Lung compliance can often be normal/high (L type) but can progress to low compliance (H type) 
Berlin Criteria:
Timing: w/i 7d
Imaging: Bilateral infiltrates
Origin: Not explained by cardiac failure/fluid overload
Mild: P/F <200mmHg (≤300 with PEEP/CPAP ≥5cmH20)
Mod: P/F <100mmHg (≤200 with PEEP/CPAP ≥5cmH20)
Severe: P/F ≤100mmHg with PEEP ≥5cmH20
Timing: w/i 7d
Imaging: Bilateral infiltrates
Origin: Not explained by cardiac failure/fluid overload
Mild: P/F <200mmHg (≤300 with PEEP/CPAP ≥5cmH20)
Mod: P/F <100mmHg (≤200 with PEEP/CPAP ≥5cmH20)
Severe: P/F ≤100mmHg with PEEP ≥5cmH20
