Second-degree atrioventricular block

Second-degree atrioventricular block (AV block), also known as Mobitz Type I (Wenckebach block) and Mobitz Type II, is a disorder of the heart's electrical conduction system. This condition is characterized by a disruption in the transmission of electrical impulses between the atria and ventricles. It is diagnosed when one or more, but not all, atrial impulses fail to reach the ventricles. This type of AV block falls between the first-degree (delayed conduction) and third-degree blocks (complete block).

AV block ECG

Classification

Second-degree AV block is divided into two types based on the conduction characteristics:

• Mobitz Type I (Wenckebach block): This type is characterized by progressive prolongation of the PR interval on the electrocardiogram (ECG) until an atrial impulse is completely blocked (fails to conduct to the ventricles). This is often due to a dysfunction in the AV node itself.
• Mobitz Type II: In this type, some of the atrial impulses fail to conduct to the ventricles without preceding prolongation of the PR interval. This is typically due to a disease in the distal conduction system (His-Purkinje system).

Etiology

Second-degree AV block can result from a variety of conditions that affect the conduction system of the heart, such as ischemic heart disease, myocarditis, Lyme disease, certain medications (e.g., beta-blockers, calcium channel blockers, and digoxin), and cardiac surgery.

Clinical Presentation

The symptoms of second-degree AV block depend on the type and the rate of ventricular response. They can range from being asymptomatic to experiencing dizziness, fatigue, lightheadedness, palpitations, syncope (fainting), or even heart failure in severe cases.

Diagnosis

The diagnosis of second-degree AV block is made by analyzing the ECG, where the characteristic findings of blocked atrial impulses can be observed. It's important to distinguish between the two types of second-degree AV block, as Mobitz Type II can be more unstable and progress to third-degree AV block, requiring pacemaker implantation.

Treatment

Management of second-degree AV block depends on the type, the cause, and the symptoms. Asymptomatic patients with Mobitz Type I often do not require treatment. In symptomatic cases or in cases of Mobitz Type II, treatment may involve discontinuation of offending drugs, treatment of underlying conditions, or in more severe cases, the placement of a pacemaker to ensure adequate heart rate.

Prognosis

Prognosis varies depending on the underlying condition and the type of second-degree AV block. While Mobitz Type I has a relatively benign course, Mobitz Type II can potentially progress to complete heart block, necessitating immediate intervention.

References

• [1] Epstein AE, DiMarco JP, Ellenbogen KA, et al. (2012). 2012 ACCF/AHA/HRS focused update incorporated into the ACCF/AHA/HRS 2008 guidelines for device-based therapy of cardiac rhythm abnormalities. Journal of the American College of Cardiology, 61(3), e6-75.
• [2] Tracy CM, Epstein AE, Darbar D, et al. (2012). 2012 ACCF/AHA/HRS focused update of the 2008 guidelines for device-based therapy of cardiac rhythm abnormalities. Circulation, 126(14), 1784-1800.

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