A 22-year-old woman with type 1 diabetes presents with abdominal pain, nausea and polyuria for 24 hours. Capillary blood glucose is 24 mmol/L, venous blood gas shows pH 7.18 and bicarbonate 12 mmol/L, and serum ketones are 4.5 mmol/L. She is tachycardic but normotensive. What is the most appropriate initial fluid management?