Second degree AV block overview

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• Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Mohammed Salih, M.D. Cafer Zorkun, M.D., Ph.D. [2] Syed Musadiq Ali M.B.B.S.[3]

Overview

• Second degree AV block is a disease of the electrical conduction system of the heart.

• It refers to a conduction block between the atria and ventricles.
• The presence of second degree AV block is diagnosed when one or more (but not all) of the atrial impulses fail to conduct to the ventricles due to impaired conduction.

Historical perspective

• Second degree AV block was first described as a progressive delay between the atrial and ventricular contraction by Dr. Wenckebach in 1899.
• Dr. Mobitz then divided the second degree AV block into two sub types.
• In 1905, Dr. Hay figured out the pause following a wave was due to failure of ventricular muscles to respond to a stimulus.

Classification

Second-degree AV block can be of one of the two types:

Mobitz I and Mobitz II.

1. Mobitz I, or Wenckebach block : consists of progressive prolongation of PR interval, until loss of conduction to the ventricle occurs (missed beat). Mobitz I block is rarely symptomatic and does not require treatment.

2. Mobitz II AV block : is characterized by a constant PR interval with intermittent missed beats. The missed beats can occur with varying frequency such as occasional to 3:1 or 2:1.

Pathophysiology

1.Mobitz type I (Wenckebach) : there is a progressive prolongation of the PR interval (AV conduction) until eventually an atrial impulse is completely blocked.

• When an atrial impulse is completely blocked there will be a P wave without a QRS complex.
• This pattern is often referred to as a “dropped beat.”
• Mobitz type I occurs because each depolarization results in the prolongation of the refractory period of the atrioventricular (AV) node. When an atrial impulse comes through the AV node during the relative refractory period, the impulse will be conducted more slowly, resulting in a prolongation of the PR interval.
• Eventually, an impulse comes when the AV node is in its absolute refractory period and will not be conducted.
• This will manifest on the ECG as a P wave that is not followed by a QRS complex.
• This non-conducted impulse allows time for the AV node to reset, and the cycle continues.
• This phenomenon leads to a grouped beating.

2. Mobitz type II : there is a constant PR interval across the rhythm strip both before and after the non-conducted atrial beat.

• Each P wave is associated with a QRS complex until there is one atrial conduction or P wave that is not followed by a QRS.
• Mobitz type II is often a problem in the infra-nodal conduction system, and therefore, is associated with a widened QRS complex, bundle-branch block, or fascicular block.

3. If more than one P wave is not conducted this is no longer a Mobitz type II and is considered a high degree AV block.

Causes

Life threatening causes of second degree AV block :

• acute myocardial infarction
• acute rheumatic fever
• myocarditis
• severe hypothermia

Common causes :

• endocarditis
• digoxin
• dilated cardiomyopathy
• diltiazem
• calcific aortic stenosis

Differentiating second degree AV block from Other Diseases

Differential Diagnosis: abnormal and irregular cardiac rhythm as atrial fibrillation, atrial flutter, atriventricular nodal reentrant tachycardia, multifocal atrial tachycardia, and paroxysmal supraventricular tachycardia.

Epidemiology and Demographics

There have not been large population-based studies on the prevalence of Mobitz type I or II atrioventricular blocks.

• At this time, there is no associated age, racial, or gender correlation.
• AV block is sometimes seen in athletes and in patients with congenital heart disorders.
• In the United States, the prevalence of second-degree AV block is believed to be 3 in 100,000 individual.
• Men and women are affected equally by second degree AV block.

Risk Factors

Common risk factors associated with second degree AV block :

• Intrinsic atrioventricular node disease
• Myocarditis ,
• Acute myocardial infarction
• Prior cardiac surgery
• Older age
• Heart attack or coronary artery disease
• Cardiomyopathy
• Sarcoidosis
• Lyme disease
• High potassium levels
• Severe hypothyroidism
• Certain inherited neuromuscular diseases
• Medicines that slow the heart rate
• Open heart surgery.

Natural History, Complications, and Prognosis

Patients with Mobitz type II second degree AV block - Hemodynamically stable : do not require urgent therapy with atropine or temporary cardiac pacing.

