second degree av block

3 min read 11-03-2025

Meta Description: Dive deep into second-degree AV block, exploring its two types (Mobitz I and Mobitz II), underlying causes, diagnostic methods, and treatment approaches. Learn how this heart condition affects the heart's rhythm and what to expect. This comprehensive guide provides valuable insights for patients, families, and healthcare professionals.

What is a Second-Degree AV Block?

A second-degree atrioventricular (AV) block is a type of heart block that disrupts the normal electrical conduction pathway between the atria and ventricles of the heart. This disruption causes some atrial impulses to fail to conduct to the ventricles, leading to an irregular heartbeat. Unlike a first-degree AV block where all atrial impulses are delayed, in a second-degree AV block, some atrial impulses are completely blocked. This results in fewer ventricular contractions than atrial contractions. Understanding the different types is crucial for proper diagnosis and management.

Types of Second-Degree AV Block

There are two main types of second-degree AV block: Mobitz type I (Wenckebach) and Mobitz type II. The distinction lies in the pattern of blocked beats on an electrocardiogram (ECG).

Mobitz Type I (Wenckebach) AV Block

ECG Characteristics: In Mobitz I, the PR interval (the time it takes for the electrical impulse to travel from the atria to the ventricles) progressively lengthens with each beat until a beat is completely blocked. Then, the cycle starts again. Think of it like a lengthening delay until a beat is dropped.
Causes: This type often results from transient disturbances in AV nodal conduction, potentially due to:
- Increased vagal tone (parasympathetic nervous system activity).
- Electrolyte imbalances (e.g., hypokalemia).
- Medications (e.g., beta-blockers, calcium channel blockers).
Severity: Generally considered less severe than Mobitz II, often resolving spontaneously or with simple treatment adjustments.

Mobitz Type II AV Block

ECG Characteristics: Mobitz II shows a constant PR interval, but some P waves are not followed by a QRS complex (ventricular contraction). The blocked beats occur unpredictably. The pattern is more concerning because it suggests a problem in the His-Purkinje system – the lower part of the heart's conduction system.
Causes: This type is often associated with more serious underlying conditions, including:
- Structural heart disease: Such as myocardial infarction (heart attack), cardiomyopathy, or congenital heart defects.
- Increased risk of heart block progression: Can lead to complete heart block.
- Infarction of the AV node or Bundle of His: Damage to the heart's conduction system.
Severity: Mobitz II is more serious and often requires closer monitoring and potentially pacing therapy.

What Causes Second-Degree AV Block?

The underlying causes of second-degree AV block are diverse and depend on the type. As noted above, Mobitz I is often associated with temporary factors, while Mobitz II points to more serious underlying heart conditions. Other potential causes include:

Infections: Myocarditis (heart muscle inflammation) and other infections can affect the heart's electrical system.
Degenerative changes: Age-related changes in the heart's conduction system can lead to AV blocks.
Surgical trauma: Heart surgery can sometimes damage the conduction system.
Certain medications: Some medications can interfere with the heart's electrical activity.

Diagnosing Second-Degree AV Block

Diagnosis typically involves:

Electrocardiogram (ECG): The ECG is the primary diagnostic tool, clearly showing the characteristic PR interval changes and blocked beats.
Holter monitoring: A 24-hour or longer ECG recording to identify intermittent or infrequent blocks.
Exercise stress test: To assess the heart's response to increased workload and identify AV block during exertion.

Treatment for Second-Degree AV Block

Treatment for second-degree AV block depends on the type, the severity of symptoms, and the presence of underlying conditions.

Mobitz Type I: Often requires no specific treatment, especially if asymptomatic. Underlying causes like electrolyte imbalances should be addressed.
Mobitz Type II: Often requires a pacemaker, especially if there are symptoms like dizziness or syncope (fainting). A pacemaker helps maintain a regular heart rhythm and prevent potentially life-threatening bradycardia (slow heart rate). Medications might also be used to manage underlying conditions.

Living with Second-Degree AV Block

The prognosis for second-degree AV block varies greatly depending on the type and underlying causes. Regular medical follow-up is essential, including ECG monitoring and potential adjustments to medication or pacing therapy. Lifestyle modifications, such as managing stress and maintaining a healthy diet and exercise routine, can be beneficial.

Disclaimer: This information is for educational purposes only and should not be considered medical advice. Always consult with your physician or a qualified healthcare professional for diagnosis and treatment of any medical condition.