A 28-year-old woman is brought to the ICU 2 hours after intentional ingestion of an organophosphate insecticide (parathion). She is intubated for airway protection due to severe bronchospasm and hypersalivation. After atropine 15 mg IV (total) and pralidoxime 1 g IV, she remains paralyzed with visible fasciculations and is unable to lift her head off the bed despite adequate sedation reversal. Vital signs are stable. What is the most appropriate next step?