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Atrioventricular Block (AV Block)

What is Atrioventricular Block?

An atrioventricular (AV) block is a type of cardiac conduction abnormality in which the electrical impulses traveling from the atria (the heart’s upper chambers) to the ventricles (the lower chambers) are delayed or completely stopped. This interruption can cause the heart to beat more slowly, irregularly, or with reduced strength, potentially leading to dizziness, fainting, or even heart failure if left untreated. AV block is classified into three main degrees based on severity:

• First‑degree AV block: All impulses reach the ventricles, but the conduction time (PR interval) is prolonged (>200 ms).
• Second‑degree AV block: Some impulses fail to reach the ventricles. It is further divided into:
  • Mobitz type I (Wenckebach) – progressive PR prolongation until a beat is dropped.
  • Mobitz type II – sudden, unpredictable dropped beats without prior PR lengthening.
• Third‑degree (complete) AV block: No atrial impulses conduct to the ventricles; the ventricles generate an independent, slower rhythm (escape rhythm).

While a first‑degree block is often benign and discovered incidentally, second‑ and third‑degree blocks can be life‑threatening and usually require prompt evaluation and therapy.

Common Causes

AV block may develop from a wide range of cardiac and non‑cardiac conditions. The most frequent causes include:

• Ischemic heart disease: Myocardial infarction, especially inferior wall infarctions, can damage the AV node.
• Degenerative conduction system disease: Age‑related fibrosis or sclerosis of the AV node and His‑Purkinje system.
• Congenital heart defects: Such as atrial septal defect, Ebstein’s anomaly, or corrected transposition of the great arteries.
• Cardiomyopathies: Hypertrophic, dilated, or restrictive types may involve the conduction tissue.
• Inflammatory or infiltrative diseases: Sarcoidosis, amyloidosis, Lyme disease, or rheumatic fever.
• Medications: Beta‑blockers, calcium‑channel blockers (verapamil, diltiazem), digoxin, and certain antiarrhythmic drugs (e.g., amiodarone, flecainide).
• Electrolyte disturbances: Hyperkalemia, severe hypocalcemia, or hypomagnesemia.
• Cardiac surgery or procedures: Valve replacements, coronary artery bypass grafting, or catheter ablation can unintentionally injure the conduction pathways.
• Infections: Endocarditis or myocarditis may compromise the AV node.
• Autoimmune disorders: Systemic lupus erythematosus, Sjögren’s syndrome, or vasculitis affecting cardiac tissue.

Associated Symptoms

Symptoms may vary with the degree of block and the underlying heart rate. Commonly reported manifestations include:

• Dizziness or light‑headedness, especially upon standing.
• Syncope (fainting) or near‑syncope.
• Fatigue or reduced exercise tolerance.
• Palpitations – perception of “skipped” beats or a slow heart rhythm.
• Shortness of breath, especially during activity.
• Chest discomfort or pain that is not typical of angina.
• Confusion or memory lapses in older adults (often due to low cardiac output).
• Peripheral edema (rare, usually when AV block is accompanied by heart failure).

When to See a Doctor

Because AV block can progress from benign to dangerous, timely medical attention is essential. Seek care promptly if you experience:

• Unexplained fainting or near‑fainting episodes.
• Persistent dizziness, especially with activities like climbing stairs.
• Sudden, new‑onset shortness of breath.
• Palpitations accompanied by a slow or irregular pulse.
• Chest pain or pressure that does not resolve.
• Signs of heart failure (e.g., swelling of ankles, rapid weight gain, shortness of breath when lying flat).

Even if symptoms are mild, a routine check‑up or an abnormal ECG during a physical exam warrants a follow‑up with a cardiologist or primary‑care provider.

Diagnosis

Diagnosing AV block involves a combination of clinical assessment, electrocardiographic testing, and sometimes imaging.

1. Electrocardiogram (ECG)

The 12‑lead ECG is the cornerstone test. It reveals:

• Prolonged PR interval (first‑degree).
• Pattern of dropped beats and PR progression (second‑degree type I vs. type II).
• Complete dissociation of P‑waves and QRS complexes (third‑degree).
• Associated arrhythmias or signs of ischemia.

