Third degree atrioventricular block

A 60-year-old male presents with fatigue, dizziness, lightheadedness, and presyncope over several days. He also reports chest pain and shortness of breath on exertion. On examination, the jugular venous pulse shows cannon a waves. Vital signs reveal severe bradycardia at 35/min with a regular pulse and a blood pressure of 110/70 mmHg. Electrocardiography demonstrates complete dissociation between P waves and QRS complexes, with constant P-P and R-R intervals. The P waves march through independently. The QRS complexes are wide, suggesting a ventricular escape rhythm. Diagnosis?

Diagnosis is Third-degree (complete) atrioventricular block.

1. Definition

Third-degree AV block is characterized by complete failure of atrial impulses to conduct to the ventricles, resulting in atrioventricular dissociation.

The atria and ventricles therefore depolarize independently, with the atria usually controlled by the SA node and the ventricles maintained by a junctional or ventricular escape rhythm.

2. Etiology

1. Degenerative fibrosis and sclerosis of the cardiac conduction system
• A common cause of acquired chronic AV block in older adults
2. Ischemic heart disease
• Acute myocardial infarction
• Chronic ischemic conduction system disease
3. Drugs that impair AV conduction
• Beta blockers
• Non-dihydropyridine calcium channel blockers
• Digoxin
• Amiodarone
4. Electrolyte and metabolic abnormalities
• Hyperkalemia
5. Inflammatory or infectious cardiac disease
• Myocarditis
• Lyme carditis
6. Infiltrative or systemic cardiac disease
• Cardiac sarcoidosis
• Amyloidosis
• Hemochromatosis
7. Iatrogenic or postprocedural conduction system injury
• Cardiac surgery
• Transcatheter or catheter-based cardiac procedures
8. Congenital complete AV block

3. Pathophysiology

1. There is complete interruption of conduction from the atria to the ventricles.
2. The atria continue to depolarize independently, usually under SA node control.
3. Ventricular activation is maintained by an escape pacemaker.
• Junctional escape rhythm
• Usually narrow QRS complexes
• Typically faster and more reliable
• Usually approximately 35 to 50/min
• Ventricular escape rhythm
• Usually wide QRS complexes
• Typically slower and less reliable
• Usually approximately 20 to 40/min
4. A more distal ventricular escape focus generally produces a slower and less reliable escape rhythm.
5. Severe bradycardia reduces cardiac output, which may cause cerebral hypoperfusion, myocardial ischemia, heart failure, presyncope, or syncope.

4. Clinical Features

4.1 Core Features

1. Severe bradycardia
2. AV dissociation
3. Cannon a waves

Cannon a waves occur when the atria intermittently contract against a closed tricuspid valve because atrial and ventricular contractions are dissociated.

4.2 Additional Features

1. Fatigue
2. Dizziness or lightheadedness
3. Presyncope or syncope
4. Exercise intolerance
5. Chest pain
6. Dyspnea
7. Heart failure in severe cases

5. Diagnosis

5.1 Electrocardiography

Typical ECG findings include:

1. No fixed relationship between P waves and QRS complexes
2. P waves march through independently
3. Atrial and ventricular rhythms are independent
4. P-P intervals are usually regular
5. R-R intervals of the escape rhythm are usually regular
6. The atrial rate is greater than the ventricular rate
7. The PR interval varies because there is no consistent relationship between atrial and ventricular depolarization

The QRS morphology depends mainly on the escape focus:

• Narrow QRS usually suggests a junctional escape focus
• Wide QRS usually suggests a distal ventricular escape focus

A wide QRS supports a distal escape mechanism but does not by itself definitively localize the site of AV block, because pre-existing bundle branch or intraventricular conduction disease can also widen the QRS complex.

In this patient, the ventricular rate of approximately 35/min with wide QRS complexes strongly suggests a ventricular escape rhythm.

5.2 Additional Evaluation

Evaluation should identify reversible or treatable causes and associated cardiac disease.

1. 12-lead ECG
2. Serum electrolytes, particularly potassium
3. Cardiac biomarkers when myocardial ischemia or infarction is suspected
4. Medication review for drugs that impair AV conduction
5. Evaluation for myocardial ischemia or infarction
6. Echocardiography when structural heart disease is suspected
7. Additional testing for specific causes such as Lyme carditis, myocarditis, or infiltrative disease when clinically indicated

6. Management

6.1 Initial Assessment

Assess whether the bradycardia is producing cardiopulmonary compromise, including:

1. Hypotension
2. Acutely altered mental status
3. Signs of shock
4. Ischemic chest discomfort
5. Acute heart failure

At the same time, identify and treat potentially reversible causes.

6.2 Acute Management

For persistent symptomatic bradycardia with cardiopulmonary compromise:

1. Atropine
• May be administered according to the adult bradycardia algorithm
• It is less likely to be effective when the block is located in the His-Purkinje system
2. If atropine is ineffective:
• Transcutaneous pacing
• Dopamine infusion
• Epinephrine infusion
3. Temporary transvenous pacing
• If hemodynamically unstable bradycardia persists despite medical therapy, temporary transvenous pacing is reasonable
• It may be particularly important when the escape rhythm is slow, unstable, or unreliable

6.3 Definitive Management

Permanent pacemaker implantation is recommended in patients with acquired third-degree AV block that is not caused by a reversible or physiologic condition, regardless of the presence or absence of symptoms.

6.4 Additional Measures

1. Treat reversible causes
2. Discontinue or adjust offending medications when appropriate
3. Correct electrolyte abnormalities
4. Treat myocardial ischemia or infarction when present
5. Treat specific infectious, inflammatory, or infiltrative diseases when identified

7. Key Clinical Insight

Severe bradycardia + cannon a waves + complete AV dissociation on ECG strongly indicates third-degree AV block.

In this patient, the independent P waves and QRS complexes, regular atrial and ventricular rhythms, severe bradycardia, and wide QRS escape complexes establish the diagnosis of third-degree AV block with a likely ventricular escape rhythm.

References

1. Kusumoto FM, Schoenfeld MH, Barrett C, et al. 2018 ACC/AHA/HRS guideline on the evaluation and management of patients with bradycardia and cardiac conduction delay. Circulation. 2019;140(8):e382-e482. doi:10.1161/CIR.0000000000000628.
2. Wigginton JG, Agarwal S, Bartos JA, et al. Part 9: Advanced Life Support: 2025 American Heart Association Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiovascular Care. Circulation. 2025;152(Suppl 2):S538-S577. doi:10.1161/CIR.0000000000001376.
3. Glikson M, Nielsen JC, Kronborg MB, et al. 2021 ESC Guidelines on cardiac pacing and cardiac resynchronization therapy. European Heart Journal. 2021;42(35):3427-3520. doi:10.1093/eurheartj/ehab364.