Atrioventricular (AV) Block: Causes and Risk Factors

Introduction

Atrioventricular (AV) block is a condition where the electrical signals between the atria and ventricles of the heart are partially or completely blocked. This disturbance can lead to irregular heart rhythms, slow heart rates (bradycardia), and in severe cases, heart failure.

Classification of AV Block

AV block is classified into three degrees: - First-degree AV block: Electrical conduction is delayed but all signals reach the ventricles. - Second-degree AV block: Some electrical impulses fail to reach the ventricles, causing dropped beats. - Third-degree (complete) AV block: None of the electrical impulses from the atria reach the ventricles, requiring emergency intervention.

Causes of AV Block

1. Intrinsic Heart Disease

• Ischemic heart disease (e.g., myocardial infarction)
• Cardiomyopathies (e.g., dilated or hypertrophic cardiomyopathy)
• Valvular heart disease (e.g., aortic stenosis)

2. Degenerative Fibrosis and Aging

• Aging can lead to fibrosis of the conduction system, particularly in the bundle of His and AV node, causing progressive conduction disturbances.

3. Medications

• Beta-blockers
• Calcium channel blockers
• Digoxin
• Antiarrhythmic drugs (e.g., amiodarone, flecainide)

4. Electrolyte and Metabolic Disorders

• Hyperkalemia and hypokalemia
• Hypothyroidism

5. Infectious and Inflammatory Conditions

• Myocarditis (e.g., Lyme carditis, viral myocarditis)
• Endocarditis affecting the conduction system
• Sarcoidosis

6. Genetic and Congenital Causes

• Congenital AV block can be associated with maternal autoimmune diseases (e.g., lupus)
• Inherited conduction system diseases

7. Iatrogenic Causes

• Cardiac surgery
• Radiofrequency ablation
• Pacemaker lead-induced trauma

Clinical Importance

Depending on severity, AV block may be asymptomatic or cause symptoms like dizziness, syncope, fatigue, and even sudden cardiac arrest in severe cases. It is crucial to identify the underlying cause to guide proper treatment, which may include medication adjustments, treatment of underlying conditions, or implantation of a pacemaker.

Source recommendations

1. American Heart Association Guidelines on Bradycardia and Conduction Disorders

1. https://www.heart.org/en/health-topics/arrhythmia/about-arrhythmia/conduction-disorders
2. https://www.ahajournals.org/doi/10.1161/CIR.0000000000000628
3. https://www.heart.org/en/health-topics/arrhythmia/about-arrhythmia/bradycardia--slow-heart-rate
4. https://pubmed.ncbi.nlm.nih.gov/30412709/
5. https://www.ahajournals.org/doi/10.1161/CIR.0000000000000627

2. European Society of Cardiology Guidelines on Cardiac Pacing and Resynchronization Therapy

1. https://www.escardio.org/Guidelines/Clinical-Practice-Guidelines/Cardiac-Pacing-and-Cardiac-Resynchronization-Therapy
2. https://academic.oup.com/eurheartj/article/42/35/3427/6358547
3. https://pubmed.ncbi.nlm.nih.gov/34455430/
4. https://academic.oup.com/eurheartj/article/34/29/2281/401445
5. https://pubmed.ncbi.nlm.nih.gov/23801827/