Third degree atrioventricular block

A 60 year old male presents with fatigue, dizziness, lightheadedness, and near syncope 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 (35/min) with a regular pulse and blood pressure 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, indicating a ventricular escape rhythm. Diagnosis?

Diagnosis is Third degree (complete) atrioventricular block.

1. Definition

Third degree AV block is characterized by complete atrioventricular dissociation, in which no atrial impulses are conducted to the ventricles, resulting in independent atrial and ventricular rhythms.

2. Etiology

1. Degenerative fibrosis of the conduction system (most common in chronic cases)
2. Ischemic heart disease (especially acute myocardial infarction)
3. AV nodal blocking drugs
• Beta blockers
• Non dihydropyridine calcium channel blockers
• Digoxin
• Amiodarone
4. Electrolyte abnormalities
• Hyperkalemia
5. Inflammatory conditions
• Myocarditis
• Lyme disease
6. Infiltrative diseases
• Sarcoidosis
• Amyloidosis
• Hemochromatosis
7. Autoimmune disorders
8. Congenital AV block

3. Pathophysiology

1. Complete interruption of AV conduction
2. Atria depolarize under SA node control
3. Ventricles are driven by escape rhythms:
• Junctional escape → narrow QRS, rate 40 to 60
• Ventricular escape → wide QRS, rate 20 to 40
4. More distal block → slower rate and worse prognosis
5. Leads to reduced cardiac output and symptoms

4. Clinical Features

4.1 Core Features

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

4.2 Additional Features

1. Fatigue
2. Dizziness
3. Syncope or near syncope
4. Chest pain
5. Dyspnea

5. Diagnosis

5.1 Electrocardiography

1. No relationship between P waves and QRS complexes
2. P waves march through independently
3. Constant P–P and R–R intervals
4. QRS width depends on escape rhythm
• Wide QRS → infranodal block (more dangerous)

5.2 Additional Evaluation

1. Cardiac biomarkers to assess for ischemia
2. Electrolytes to identify reversible causes

6. Management

6.1 Initial Step

1. Assess hemodynamic stability

6.2 Acute Management

1. Immediate transcutaneous pacing if unstable
2. Atropine may be attempted but often ineffective in infranodal block
3. Epinephrine or dopamine infusion if needed

6.3 Definitive Management

1. Transvenous pacing
2. Permanent pacemaker implantation

6.4 Additional Measures

1. Treat reversible causes
2. Discontinue offending drugs

7. Key Clinical Insight

Severe bradycardia + cannon a waves + AV dissociation on ECG = Third degree AV block

8. Exam Level Pearls

1. Wide QRS suggests infranodal block and worse prognosis
2. Mobitz type II can progress to complete heart block
3. Inferior MI → AV nodal block (better prognosis)
4. Anterior MI → infranodal block (worse prognosis)
5. Third degree AV block is a medical emergency requiring pacing