Second degree AV block

Second Degree AV Block is a type of heart block that affects the electrical conduction system of the heart. It is characterized by an intermittent failure of the electrical impulses generated by the sinoatrial node to be conducted through the atrioventricular node (AV node) to the ventricles. This condition is classified into two types, Type I (Mobitz I or Wenckebach) and Type II (Mobitz II), based on the pattern of conduction interruption observed on an electrocardiogram (ECG).

Types

Type I (Mobitz I/Wenckebach)

Type I Second Degree AV Block is identified by a progressive lengthening of the PR interval on consecutive beats followed by a dropped beat (a beat that does not result in ventricular contraction). This type of block is usually located in the AV node and is often considered less serious than Type II. It may occur in healthy individuals, particularly during sleep, and can be seen in conditions such as increased vagal tone, myocarditis, or after medication use that affects the AV node.

Type II (Mobitz II)

Type II Second Degree AV Block is characterized by the sudden failure of some atrial impulses to conduct to the ventricles without a preceding change in the PR interval, as seen in Type I. This type of block is more likely to be associated with structural heart disease and carries a higher risk of progression to complete heart block. It often requires immediate attention and the possible implantation of a pacemaker.

Causes

Second Degree AV Block can be caused by a variety of factors, including:

• Ischemic heart disease
• Cardiomyopathy
• Increased vagal tone
• Medications such as beta-blockers, calcium channel blockers, and digoxin
• Lyme disease
• Myocarditis

Symptoms

Symptoms of Second Degree AV Block may include:

• Fatigue
• Dizziness
• Fainting (syncope)
• Shortness of breath
• Palpitations

Diagnosis

Diagnosis of Second Degree AV Block involves a thorough medical history, physical examination, and the use of diagnostic tools such as:

• Electrocardiogram (ECG) - to identify the pattern of heartbeats and blocks.
• Holter monitor - a continuous ECG recording over 24 to 48 hours.
• Exercise stress test - to observe the heart's activity under physical stress.

Treatment

Treatment of Second Degree AV Block depends on the type, symptoms, and underlying cause. Options may include:

• Observation and monitoring for asymptomatic cases or those caused by reversible factors.
• Medication adjustments for blocks induced by drugs.
• Pacemaker implantation, especially for symptomatic Type II Second Degree AV Block or in cases where there is a high risk of progression to complete heart block.

Prognosis

The prognosis for individuals with Second Degree AV Block varies depending on the type, underlying cause, and presence of other cardiac conditions. Type I often has a benign course, while Type II may require more aggressive management due to its association with more significant heart disease.

See Also

• Heart block
• Atrioventricular node
• Electrocardiogram
• Pacemaker

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