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First-Degree AV Block – Causes, Symptoms & Treatment

First-degree AV block is a mild cardiac conduction disorder in which the electrical signal from the atria to the ventricles is slowed. It is usually detected on an ECG and is often harmless.

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Things worth knowing about "First-Degree AV Block"

First-degree AV block is a mild cardiac conduction disorder in which the electrical signal from the atria to the ventricles is slowed. It is usually detected on an ECG and is often harmless.

What is a First-Degree AV Block?

First-degree atrioventricular (AV) block is a type of cardiac conduction disturbance in which the electrical impulse from the atria (upper chambers of the heart) to the ventricles (lower chambers) is delayed. The impulse still reaches the ventricles with every heartbeat, but it takes longer than normal to pass through the AV node, the specialized relay station in the heart´s conduction system.

Compared to second- and third-degree AV block, first-degree AV block is the mildest form and, in most cases, does not cause any clinical symptoms. However, medical evaluation is recommended, as it can occasionally indicate an underlying heart condition.

Causes

First-degree AV block can have a variety of causes. It is frequently an incidental finding with no pathological significance. Possible causes include:

Increased vagal tone: A slowed AV conduction is physiologically normal in athletes or during sleep.
Heart disease: Coronary artery disease, myocarditis (inflammation of the heart muscle), or cardiomyopathies can cause conduction disturbances.
Medications: Certain drugs such as beta-blockers, calcium channel blockers (e.g., verapamil, diltiazem), digoxin, or antiarrhythmic agents can slow AV conduction.
Electrolyte imbalances: Changes in potassium or calcium levels in the blood can affect the conduction system.
Myocardial infarction: Particularly inferior wall myocardial infarction can cause AV block.
Infectious diseases: For example, Lyme disease or rheumatic fever.
Age-related degeneration: Degenerative changes in the conduction system can occur with advancing age.

Symptoms

First-degree AV block typically causes no symptoms. It is most often discovered incidentally during a routine electrocardiogram (ECG). In rare cases, particularly when the PR interval is markedly prolonged, the following symptoms may occasionally occur:

Mild palpitations or a sensation of irregular heartbeats
Fatigue or reduced exercise tolerance (in pronounced cases)

However, if significant symptoms such as dizziness, fainting, or severe palpitations occur, these may point to a higher-degree AV block or another heart condition and require prompt medical evaluation.

Diagnosis

The diagnosis of first-degree AV block is made exclusively by electrocardiogram (ECG). The characteristic finding is a prolonged PR interval of more than 200 milliseconds (0.2 seconds) in adults. Importantly, every P wave is followed by a QRS complex, meaning no impulse is completely blocked.

Depending on the clinical context, additional diagnostic tests may be appropriate:

Holter monitor (ambulatory ECG): To assess heart rhythm over 24 hours or longer.
Echocardiography: Ultrasound examination of the heart to assess cardiac structure and function.
Laboratory tests: Blood tests for electrolytes, thyroid function, and inflammatory markers.
Medication review: Assessment of all current medications for their effects on the conduction system.

Treatment

In the majority of cases, no specific treatment is required for first-degree AV block, as it has no hemodynamic consequences and does not impair cardiac function. Management is primarily directed at the underlying cause:

Medication adjustment: If a drug is causing the AV block, a dose reduction or change in medication may be appropriate.
Treatment of the underlying condition: In cases of myocarditis, Lyme disease, or other triggers, the underlying condition is treated.
Regular monitoring: Periodic ECG follow-up is recommended to ensure the block does not progress to a higher degree.

A cardiac pacemaker is generally not necessary for first-degree AV block. Pacemaker implantation is reserved for higher-degree AV blocks (grade 2 or 3) when cardiac function is significantly impaired.

Prognosis

The prognosis of first-degree AV block is generally favorable. For most individuals, the finding remains stable and clinically insignificant. Nevertheless, regular cardiological follow-up is advisable to monitor for possible progression. In physically active individuals, first-degree AV block is often a normal physiological adaptation to training and typically requires no further intervention.

References

Braunwald's Heart Disease: A Textbook of Cardiovascular Medicine, 12th ed., Zipes et al., Elsevier (2022).
European Society of Cardiology (ESC): Guidelines on Cardiac Pacing and Cardiac Resynchronization Therapy (2021). Available at: www.escardio.org
Page R.L. et al. - 2015 ACC/AHA/HRS Guideline for the Management of Adult Patients With Supraventricular Tachycardia. Journal of the American College of Cardiology, 67(13), e27-e115 (2016).

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