A 61-year-old alcoholic man presents with severe epigastric pain radiating to his back. His amylase and lipase are elevated, and he is diagnosed with acute pancreatitis. Over the first 48 hours, he is determined to have 6 Ranson's criteria, including a PaO2 less than 60 mm Hg. His chest x-ray reveals bilateral pulmonary infiltrates, and his wedge pressure is low. Which of the following criteria must be met to make a diagnosis of adult respiratory distress syndrome (ARDS)?