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?

Explanation

## Diagnosis: First-Degree AV Block ### Clinical Context This patient presents with an inferior MI complicated by conduction delay. The key ECG finding is a **prolonged but constant PR interval (0.28 seconds; normal <0.20 s) with 1:1 AV conduction** — every P wave is followed by a QRS complex. ### ECG Interpretation **Key Point:** First-degree AV block is defined as **PR interval >0.20 seconds with maintained 1:1 conduction**. There is no dropped beat. ### Pathophysiology in Inferior MI **High-Yield:** Inferior MI often damages the AV node (supplied by the RCA in ~90% of people) because: - The AV nodal artery typically arises from the RCA - This causes slowed conduction through the AV node - Results in prolonged PR interval but preserved conduction ### Clinical Significance **Clinical Pearl:** First-degree AV block in the setting of acute inferior MI is usually: - Transient and benign - Resolves within 3–7 days as ischemia resolves - Does NOT require pacing unless symptomatic (which is rare) - Hypotension here is likely from the MI itself, not the conduction delay ### Management - Continuous cardiac monitoring - Atropine if symptomatic bradycardia develops - Temporary pacing only if progression to higher-degree block occurs - Avoid drugs that slow AV conduction (beta-blockers, calcium channel blockers) acutely **Mnemonic:** **PR-1-1** = First-degree block: **P**rolonged **R**egular **1**:1 conduction