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Second-Degree AV Block – Causes, Symptoms & Treatment

Second-degree AV block is a cardiac conduction disorder in which some electrical impulses from the atria are partially or intermittently blocked before reaching the ventricles.

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Second-degree AV block is a cardiac conduction disorder in which some electrical impulses from the atria are partially or intermittently blocked before reaching the ventricles.

What Is a Second-Degree AV Block?

Second-degree atrioventricular (AV) block is a cardiac arrhythmia in which the electrical conduction between the atria (upper chambers) and the ventricles (lower chambers) of the heart is impaired. Unlike a first-degree AV block, not every electrical impulse is conducted to the ventricles – some impulses are completely blocked. There are two main subtypes: Type I (Wenckebach) and Type II (Mobitz II).

Causes

Second-degree AV block can result from various underlying conditions:

Coronary artery disease (CAD) and myocardial infarction, especially inferior wall infarction
Myocarditis (inflammation of the heart muscle)
Cardiomyopathies (diseases of the heart muscle)
Degenerative changes of the conduction system (e.g., Lenegre disease)
Medication side effects (e.g., digoxin, beta-blockers, calcium channel blockers)
Electrolyte imbalances (e.g., hyperkalemia)
Increased vagal tone (Type I can be physiological in well-trained athletes)

Types of Second-Degree AV Block

Type I – Wenckebach Block (Mobitz I)

In the Wenckebach block, the PR interval on the ECG progressively lengthens with each heartbeat until one impulse is completely blocked and a ventricular beat is dropped. The cycle then resets and repeats. This type is often benign and may be seen in athletes or during sleep.

Type II – Mobitz II Block

In the Mobitz II block, a ventricular conduction suddenly fails without any prior prolongation of the PR interval. This type is clinically more significant because it can be unpredictable and carries a higher risk of progressing to complete (third-degree) AV block. It typically requires close monitoring or treatment.

Symptoms

Symptoms depend on the frequency of blocked impulses and the presence of underlying heart disease:

Dizziness and lightheadedness
Heart palpitations or perceived pauses in heartbeat
Shortness of breath (dyspnea), especially during exertion
Fatigue and reduced exercise tolerance
Syncope (brief fainting episodes), particularly with Mobitz II
In severe cases: Adams-Stokes attacks (sudden loss of consciousness due to cardiac arrest)

Diagnosis

Diagnosis is primarily made using the electrocardiogram (ECG). Characteristic ECG findings include:

Type I (Wenckebach): Progressively prolonged PR interval until a P wave fails to conduct and a QRS complex is dropped
Type II (Mobitz II): Constant PR interval with sudden non-conducted P waves and absent QRS complexes

Additional diagnostic tools may include:

Holter monitor (24–48-hour ECG): To capture intermittent conduction disturbances
Echocardiography: To assess cardiac structure and function
Electrophysiology study (EPS): To precisely localize the level of block within the conduction system
Laboratory tests: Electrolytes, thyroid function, medication levels

Treatment

Treatment depends on the type of block, the presence of symptoms, and the underlying cause:

Watchful Waiting

Asymptomatic Wenckebach block (Type I) in young, healthy individuals or endurance athletes often requires no treatment beyond regular monitoring.

Treating the Underlying Cause

If the block is caused by medications such as beta-blockers or digoxin, dose reduction or discontinuation may resolve the conduction disturbance. Electrolyte imbalances are corrected accordingly.

Pacemaker Therapy

For symptomatic Mobitz II block or when there is a high risk of progression to complete AV block, implantation of a permanent pacemaker is the standard treatment. The pacemaker takes over impulse conduction and ensures a regular heart rhythm.

Acute Management

In acute, hemodynamically significant block, atropine (an anticholinergic agent) may be administered intravenously to temporarily increase heart rate. In emergency situations, a temporary pacemaker may be used.

Prognosis

Prognosis is closely tied to the type of AV block and the underlying condition. The Wenckebach block generally carries a favorable outlook, while Mobitz II block, if left untreated, poses a risk of progression to complete AV block and potentially life-threatening situations. With appropriate pacemaker therapy, the quality of life for affected patients is typically significantly improved.

References

Brugada J. et al. – 2019 ESC Guidelines on Supraventricular Tachycardia and Bradycardia Management. European Heart Journal, 2019.
Zipes D. P. et al. – Braunwald's Heart Disease: A Textbook of Cardiovascular Medicine. 11th Edition, Elsevier, 2018.
Kusumoto F. M. et al. – 2018 ACC/AHA/HRS Guideline on the Evaluation and Management of Patients With Bradycardia and Cardiac Conduction Delay. Journal of the American College of Cardiology, 2019.

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