First-Degree Atrioventricular (AV) Block

Introduction

First-degree AV block is a condition in which electrical conduction through the atrioventricular node (AV node) is delayed, but all impulses still reach the ventricles. This condition is often benign but can indicate underlying heart disease or systemic conditions.

Causes of First-Degree AV Block

First-degree AV block can be caused by various factors, including:

1. Intrinsic Cardiac Causes

• Aging and Fibrosis of the Conduction System – The electrical system of the heart may undergo natural degeneration with age.
• Ischemic Heart Disease – Coronary artery disease and myocardial infarction can damage the AV node.
• Myocarditis – Inflammation of the heart muscle can affect conduction.
• Cardiomyopathies – Diseases like hypertrophic or dilated cardiomyopathy can affect electrical conduction.
• Congenital Heart Disease – Some patients are born with abnormal AV conduction.

2. Medication-Induced Causes

Certain medications can slow conduction through the AV node, including: - Beta-blockers (e.g., metoprolol, atenolol) - Calcium channel blockers (e.g., verapamil, diltiazem) - Digoxin - Antiarrhythmic medications (e.g., amiodarone, sotalol)

3. Electrolyte Imbalances and Metabolic Disorders

• Hyperkalemia – Increased potassium levels can slow conduction.
• Hypermagnesemia – High magnesium levels can depress AV nodal conduction.
• Hypothyroidism – Reduced metabolic activity may contribute to electrical slowing.

4. Autonomic Nervous System Influences

• Increased Vagal Tone – Common in athletes and during sleep, vagal stimulation can lead to first-degree AV block.

5. Infiltrative and Systemic Diseases

• Sarcoidosis – Can lead to granulomas affecting the conduction system.
• Amyloidosis – Deposition of amyloid proteins can interfere with normal conduction.
• Hemochromatosis – Iron deposition in the heart can disrupt electrical activity.
• Lyme Disease – Can cause AV block, sometimes progressing to higher degrees.

When to Be Concerned

First-degree AV block is often asymptomatic and discovered incidentally on an ECG. However, it may warrant further evaluation if: - It progresses over time. - It is associated with symptoms like dizziness, syncope, or fatigue. - It occurs in the presence of other heart disease.

Conclusion

Most cases of first-degree AV block do not require treatment unless they progress to higher-degree heart blocks or cause symptoms. Identifying and addressing the underlying cause is essential for management.

Source recommendations

1. American Heart Association Guidelines on Bradyarrhythmias

1. https://cpr.heart.org/-/media/cpr-files/cpr-guidelines-files/algorithms/algorithmacls_bradycardia_200612.pdf
2. https://www.heart.org/en/health-topics/arrhythmia/about-arrhythmia/bradycardia--slow-heart-rate
3. https://www.ahajournals.org/doi/10.1161/CIR.0000000000000628
4. https://cpr.heart.org/en/resuscitation-science/cpr-and-ecc-guidelines/algorithms
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/