A 35-year-old woman presents with ptosis of both eyelids for 6 months. Recently, she developed quadriparesis and dysphagia. No fever or diarrhea. Chest CT revealed thymic hyperplasia. On admission, she developed cyanosis of the lips and tachypnea. She had been taking pyridostigmine 60mg four times daily for the past 3 months. On admission, after intramuscular neostigmine 1mg, her symptoms worsened after 1 hour. The most appropriate emergency management is: