A 24-year-old man with type 1 diabetes presents with abdominal pain, nausea and vomiting. He has polyuria and polydipsia. On examination, he is tachycardic, hypotensive and tachypnoeic with Kussmaul respirations. Capillary glucose is 26 mmol/L, venous pH 7.10 and serum ketones are elevated. What is the most appropriate initial management priority after airway assessment?