A 6-year-old girl with sickle cell anemia (HbSS), not on hydroxyurea, presents with a temperature of 39.4°C (103°F), cough, and respiratory distress. A chest x-ray reveals a new infiltrate in the left lower lobe. Continuous pulse oximetry shows her saturation has dropped to 91%. In addition to antibiotics and oxygen, which intervention is most critical for aborting a rapidly progressing acute chest syndrome?