A 16-year-old male presents with muscle strength grade 3 in all four limbs and numbness for 10 days. He had diarrhea 10 days prior, which resolved in 2 days. Examination: clear consciousness, bilateral peripheral facial palsy. Limb muscle strength grade 2, decreased muscle tone, absent tendon reflexes, calf tenderness, and stocking-glove pattern sensory disturbance. CSF findings: WBC count 4×10<sup>6</sup>/L, protein 0.6g/L, glucose 3.2mmol/L, chloride 122mmol/L. On day 3 of hospitalization, the patient develops shortness of breath, dyspnea, weak cough, and cyanosis of the lips. Arterial oxygen tension is 60mmHg despite oxygen therapy (5L/min). The immediate action required is: