A 23-year-old woman with type 1 diabetes presents with abdominal pain, vomiting and polyuria. Capillary glucose is 22 mmol/L, venous pH 7.15, bicarbonate 12 mmol/L and ketones 5.5 mmol/L. She is tachycardic but normotensive. According to current diabetic ketoacidosis (DKA) protocols, what is the most appropriate initial fluid resuscitation?