A 23-year-old woman with type 1 diabetes presents with 24 hours of polyuria, vomiting and abdominal pain. She has Kussmaul respirations and acetone breath. Capillary glucose is 24 mmol/L, ketones 5 mmol/L and venous blood gas shows pH 7.12 and bicarbonate 10 mmol/L. What is the most important immediate management step after ABC assessment and establishing IV access?