A 28-year-old man with known generalized myasthenia gravis on pyridostigmine presents to the emergency department with acute onset of severe weakness, respiratory distress, and difficulty swallowing over the past 6 hours. Vital signs: RR 28/min, SpO₂ 88% on room air, BP 140/90 mmHg. There is no recent infection or medication change. What is the most appropriate immediate action?