A 32-year-old man is brought to the emergency department 2 hours after sustaining a 40% total body surface area (TBSA) deep partial-thickness burn to his trunk and bilateral lower limbs from a house fire. He is alert, breathing spontaneously at 24/min, and has singed nasal hairs. His radial pulse is 118/min and blood pressure is 110/70 mmHg. Urine output is not yet documented. What is the most appropriate immediate next step in management?