A 68-year-old man with a history of inferior wall myocardial infarction 3 days ago presents to the emergency department with syncope. On examination, blood pressure is 88/54 mmHg, heart rate is 42 bpm and regular. Jugular venous pressure is elevated. Auscultation reveals variable intensity of the first heart sound and occasional cannon 'a' waves in the JVP. ECG shows P waves at a rate of 90/min and QRS complexes at 42/min, with no fixed relationship between them. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Complete Heart Block (Third-Degree AV Block) ### Key Clinical Features Presented **Key Point:** The combination of: - Atrial rate (90/min) completely independent of ventricular rate (42/min) - No fixed PR interval relationship - Variable intensity of S1 and cannon 'a' waves in JVP - Hemodynamic compromise (syncope, hypotension, bradycardia) All point unequivocally to **complete dissociation** between atrial and ventricular activity. ### ECG Interpretation | Feature | Third-Degree Block | Mobitz I | Mobitz II | |---------|-------------------|----------|----------| | **P-QRS relationship** | No fixed relationship; P's march through | Progressive PR prolongation then dropped beat | Fixed PR, then sudden dropped beat | | **Atrial rate** | Independent from ventricular | Faster than ventricular | Faster than ventricular | | **QRS width** | Usually wide (escape rhythm) | Normal | Usually wide | | **Escape pacemaker** | Ventricular (40–60 bpm) | AV nodal (60–80 bpm) | Ventricular (40–60 bpm) | **High-Yield:** In third-degree block, the ventricles are driven by a **subsidiary escape pacemaker** (either junctional or ventricular), which is inherently slower and less reliable than sinus rhythm. ### Why This Presentation Screams Complete Block 1. **Cannon 'a' waves** — Atria contract against closed tricuspid valve (no AV synchrony) → giant JVP waves. 2. **Variable S1 intensity** — Ventricular contraction occurs at different phases of atrial systole, changing the position of the mitral valve at the moment of ventricular contraction. 3. **Hemodynamic collapse** — Loss of atrial "kick" + slow ventricular rate (42 bpm) → reduced cardiac output → syncope and hypotension. 4. **Post-MI context** — Inferior MI can cause ischemia of the AV node or proximal bundle, leading to complete block. ### Escape Rhythm Characteristics **Clinical Pearl:** In acute inferior MI with complete heart block: - Escape rhythm is often **junctional** (narrower QRS, rate 40–60 bpm). - If the escape is **ventricular** (wide QRS, rate 20–40 bpm), prognosis is worse. - In this case, the 42 bpm rate suggests a junctional escape, which is relatively favorable. ### Management Implications **Key Point:** Acute complete heart block in the setting of MI is a **medical emergency** requiring: - Immediate transcutaneous pacing (TCP) or transvenous pacing (TVP). - Atropine may be tried (though often ineffective in infranodal block). - Temporary pacing is the definitive bridge; permanent pacemaker insertion if block persists >3–7 days. [cite:Harrison 21e Ch 297]