A 40-year-old man presents to the ED with severe epigastric pain radiating to his back, associated with nausea and vomiting. He admits to heavy alcohol consumption over the weekend. Observations: HR 120/min, BP 100/60 mmHg, RR 22/min, Temp 37.8°C. Serum amylase is 1500 U/L (normal <100 U/L). According to management guidelines (e.g., BSG), what is the most important initial management step?