ≥5 cmH2O
P/F Ratio = PaO2 / FiO2
Where FiO2 is expressed as a decimal (e.g., 0.40)
Berlin Criteria also require:
Normal P/F ratio is approximately 400-500 on room air (PaO2 ~100 / FiO2 0.21 ≈ 475). Values below 300 suggest oxygenation impairment.
PEEP ≥5 cmH2O ensures consistent assessment. Without PEEP, some patients would not meet criteria when actually having ARDS. The Berlin Definition standardizes this.
P/F ratio is simpler (just PaO2/FiO2). A-a gradient is more precise for identifying the cause of hypoxemia but requires more calculation. P/F is preferred for ARDS classification.
Yes. P/F ratio can change within minutes to hours based on disease progression, treatment response, or ventilator settings. Serial measurements are important.
Prone positioning improves V/Q matching and has proven mortality benefit in moderate-severe ARDS (P/F <150). Should be done for ≥16 hours/day.
Consider ECMO when P/F <80 despite optimal ventilation, or P/F <50 for >3 hours. Refer early to ECMO center for severe refractory hypoxemia.
P/F ratio is sensitive to FiO2 changes. A patient on 100% O2 with PaO2 of 200 (P/F=200) is sicker than someone on 40% with PaO2 of 80 (P/F=200). Always consider the full clinical picture.