A 23-year-old woman with type 1 diabetes presents with nausea, abdominal pain and increasing lethargy. She has stopped taking insulin for 2 days due to intercurrent illness. On examination she is tachycardic, tachypnoeic with Kussmaul respirations and mildly hypotensive. Capillary blood glucose is 26 mmol/L, venous pH 7.12 and serum ketones 6.0 mmol/L. According to standard DKA protocols, which is the most appropriate initial management step after immediate ABC assessment?