A 40-year-old man presents with severe epigastric pain radiating to his back, associated with nausea and vomiting for 12 hours. He admits to drinking heavily over the weekend. On examination, he is tachycardic (110 bpm) and tender in the epigastrium. His serum amylase level is 1500 U/L (Reference range <100 U/L). What is the most important initial management priority?