A 66-year-old man presents with acute, severe, constant epigastric pain radiating through to his back, associated with nausea and vomiting. He drinks alcohol heavily. On examination, he is tachycardic (110 bpm), hypotensive (95/60 mmHg), and has marked epigastric tenderness. Amylase level is 1500 U/L (normal <100 U/L). What is the most important initial management step?