According to Berlin criteria, which PaO2/FiO2 ratio defines severe ARDS when PEEP is at least 5 cm H2O?

• A. PaO2/FiO2 ≤200 (moderate)
• B. PaO2/FiO2 ≤100 (severe)
• C. PaO2/FiO2 ≤300 (mild)
• D. PaO2/FiO2 ≤50 (extreme)

Answer: (B)

The main idea here is that ARDS severity is determined by how well the lungs oxygenate when a standard amount of ventilatory support is used. Under the Berlin definition, the PaO2/FiO2 ratio is used to stage severity, and this measurement is taken with at least 5 cm H2O of PEEP to keep the assessment consistent across patients. The severe category corresponds to a very poor oxygenation level, specifically a PaO2/FiO2 ratio of one hundred or less. The other ranges define milder levels of impairment: roughly between two hundred and three hundred for mild, and between one hundred and two hundred for moderate. So the threshold that marks severe ARDS is the low end of the scale, at or below one hundred, when the patient is on a minimum PEEP of five.