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AV Block – Causes, Symptoms and Treatment

AV block is a cardiac conduction disorder in which the electrical signal between the atria and ventricles is delayed or completely interrupted.

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Things worth knowing about "AV Block"

AV block is a cardiac conduction disorder in which the electrical signal between the atria and ventricles is delayed or completely interrupted.

What is an AV Block?

AV block (atrioventricular block) is a type of cardiac arrhythmia in which the electrical conduction between the atria (upper chambers) and the ventricles (lower chambers) of the heart is impaired. The electrical impulse normally travels through the AV node (atrioventricular node), which acts as a relay station between the atria and ventricles. In AV block, this impulse is delayed, partially blocked, or completely interrupted.

Causes

AV block can result from a variety of causes:

Age-related degeneration of the cardiac conduction system
Myocardial infarction (heart attack), especially involving the right coronary artery
Inflammatory heart conditions such as myocarditis or endocarditis
Medications such as beta-blockers, digoxin, or calcium channel blockers
Congenital heart defects
Electrolyte imbalances, such as high potassium levels (hyperkalemia)
Autoimmune diseases such as lupus or Lyme disease

Classification and Degrees of Severity

AV block is classified into three degrees of severity:

First-Degree AV Block

The conduction from the atria to the ventricles is slowed but not interrupted. Every atrial impulse reaches the ventricles, though with a prolonged delay. On the ECG, this appears as a PR interval greater than 200 milliseconds. This degree is usually asymptomatic and often considered benign.

Second-Degree AV Block

Not all atrial impulses are conducted to the ventricles. There are two subtypes:

Mobitz Type I (Wenckebach): The PR interval progressively lengthens with each beat until one impulse is completely blocked. The cycle then resets and repeats.
Mobitz Type II: Conduction is blocked suddenly and without warning. This pattern is considered more dangerous, as it may progress to complete heart block.

Third-Degree AV Block (Complete Heart Block)

There is a complete interruption of conduction between the atria and ventricles. The atria and ventricles beat independently of each other. The ventricles are maintained by a slow escape rhythm, which often leads to significantly reduced cardiac output.

Symptoms

Symptoms depend on the degree of block:

First degree: Usually no symptoms
Second and third degree: Dizziness, fainting (syncope), fatigue, shortness of breath, palpitations, or loss of consciousness
Severe cases: Adams-Stokes attacks (sudden loss of consciousness due to cardiac standstill or very slow heart rate)

Diagnosis

The primary diagnostic tool is the electrocardiogram (ECG), which records the electrical activity of the heart and identifies the specific characteristics of AV block. Additional investigations include:

Holter monitor (24-48 hour ECG): Continuous recording to detect intermittent blocks
Echocardiography: Ultrasound imaging to assess cardiac structure and function
Electrophysiological study (EPS): Invasive measurement of the conduction system
Laboratory tests: Electrolytes, thyroid function, drug levels

Treatment

Treatment depends on the degree and underlying cause:

First-degree AV block: Usually no treatment required; regular monitoring is recommended
Second-degree (Mobitz II) and third-degree AV block: Typically requires implantation of a pacemaker to replace the missing electrical impulse conduction
Treatment of the underlying cause: Discontinuing offending medications, treating infections or electrolyte imbalances
Emergency treatment: In acute complete AV block, medications such as atropine or temporary pacing may be used

Prognosis

The outlook for AV block depends significantly on its degree and cause. First-degree AV block is generally benign and requires no treatment. Complete AV block, however, usually requires permanent pacemaker therapy, which can substantially improve the quality of life for affected individuals.

References

Zipes, D.P. et al. - Braunwald's Heart Disease: A Textbook of Cardiovascular Medicine. 12th edition, Elsevier (2022).
Kasper, D.L. et al. - Harrison's Principles of Internal Medicine. 21st edition, McGraw-Hill (2022).
European Society of Cardiology (ESC) - Guidelines on Cardiac Pacing and Cardiac Resynchronization Therapy. European Heart Journal (2021). Available at: https://www.escardio.org

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