A client is admitted to the intensive care unit (ICU) after surgical repair of an aortic abdominal aneurysm (AAA). Sodium nitroprusside is infusing for severe postoperative hypertension. Normal saline is also infusing at 50 mL/hr. When the client suddenly reports back pain, the nurse measures the client's vital signs to be a heart rate of 38 beats/minute and a blood pressure of 80/60 mm Hg. Which intervention should the nurse implement first?