A 42-year-old woman with a history of shellfish allergy and asthma presents for contrast-enhanced MRI of the brain for evaluation of chronic headaches. During contrast injection with gadolinium-based contrast agent (GBCA), she develops urticaria, facial edema, and mild bronchospasm within 2 minutes. Her blood pressure is 115/70 mmHg and oxygen saturation is 94% on room air. The MRI is immediately halted. What is the most appropriate immediate management?