A 28-year-old woman is brought to the emergency department 2 hours after sustaining a 40% total body surface area (TBSA) deep partial-thickness burn to her trunk and bilateral lower limbs from a kitchen fire. She is alert, breathing spontaneously at 24/min, and complaining of severe pain. Her blood pressure is 110/70 mmHg and heart rate is 118/min. What is the most appropriate immediate next step in management?