A 23-year-old woman with type 1 diabetes presents with nausea, vomiting, abdominal pain and polyuria. She has missed several doses of insulin. On examination she is tachycardic, tachypnoeic with Kussmaul respirations and mildly hypotensive. Capillary glucose is 24 mmol/L and capillary ketones are high. What is the most important immediate initial management step?