A 23-year-old man with type 1 diabetes is admitted with vomiting and abdominal pain. He is drowsy and dehydrated, respiratory rate 30/min with deep sighing respirations, and his capillary blood glucose is 32 mmol/L. Ketones are 5.5 mmol/L and arterial blood gas shows pH 7.10 and bicarbonate 10 mmol/L. What is the most important initial fluid management step?