A 68-year-old man with a history of inferior wall myocardial infarction 2 days ago presents to the cardiology ward with complaints of dizziness and syncope. On examination, blood pressure is 88/56 mmHg, heart rate is 42 bpm and regular. Cardiovascular examination reveals clear S1 and S2 with no murmurs. An ECG is obtained and shows a regular rhythm with a PR interval of 0.28 seconds, each P wave is followed by a QRS complex, but the PR interval is constant and prolonged. What is the most likely diagnosis?