A 24-year-old woman with type 1 diabetes presents with abdominal pain, vomiting and deep breathing. Her blood glucose concentration is 24 mmol/L, venous pH is 7.18 and serum ketones are 5.6 mmol/L. She is dehydrated but alert, and her serum potassium concentration is 2.9 mmol/L (3.5–5.2). Which one of the following is the most appropriate initial management?