Upon arrival at the ED, DB scored 13 on the Glasgow Coma Scale. His airway was patent and his breath sounds were clear and equal bilaterally. Heart rate was 92, respiratory rate was 20, blood pressure was 144/67, temperature was 37.1 C, and SpO2 on room air was 100%.

Initial physical examination revealed bruising and swelling around the left eye orbit and an opacified iris with no vision of the same eye. In addition, he complained of abdominal and chest pain, which correlated clinically with bruising and swelling of the same area. Chest radiograph and c-spine images did not show any signs of trauma. Following the initial exam, DB scored 15 on the GCS. However, he did exhibit gross hematuria, which prompted an irrigation of his bladder and CT scans of the head and abdomen.

DB was admitted to the hospital. CT scan suggested a spleen laceration, a kidney contusion, and splenic pseudoaneurysm, which would eventually require dialysis. Not long after admission, he experienced a large hypotensive event. Four units of blood were required to stabilize. Ativan and morphine were ordered for pain and anxiety.

Two days later, the patient's chest radiograph showed right upper lobe collapse and bilateral infiltrates. This and his physical assessment suggested the development of adult respiratory distress syndrome. The decision was made to place the patient on ventilatory support via the Puritan-Bennett 7200 ventilator, with the following settings:

Mode	CMV	Rate	14
Tidal Volume	650 mL	Flow	60 Lpm
FiO2	0.50	PEEP	+5 cmH2O

The patient did not respond well to mechanical ventilation, with repeated episodes of refractory hypoxemia and general agitation. Therefore, he was given neuromuscular blocking agent and the ventilator settings were changed to:

Mode	CMV	Rate	28
Tidal Volume	420 mL	Flow	60 Lpm
FiO2	0.40	PEEP	+7.5 cmH2O

Within three days of initiation of mechanical ventilation, the PEEP level was increased, in gradual increments, to a maximum of 22.5 cmH2O, with an FiO2 of 0.80, in order to maintain a PaO2 of 90 mmHg and a SaO2 of 92%.

Over the next several days, the patient began to improve and the PEEP was eventually reduced to +5 and the FiO2 decreased to .40. Arterial blood gases drawn on these settings (tidal volume 420 mL, rate 28) were: