Third-Degree AV Block – Causes, Symptoms & Treatment
Third-degree AV block is a severe cardiac arrhythmia in which electrical conduction between the atria and ventricles is completely interrupted.
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Third-degree AV block is a severe cardiac arrhythmia in which electrical conduction between the atria and ventricles is completely interrupted.
What is Third-Degree AV Block?
Third-degree atrioventricular (AV) block, also known as complete heart block, is the most severe form of AV conduction disturbance. In this condition, no electrical impulses from the atria (upper chambers) are conducted to the ventricles (lower chambers). The heart normally relies on a specialized conduction system, in which electrical signals travel from the sinoatrial (SA) node through the AV node to coordinate the heartbeat. In complete AV block, this pathway is entirely severed.
To compensate, a secondary escape rhythm takes over pacing of the ventricles. This escape rhythm is significantly slower and less reliable than the normal sinus rhythm, often resulting in a marked reduction in cardiac output and potentially life-threatening symptoms.
Causes
Third-degree AV block can arise from a variety of underlying conditions:
- Coronary artery disease (CAD) and myocardial infarction: Ischemia affecting the AV node, most commonly in inferior wall myocardial infarction
- Degenerative disease: Age-related fibrosis and calcification of the conduction system (Lenegre-Lev disease)
- Inflammatory conditions: Myocarditis, endocarditis, sarcoidosis, and Lyme carditis
- Congenital heart defects: Congenital complete AV block, often associated with maternal autoimmune diseases such as systemic lupus erythematosus
- Medications: Overdose or toxicity of beta-blockers, calcium channel blockers, digoxin, or antiarrhythmic drugs
- Cardiac surgery or ablation: Iatrogenic damage to the conduction system
- Electrolyte imbalances: Severe hyperkalemia
Symptoms
The severity of symptoms depends on the rate of the escape rhythm and the underlying cause. Common symptoms include:
- Dizziness and syncope (brief loss of consciousness, known as Adams-Stokes attacks)
- Severe fatigue and reduced exercise tolerance
- Shortness of breath (dyspnea), especially on exertion
- Signs of heart failure: Peripheral edema, orthopnea
- Bradycardia: Heart rate often below 40 beats per minute
- Sudden cardiac death: Possible in severe cases if no stable escape rhythm emerges
Diagnosis
The diagnosis is primarily established by electrocardiography (ECG). The hallmark finding is complete AV dissociation, meaning atrial and ventricular activity are entirely independent of each other.
- P waves: Regular, at normal sinus rate
- QRS complexes: Regular, but at a significantly slower rate (ventricular escape rhythm)
- No fixed PR interval: P waves and QRS complexes bear no consistent temporal relationship
- Wide QRS complexes suggest a ventricular escape rhythm; narrow QRS complexes suggest a junctional escape rhythm
Additional investigations include echocardiography, Holter monitoring, laboratory tests (electrolytes, thyroid function, inflammatory markers), and coronary angiography if ischemia is suspected.
Treatment
Acute Management
Symptomatic third-degree AV block is a medical emergency. Immediate treatment options include:
- Intravenous atropine: Temporarily increases heart rate by blocking vagal tone (most effective in supra-nodal block)
- Temporary pacing: Transcutaneous or transvenous cardiac pacing as a bridging measure
- Isoprenaline or epinephrine: Used as pharmacological bridging in refractory bradycardia
- Discontinuation of all causative medications
Long-Term Management
The definitive treatment for permanent third-degree AV block is implantation of a permanent cardiac pacemaker. The pacemaker reliably restores coordinated cardiac conduction, prevents syncope, and reduces the risk of sudden cardiac death. According to the European Society of Cardiology (ESC) guidelines, pacemaker implantation is recommended in all symptomatic patients and in asymptomatic patients with confirmed complete AV block.
References
- Brignole M et al. - 2021 ESC Guidelines on cardiac pacing and cardiac resynchronization therapy. European Heart Journal, 2021.
- Zipes DP et al. - Braunwald's Heart Disease: A Textbook of Cardiovascular Medicine. 12th ed. Elsevier, 2022.
- Trappe HJ, Gummert J - Current Diagnosis and Treatment: Cardiology. Deutsches Ärzteblatt International, 2023.
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