A HEALTHY 42-YEAR-OLD farmer lost his balance while riding a tractor, which then ran over his chest and upper abdomen. A computed tomographic scan of the abdomen demonstrated a grade III liver laceration that was treated nonoperatively. However, during the next 12 hours, he developed progressive shortness of breath and required intubation. An arterial blood gas measurement gave the following results: pH, 7.31; PaO2, 57; and PCO2, 46, while the patient was receiving a fraction of inspired oxygen (FIO2) of 80%. Chest x-ray films obtained at admission (A) and after intubation (B) are shown in Figure 1.