A 26-year-old man with type 1 diabetes presents with 1 day of polyuria, vomiting and abdominal pain. He appears dehydrated and drowsy. Capillary glucose is 28 mmol/L, venous pH 7.12, bicarbonate 10 mmol/L and blood ketones 6.0 mmol/L. Which of the following is the most important initial management step after confirming diabetic ketoacidosis and starting standard monitoring?