Second degree AV block overview

# 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. 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 subtypes. In 1905, Dr. Hay figured out the pause following a wave was due to the failure of ventricular muscles to respond to a stimulus. There are 4 distinct types of second degree AV block.  The distinction is made between them because type 1 second-degree heart block is considered a more benign entity than the other types. In mobitz type 1 second degree AV block there is evidence of gradually PR prolongation and dropped beat and grouped beating pattern. In mobitz type 2 AV block there is suddenly dopped beats without evidence of preceding PR prolongation. In atrioventricular block with the pattern of 2:1, there is every other beat without conducting down to the ventricle. In a high-grade AV block, there are two or more consecutive P waves without conducting down to the ventricle. It is important to determine the anatomic site of AV block. In Mobitz type 1 AV block, the site is usually within the AV node, but in Mobitz type II AV block the site is almost always below the AV node. In the presence of wide QRS complex and 2:1 AV conduction it is more likely that the site of  AV block is intranodal or infranodal. In some cases, second-degree atrioventricular block must be differentiated from other causes of pauses such as  non-conducted premature atrial contractions or atrial tachycardia with block.In 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. In 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 a fascicular block. When more than one P wave is not conducted this is no longer a Mobitz type II and is considered a high degree AV block.Common causes of second degree AV block include acute myocardial infarction, acute rheumatic fever, myocarditis, and severe hypothermia, endocarditis, digoxin toxicity, dilated cardiomyopathy, betablockers, calcium channel blockers and calcific aortic stenosis.Second degree AV block  must be differentiated from different abnormal and irregular cardiac rhythms as atrial fibrillation with slow ventricular  response, atrial flutter, atrial tachycardia with block.There have not been large population-based studies on the prevalence of Mobitz type I or II atrioventricular blocks. In the United States, the prevalence of second-degree AV block is believed to be 3 in 100,000 individuals. Men and women are affected equally by second-degree AV block. There is no racial predilection for second- degree AV block.Common risk factors associated with progression of atioventricular block include older age, male sex, history of myocardial infarction, history of congestive heart disease, high systolic blood pressure, Increased fasting blood glucose level. There is no established screening method for atrioventricular block.Second-degree AV nodal block  commonly is seen in acute clinical settings including acute inferior wall myocardial infarction, digitalis intoxication, myocarditis, rheumatic fever), after cardiac surgery. Chronic AV nodal block is seen in the setting of ischemic heart disease, mesothelioma of the AV node, atrial septal defect, aortic valvular disease, amyloidosis, Reiter's syndrome, mitral valve prolapse, in healthy populations, and in trained athletes. Mobitz II second degree Av block  due to block inferior to the AV node (infra-Hisian structures) may progresses to complete heart block.Common complications associated with mobitz type 2 second degree AV block include progression to  Complete heart block, syncope, dizziness, chest pain, and death.Prognosis is generally good in patients with chronic second-degree AV nodal block without organic heart disease.However, in patients with heart  disease prognosis is poor and dependent on the severity of underlying heart disease. Electrocardiography (ECG) is employed to determine the type of second-degree atrioventricular  (AV) block present. Follow-up ECGs and cardiac monitoring are appropriate.Common symptoms in patients with second degree atrioventricular block include light-headedness, dizziness, fainting, fatigue, heart failure symptoms
, pre-syncope, and syncope. Mobitz type 1 second degree AV block (Wenckebach) is often asymptomatic and can be seen in active, healthy  patients without known heart disease. It may occur during exercise  causing exertional intolerance or dizziness, or syncope. In patients with intermittent atrioventricular block leading syncope, initial evaluation including resting ECG, physical exam, echocardiography may be normal and intermittent episodes of the atrioventricular block can be found with long-term monitoring. Symptoms in patients with an atrioventricular block that conducts in a 2:1 pattern include  fatigue and dizziness particularly if it persists during exertion.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. Physical examination in patients with heart failure may include lung crackles, jugular venous distension, and peripheral edema.Laboratory tests in patients with second degree AV block include checking the levels of serum electrolytes as calcium, magnesium and potassium. Myocarditis related lab tests as lyme titres, HIV tests, PCR for enteroviruses, and Chagas titres should be done also.There are no x-ray findings associated with second degree AV block.Echocardiography is useful for finding the underlying structural heart disease including left ventricular systolic dysfunction in patients with atrioventricular block, especially in the presence of LBBB pattern on resting ECG. Transesophageal echocardiography, computed tomography, cardiac magnetic resonance imaging (MRI), or nuclear imaging are other advanced imagings that can be used in suspicion of structural heart disease in patients presented with bradycardia or bundle branch block.Electrocardiographic monitoring can be used to identify the changes in QRS morphology such as alternating bundle branch block in the presence of atrioventricular conduction abnormalities. Treadmill exercise stress testing may be diagnostic to differentiate that 2:1 atrioventricular block is Mobitz type I or II in some cases or identify the presence of infranodal disease. EPS may be helpful to determine the  anatomic site of block in mobitz type 2 atrioventricular block including atrioventricular node, intra-His, or infra-His. Worsening atrioventricular  block  with  isoproterenol and atropine may be suggestive of infranodal  block. However, improvement of atrioventricular conduction  with carotid  sinus  massage  may  be  observed in patients with infranodal atrioventricular block.Treatment for a Mobitz type I second-degree AV block (Wenckebach) is often not necessary. Occasionally Mobitz type 1 second degree AV blocks may result in bradycardia leading to hypotension and responds well to medications. If unresponsive to atropine or beta-adrenergic agonists, pacing (transcutaneous or transvenous) should be initiated for stabilization. If the patient is on any beta-blockers, calcium channel blockers or digoxin, the medications should be discontinued. All patients with Mobitz 1 block should be admitted and monitored. Treatment for a Mobitz type II involves initiating pacing as soon as this rhythm is identified. Mobitz type II second-degree AV blocks may imply structural damage to the AV conduction system. This rhythm often deteriorates into a complete heart block. These patients require transvenous pacing until a permanent pacemaker is placed. Unlike Mobitz type I second degree AV block (Wenckebach), Mobitz type II AV block often do not respond to atropine or beta-adrenergic agonists.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 candidates for pacemaker insertion on initial presentation. So, 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. If no reversible causes are present, definitive treatment of Mobitz type II second degree AV block involves permanent pacemaker placement in most patients. There is no benefit of implantation of permanent pacacemaker in patients with long-standing asymptomatic persistent or permanent atrial fibrillation  with a low heart rate and appropriate chronotropic response.Effective measures for primary prevention  of atrioventricular  block include treatment of hypertension and maintenance of normal blood glucose levels. 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 without primary prevention strategy.Secondary prevention of atrioventricular block may include correction of electrolytes disturbance, ischemia, and treating decompensated heart failure.

# 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 subtypes. In 1905, Dr. Hay figured out the pause following a wave was due to the failure of ventricular muscles to respond to a stimulus.

# Classification
There are 4 distinct types of second degree AV block.  The distinction is made between them because type 1 second-degree heart block is considered a more benign entity than the other types. In mobitz type 1 second degree AV block there is evidence of gradually PR prolongation and dropped beat and grouped beating pattern. In mobitz type 2 AV block there is suddenly dopped beats without evidence of preceding PR prolongation. In atrioventricular block with the pattern of 2:1, there is every other beat without conducting down to the ventricle. In a high-grade AV block, there are two or more consecutive P waves without conducting down to the ventricle. It is important to determine the anatomic site of AV block. In Mobitz type 1 AV block, the site is usually within the AV node, but in Mobitz type II AV block the site is almost always below the AV node. In the presence of wide QRS complex and 2:1 AV conduction it is more likely that the site of  AV block is intranodal or infranodal. In some cases, second-degree atrioventricular block must be differentiated from other causes of pauses such as  non-conducted premature atrial contractions or atrial tachycardia with block.

# Pathophysiology
In 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. In 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 a fascicular block. When 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
Common causes of second degree AV block include acute myocardial ischemia or infarction, infiltrative diseases, collagen vascular disease, surgical trauma, endocrine abnormalities, autonomic effects, neuromuscular  disorders, and medications.

