A 9-year-old boy is recovering from a severe traumatic brain injury in the intensive care unit. He begins to have paroxysmal episodes of tachycardia, tachypnea, hyperthermia to 39.5°C (103.1°F), diaphoresis, and dystonic posturing, often triggered by repositioning. Which medication is reported to be ineffective for managing these episodes of paroxysmal sympathetic hyperactivity (PSH)?