A 50-year-old man with poorly controlled type 1 diabetes presents with polyuria, abdominal pain and vomiting. Vital signs: temperature 37.5°C, HR 115 beats/min, BP 100/60 mm Hg, RR 28/min, Kussmaul respirations. Capillary glucose is 25 mmol/L, and serum ketones are elevated. According to diabetic ketoacidosis management principles, what is the first priority in treatment?