A previously fit and well 70-year-old woman has been admitted due to a fractured neck of femur, and she has recently returned to the ward after surgery. You notice that she has had only 30 ml urine output in the last 3 hours. She has a 12-hourly bag of normal saline running. Her catheter is draining concentrated urine. Abdominal examination was normal. Which of the following interventions would you try first to increase the urine output?