 3. Worsening gas exchange ratio of partial pressure of arterial oxygen to fraction of inspired oxygen PaO2FiO2 240 or PaO2 60 mmHg leading to acute changes to the ventilator support system minimum daily FiO2 values increased by at least 0.20 over the daily minimum FiO2 in the preceding 48 hours, or minimum daily positive end- expiratory pressure PEEP values increased by at least 3 centimeters cm H2O over the daily minimum PEEP in the preceding 48 hours. 