A 58-year-old man presents to the emergency department with substernal chest pressure that began 45 minutes ago while watching television. He is diaphoretic, nauseated, and short of breath. His vital signs are: BP 92/60 mmHg, HR 110 beats/min, RR 24 breaths/min, SpO2 92% on room air. Lung sounds reveal crackles bilaterally. His 12-lead ECG demonstrates ST-segment elevation in leads V1 through V4. Which initial intervention is the priority?