A 72-year-old woman from Kolkata presents with palpitations and dyspnea for 3 days. On examination, her heart rate is 48/min and irregular. Blood pressure is 110/70 mmHg. ECG shows a rhythm with variable PR intervals, some P waves not followed by QRS complexes, and a QRS duration of 0.09 seconds. The atrial rate is approximately 100/min, but the ventricular rate is 48/min. What is the primary ECG abnormality?

Explanation

## Complete Heart Block (Third-Degree AV Block) **Key Point:** Complete heart block (CHB) is characterized by complete dissociation between atrial and ventricular activity. No atrial impulse conducts to the ventricles, and the ventricles are driven by an escape pacemaker (junctional or ventricular). ### Diagnostic Features of Complete Heart Block | Feature | Observation in This Case | Significance | |---------|--------------------------|---------------| | Atrial rate | ~100/min (P waves visible) | Sinus rhythm continues independently | | Ventricular rate | ~48/min (QRS complexes) | Escape rhythm (junctional or ventricular) | | PR interval | Variable/absent relationship | No fixed PR interval; P and QRS dissociated | | QRS duration | 0.09 sec (narrow) | Junctional escape (narrow QRS) | | P-QRS relationship | **Complete dissociation** | Pathognomonic for CHB | **High-Yield:** The key finding is **complete AV dissociation** — P waves march through the QRS complexes at their own rate, independent of ventricular activity. This is the hallmark of third-degree AV block. ### AV Block Classification ```mermaid flowchart TD A[AV Block]:::outcome --> B{Degree?}:::decision B -->|First-degree| C[Prolonged PR > 0.20 sec]:::action C --> C1[All impulses conduct, just delayed]:::outcome B -->|Second-degree| D{Type?}:::decision D -->|Mobitz I| E[PR progressively lengthens, then drop]:::action D -->|Mobitz II| F[Fixed PR, sudden QRS drop]:::action B -->|Third-degree| G[Complete dissociation]:::urgent G --> G1[Atrial and ventricular rhythms independent]:::outcome G --> G2[Escape rhythm: junctional or ventricular]:::outcome ``` **Clinical Pearl:** In complete heart block with a **narrow QRS escape rhythm** (0.06–0.10 sec), the escape pacemaker is in the **AV junction** (junctional escape rhythm), typically firing at 40–60/min. If the QRS is **wide** (>0.12 sec), the escape is **ventricular**, firing at 20–40/min and is less reliable. **Mnemonic:** **CHB Features** — - **C**omplete dissociation of P and QRS - **H**eart rate slow (escape rhythm rate) - **B**oth atria and ventricles have independent rates ### Why This Is Complete Heart Block, Not Mobitz II Mobitz II block shows: - Fixed PR interval in conducted beats - Sudden, unpredictable drop of QRS (some P waves not followed by QRS) - But when conduction occurs, PR is constant In **complete heart block**: - **No fixed PR interval** — P waves and QRS are completely independent - **Every P wave is not followed by a QRS** — because no conduction occurs - **Atrial and ventricular rates are different** — atrial ~100, ventricular ~48 - This dissociation is the diagnostic hallmark