A 19-year-old man is admitted to the hospital because of the sickle cell pain crisis. Over the next 48 hours, he develops worsening dyspnea, chest pain and fever. On examination, temp 38C, BP 123/65 HR 120/min, oxygen saturation 86% on 6 L of oxygen/ min by nasal cannula. Chest examination discloses decreased bilateral breath sounds at the lung bases but no crackles. His initial investigations show a hemoglobin level of 7.4 g/dl, white blood cell count 6900/ ul with normal differential. Chest x-ray shows new multilobar infiltrates. Broad spectrum antibiotics are started. Which of the following is the most appropriate additional treatment?