2. Ambulatory Monitoring

If symptoms are intermittent, a Holter monitor (24‑48 h) or an event recorder can capture episodes that a single ECG might miss.

3. Electrophysiology Study (EPS)

In complex cases, an invasive EPS maps the exact site of conduction delay and helps decide on pacing strategies.

4. Blood Tests

Labs may identify reversible contributors such as:

• Serum electrolytes (potassium, calcium, magnesium).
• Thyroid function (hypothyroidism can slow conduction).
• Cardiac biomarkers (troponin) if an acute coronary syndrome is suspected.
• Serology for Lyme disease or inflammatory markers when infection/inflammation is a concern.

5. Imaging

Transthoracic echocardiography evaluates structural heart disease, ventricular function, and valve pathology that may predispose to AV block.

Treatment Options

Therapy is guided by the block’s severity, symptom burden, and underlying cause.

Medical Management

• Address reversible causes: Correct electrolyte abnormalities, discontinue offending drugs, treat infections (e.g., antibiotics for Lyme disease).
• Medication adjustments: Reduce or stop AV‑node‑blocking agents (beta‑blockers, calcium‑channel blockers, digoxin) under physician guidance.
• Rate‑supportive drugs: In rare cases, agents like atropine can temporarily increase AV nodal conduction during acute symptomatic episodes.

Device Therapy

Permanent cardiac pacing is the definitive treatment for symptomatic second‑ or third‑degree AV block.

• Single‑chamber ventricular pacemaker (VVI): Most common for isolated AV block.
• Dual‑chamber pacemaker (DDD): Preferred when atrial‑ventricular synchrony is desired, especially in younger or more active patients.
• Leadless pacemakers: Emerging option for patients with limited venous access or high infection risk.
• Implantable cardioverter‑defibrillator (ICD): Considered if there is co‑existing ventricular arrhythmia risk.

Home & Lifestyle Measures

• Maintain adequate hydration and electrolytes.
• Avoid over‑the‑counter cold or sinus medications that contain decongestants which may affect heart rate.
• Monitor your pulse regularly if you have a known block; a resting rate < 50 bpm with symptoms warrants a call to your doctor.
• Engage in moderate, physician‑approved aerobic activity; avoid sudden intense exertion until a stable rhythm is confirmed.

Prevention Tips

While some causes (age‑related degeneration, congenital anomalies) cannot be prevented, you can reduce risk for acquired AV block:

• Control cardiovascular risk factors: Manage hypertension, diabetes, and cholesterol to lower the chance of coronary artery disease.
• Quit smoking and limit alcohol: Both accelerate atherosclerosis and can aggravate conduction disturbances.
• Use medications wisely: Discuss any new prescription or herbal supplement with your clinician, especially if you have a known conduction issue.
• Promptly treat infections: Early antibiotic therapy for Lyme disease or bacterial endocarditis can prevent cardiac involvement.
• Regular follow‑up: If you have a prior ECG abnormality or a pacemaker, attend scheduled check‑ups to monitor device function and rhythm.
• Maintain electrolyte balance: Especially important during illness, diuretic therapy, or intense exercise.

Emergency Warning Signs

Key Takeaways

• AV block is a disturbance in the heart’s electrical pathway that ranges from benign (first‑degree) to life‑threatening (third‑degree).
• Common causes include heart disease, medications, electrolyte imbalances, infections, and aging‑related tissue changes.
• Symptoms often involve dizziness, fainting, fatigue, and palpitations; however, many patients are asymptomatic.
• Diagnosis relies on ECG, ambulatory monitoring, labs, and sometimes electrophysiology studies.
• Treatment may be as simple as correcting a reversible factor, but symptomatic second‑ or third‑degree blocks usually need a permanent pacemaker.
• Preventive strategies focus on cardiovascular health, medication vigilance, and prompt infection treatment.
• Seek emergency care immediately for syncope, severe chest pain, or very slow heart rates accompanied by weakness or confusion.

For more detailed information, consult reputable sources such as the Mayo Clinic, CDC, and the National Heart, Lung, and Blood Institute (NHLBI).

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