A 5-year-old girl with a 3-day history of fever, cough, and fast breathing is brought to the emergency department. On examination, she is irritable, with a respiratory rate of 52/min, oxygen saturation of 88% on room air, and signs of severe respiratory distress. Auscultation reveals bilateral crackles. Capillary refill is 2.5 seconds, and her extremities are cool. Blood pressure is 95/60 mmHg. After placing her on high-flow oxygen and establishing IV access, what is the most appropriate next step?