A 32-year-old male from rural Maharashtra presents to the emergency department with a 5-day history of progressive weakness starting in both lower limbs and ascending to involve the upper limbs and face. He reports a viral illness with diarrhea 2 weeks prior. On examination, he has symmetrical flaccid paralysis of lower limbs, diminished deep tendon reflexes, and mild facial weakness. Vital signs show respiratory rate 28/min, blood pressure 140/90 mmHg, and heart rate 102/min with occasional irregular beats. CSF analysis shows protein 85 mg/dL with normal cell count. Nerve conduction studies show prolonged distal latencies and conduction blocks. What is the most appropriate immediate management?