• However, Mobitz type II second degree AV block is by nature unstable and frequently progresses to third degree (complete) AV block
• Patients should be continuously monitored with transcutaneous pacing pads in place in the event of clinical deterioration.
• While stable patients are being monitored, reversible causes of Mobitz type II second degree AV block such as myocardial ischemia, increased vagal tone, hypothyroidism, hyperkalemia, and drugs that depress conduction, should be excluded in patients prior to implantation of a permanent pacemaker.
• For patients with Mobitz type II second degree AV block who do not have a reversible etiology, implantation of a permanent pacemaker (Grade 1A) is recommended.
• A dual chamber DDD pacemaker( has pacing and sensing capabilities in both the atrium and ventricle) is implanted whenever possible, so as to maintain physiologic AV synchrony.

Diagnosis

History and Symptoms

• Patients with second degree AV block are usually asymptomatic.
• Some patients may present with symptoms of reduced cardiac output.
• Symptoms include : dizziness, fatigue, pre syncope or syncope, and light headedness.

Physical Examination

• Patients with second degree AV block are usually asymptomatic.

• However, patients with previous chronic cardiac condition may appear in a distress.
• In symptomatic patients, common physical examination findings include bradycardia, hypotension, and syncope.
• Common findings in patients associated with heart failure include lung crackles, jugular venous distension, and peripheral edema.

Laboratory Findings

Patients with second degree AV block laboratory tests include:

• checking the levels of serum electrolytes as calcium, magnesium and potassium.
• Myocarditis related lab tests as Lyme titres, HIV tests, PCR for Entero viruses, and Chagas titres should also be done.

Electrocardiogram

On ECG, Type I Second degree AV block is characterized by:

• a progressive prolongation of the PR interval and progressive shortening of RR interval until a P wave is blocked.

• The RR interval containing the blocked P wave is shorter than the sum of 2 PP intervals.
• The increase in the PR interval is longest in the second conducted beat after the pause.

Type II second degree AV block is characterized by:

• a constant PR interval.
• Most patients with type II second-degree AV block have associated bundle branch block.

X-ray

There are no x-ray findings associated with second degree AV block.

Other Imaging Findings

There are no other imaging findings associated with second degree AV block.

Other Diagnostic Studies

There are no other diagnostic studies associated with second degree AV block.

Treatment

Medical Therapy

1.Treatment for a Mobitz type I (Wenckebach) is often not necessary.

• Occasionally type I blocks may result in bradycardia leading to hypotension, If hypotension and bradycardia occur, type I blocks respond well to Atropine.
• If unresponsive to atropine, pacing (transcutaneous or transvenous) should be initiated for stabilization.
• If the patient is on any beta blockers, calcium channel blockers or digoxin, the dose of these medications should be reduced or the medication discontinued.
• All patients with Mobitz 1 block should be admitted and monitored.

2.Treatment for a Mobitz type II involves initiating pacing as soon as this rhythm is identified.

• Type II blocks imply structural damage to the AV conduction system,this rhythm often deteriorates into complete heart block.
• These patients require transvenous pacing until a permanent pacemaker is placed.
• Unlike Mobitz type I (Wenckebach), patients that are bradycardic and hypotensive with a Mobitz type II rhythm often do not respond to atropine.

Surgery

• If no reversible causes are present, definitive treatment of Mobitz type II second degree AV block involves permanent pacemaker placement in most patients
• Dual-chamber pacing to maintain AV synchrony is preferred (rather than single chamber right ventricular pacing) in most patients due to the favorable hemodynamic benefits of AV synchrony.
• Unlike asymptomatic patients with Mobitz type I second degree AV block who do not require any specific therapy, patients with Mobitz type II second degree AV block have a high likelihood of progressing to symptomatic Mobitz type II second degree AV block or Complete heart block and should be considered for pacemaker insertion on initial presentation.

Primary Prevention

• Pacemaker implantation as a treatment for atrioventricular (AV) block are increasing worldwide.
• Prevention strategies for AV block are lacking because modifiable risk factors have not yet been identified.
• Atrioventricular (AV) block is a common reason for pacemaker implantation, and the number of pacemaker implantations is increasing.
• Atrioventricular block most commonly occurs in the absence of significant cardiac disease and is generally attributed to idiopathic fibrosis of the conduction system. By definition, the cause of that fibrosis remains unknown.

References

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