A 4-year-old child with diphtheria is admitted with pseudomembrane, fever, and mild stridor. Throat culture on Loeffler's serum medium shows gram-positive bacilli with metachromatic granules. On day 2 of hospitalization, the child develops tachycardia (HR 120/min), hypotension (BP 90/60 mmHg), and signs of myocarditis on ECG (prolonged PR interval, ST depression). What is the most appropriate next step in management?