A 20-year-old male patient presents with muscle strength grade 3 in all four limbs for 4 days, no bowel and bladder dysfunction, and no fever. Examination shows limb muscle strength grade 3, hypoesthesia in distal limbs, absent tendon reflexes, and no pathological reflexes. CSF is normal. During disease progression, the patient develops respiratory muscle paralysis. The most critical intervention at this stage is: