A 68-year-old man with acute inferior wall myocardial infarction presents with progressive bradycardia and hypotension. ECG shows a regular ventricular rhythm of 40 bpm with P waves marching through the QRS complexes at a rate of 100 bpm. Which of the following is NOT a characteristic feature of complete heart block (third-degree AV block)?

Explanation

## Understanding Third-Degree AV Block ### Definition Complete heart block (CHB) is a condition where there is complete electrical dissociation between atrial and ventricular activity. No atrial impulses conduct to the ventricles. ### Key Characteristics of CHB | Feature | Details | |---------|----------| | **P-QRS relationship** | Completely independent; P waves "march through" QRS and T waves | | **PR interval** | Variable and meaningless (no conduction occurring) | | **Ventricular rate** | Determined by escape pacemaker (junctional 40–60 bpm, ventricular 20–40 bpm) | | **QRS duration** | Narrow if junctional escape (< 120 ms); wide if ventricular escape (≥ 120 ms) | | **Atrial rate** | Independent, often faster than ventricular rate | **Key Point:** In complete heart block, the PR interval is NOT constant — it is variable because there is no relationship between P waves and QRS complexes. The P waves are independent and "march through" the QRS and T waves at their own rate. ### Why Option 2 (PR interval constant and < 200 ms) is INCORRECT **High-Yield:** A constant PR interval suggests a fixed conduction delay (as seen in first-degree block). In third-degree block, there is NO conduction, so the PR interval is: - **Variable** (not constant) - **Meaningless** (no relationship between P and QRS) - Can be any duration depending on where the P wave falls relative to the QRS ### Clinical Pearl **Cannon A waves** occur when the atrium contracts against a closed tricuspid valve — this happens in CHB because atrial and ventricular contractions are out of sync. This is a classic finding and IS present in CHB. ### Escape Rhythms in CHB 1. **Junctional escape** (AV junction): Rate 40–60 bpm, narrow QRS (< 120 ms) 2. **Ventricular escape** (ventricular myocardium): Rate 20–40 bpm, wide QRS (≥ 120 ms) In the clinical scenario given (inferior MI with regular rhythm at 40 bpm), the narrow QRS suggests a junctional escape pacemaker, which is favorable and may not require pacing if hemodynamically stable. ### Management Implications - **Acute inferior MI with CHB:** Often transient; may resolve with reperfusion therapy - **Anterior MI with CHB:** Usually indicates extensive damage; high mortality; pacing often required - **Symptomatic bradycardia:** Atropine (vagolytic) or temporary pacing