A 54-year-old woman is treated for symptomatic hyponatraemia that has probably been present for several days. Her initial serum sodium concentration was 108 mmol/L (135–145), but after 6 hours of saline it is 119 mmol/L and she is passing 400 mL of dilute urine each hour. She has no further seizures, but is at risk of neurological injury from the rapid correction. Which one of the following is the most appropriate next step?