A 23-year-old woman with type 1 diabetes presents to the emergency department with abdominal pain, nausea and polyuria. She has missed several insulin doses. On examination she is tachycardic and mildly hypotensive. Capillary glucose is 24 mmol/L, serum ketones are positive and venous blood gas shows pH 7.18. According to standard DKA management principles, what is the most appropriate initial fluid resuscitation?