A 21-year-old man with a T-5 spinal cord injury presents to the urgent care clinic where the nurse practitioner (NP) is working. He says he was out to dinner with a friend when he suddenly began to feel lightheaded. Now, he reports symptoms of diaphoresis, flushed face and neck, and severe headache. Upon assessment, the NP finds bradycardia and severe hypertension. Which of the following has NOT been used as treatment for autonomic dysreflexia?