A 58-year-old man with a 3-year history of stable angina presents to the emergency department with chest pain at rest lasting 20 minutes. He is on aspirin 75 mg daily and atenolol 50 mg once daily. On examination, BP is 145/92 mmHg, HR 88/min, and an ECG shows 2 mm ST depression in leads II, III, aVF with T-wave inversion. Troponin I is negative. What is the most appropriate immediate next step in management?