A 30-year-old man with type 1 diabetes presents to the emergency department with abdominal pain, nausea and polyuria. He has missed several insulin doses. Vital signs: temperature 37.5 °C, heart rate 116 beats/min, blood pressure 104/68 mmHg, respiratory rate 26 breaths/min with deep sighing respirations. Capillary glucose is 24 mmol/L, venous bicarbonate 14 mmol/L and ketones 4.5 mmol/L. You diagnose diabetic ketoacidosis (DKA). What is the most appropriate initial management step after securing airway and IV access?