A 72-year-old man with chronic hypertension and diabetes mellitus presents to the emergency department with a 3-hour history of severe chest pain radiating to the back. On examination, he is diaphoretic, blood pressure is 145/92 mmHg, heart rate is 58 bpm and irregular. Cardiac auscultation reveals an early diastolic murmur at the right upper sternal border. An ECG shows ST elevation in leads II, III, and aVF. Notably, the ECG also demonstrates some P waves that are not followed by QRS complexes, and some QRS complexes that appear without preceding P waves. The PR intervals of conducted beats vary. What is the most likely diagnosis?