A 68-year-old man with a history of acute anterior wall myocardial infarction presents to the emergency department with syncope. On examination, blood pressure is 85/55 mmHg, heart rate 35 bpm, and JVP is elevated. ECG shows complete dissociation between P waves and QRS complexes; P waves occur at a rate of 90 bpm while QRS complexes occur at 35 bpm with a wide QRS morphology (>120 ms). What is the most likely diagnosis?

Explanation

## Clinical Presentation Analysis This patient presents with the classic triad of third-degree (complete) AV block: - **Hemodynamic compromise**: Hypotension (85/55) and syncope due to inadequate cardiac output - **Bradycardia**: Heart rate 35 bpm (escape rhythm rate) - **Elevated JVP**: Cannon A waves from atrial contraction against closed tricuspid valve ## ECG Findings — The Key Discriminator **Key Point:** The width of the QRS complex determines the level of the block and the escape pacemaker: | Feature | Junctional Escape | Ventricular Escape | |---------|-------------------|--------------------| | **QRS Duration** | ≤120 ms (narrow) | >120 ms (wide) | | **Escape Rate** | 40–60 bpm | 20–40 bpm | | **Prognosis** | More stable | Unreliable, unstable | | **Typical Location** | Infranodal AV block | Infranodal block below His bundle | This patient's QRS is **>120 ms (wide)** and the rate is **35 bpm**, indicating a **ventricular escape rhythm** — the most dangerous form of third-degree block. ## Clinical Significance **High-Yield:** Complete AV block with a **wide QRS escape rhythm** in acute anterior MI indicates: 1. **Infranodal (Hisian or infra-Hisian) block** — the conduction defect is below the AV node 2. **Poor prognosis** — ventricular escape rhythms are unreliable and prone to degeneration into asystole or VF 3. **Urgent need for pacing** — temporary transvenous pacing is indicated immediately **Clinical Pearl:** Anterior MI causes third-degree block via **extensive septal necrosis** affecting the bundle branches, whereas inferior MI typically causes nodal block (usually reversible). ## Why This Is Third-Degree Block **Mnemonic: "Complete Dissociation = Complete Block" (CD=CB)** - P waves and QRS complexes are **completely independent** (no relationship) - Atrial rate (90 bpm) ≠ Ventricular rate (35 bpm) - This is pathognomonic for third-degree AV block ```mermaid flowchart TD A[Complete AV Block Diagnosed]:::outcome --> B{QRS Width?}:::decision B -->|Narrow ≤120 ms| C[Junctional Escape Rhythm]:::outcome C --> D[Rate 40-60 bpm]:::outcome D --> E[Relatively stable, may observe]:::action B -->|Wide >120 ms| F[Ventricular Escape Rhythm]:::outcome F --> G[Rate 20-40 bpm]:::outcome G --> H[Unreliable, high mortality risk]:::urgent H --> I[Emergent Transvenous Pacing]:::action ``` ## Management Implication This patient requires **immediate temporary transvenous pacing** followed by consideration of permanent pacemaker implantation, as anterior MI with complete AV block carries a mortality rate >50% if not paced.