# Differentiating second degree AV block from Other Diseases
Second degree AV block  must be differentiated from different abnormal and irregular cardiac rhythms as atrial fibrillation with slow ventricular  response, atrial flutter, atrial tachycardia with block.

# Epidemiology and Demographics
There have not been large population-based studies on the prevalence of Mobitz type I or II atrioventricular blocks. In the United States, the prevalence of second-degree AV block is believed to be 3 in 100,000 individuals. Men and women are affected equally by second-degree AV block. There is no racial predilection for second- degree AV block.

# Risk Factors
Common risk factors associated with progression of atioventricular block include older age, male sex, history of myocardial infarction, history of congestive heart disease, high systolic blood pressure, Increased fasting blood glucose level.

# Screening
There is no established screening method for atrioventricular block.

# Natural History, Complications, and Prognosis
Second-degree AV nodal block  commonly is seen in acute clinical settings including acute inferior wall myocardial infarction, digitalis intoxication, myocarditis, rheumatic fever, after cardiac surgery. Chronic AV nodal block is seen in the setting of ischemic heart disease, mesothelioma of the AV node, atrial septal defect, aortic valvular disease, amyloidosis, Reiter's syndrome, mitral valve prolapse, in healthy populations, and in trained athletes. Mobitz II second degree Av block  due to block inferior to the AV node (infra-Hisian structures) may progresses to complete heart block. Common complications associated with mobitz type 2 second degree AV block include progression to  Complete heart block, syncope, dizziness, chest pain, and death.
Prognosis is generally good in patients with chronic second-degree AV nodal block without organic heart disease.However, in patients with heart  disease prognosis is poor and dependent on the severity of underlying heart disease.

# Diagnosis

## Diagnostic Study of Choice
Electrocardiography (ECG) is employed to determine the type of second-degree atrioventricular  (AV) block present. Follow-up ECGs and cardiac monitoring are appropriate.

## History and Symptoms
Common symptoms in patients with second degree atrioventricular block include light-headedness, dizziness, fainting, fatigue, heart failure symptoms
, pre-syncope, and syncope. Mobitz type 1 second degree AV block (Wenckebach) is often asymptomatic and can be seen in active, healthy  patients without known heart disease. It may occur during exercise  causing exertional intolerance or dizziness, or syncope. In patients with intermittent atrioventricular block leading syncope, initial evaluation including resting ECG, physical exam, echocardiography may be normal and intermittent episodes of the atrioventricular block can be found with long-term monitoring. Symptoms in patients with an atrioventricular block that conducts in a 2:1 pattern include  fatigue and dizziness particularly if it persists during exertion.

## 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. Physical examination in patients with heart failure may include lung crackles, jugular venous distension, and peripheral edema.

## Laboratory Findings
Laboratory tests in patients with second degree AV block include checking the levels of serum electrolytes as calcium, magnesium and potassium. Myocarditis related lab tests as lyme titres, HIV tests, PCR for enteroviruses, and Chagas titres should be done also.

## Electrocardiogram
On ECG, Type I Second degree AV block is characterized by 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.

## Echocardiography
Echocardiography is useful for finding the underlying structural heart disease including left ventricular systolic dysfunction in patients with atrioventricular block, especially in the presence of LBBB pattern on resting ECG.

## Other Imaging Findings
Transesophageal echocardiography, computed tomography, cardiac magnetic resonance imaging (MRI), or nuclear imaging are other advanced imagings that can be used in suspicion of structural heart disease in patients presented with bradycardia or bundle branch block.

## Other Diagnostic Studies
Electrocardiographic monitoring can be used to identify the changes in QRS morphology such as alternating bundle branch block in the presence of atrioventricular conduction abnormalities. Treadmill exercise stress testing may be diagnostic to differentiate that 2:1 atrioventricular block is Mobitz type I or II in some cases or identify the presence of infranodal disease. EPS may be helpful to determine the  anatomic site of block in mobitz type 2 atrioventricular block including atrioventricular node, intra-His, or infra-His. Worsening atrioventricular  block  with  isoproterenol and atropine may be suggestive of infranodal  block. However, improvement of atrioventricular conduction  with carotid  sinus  massage  may  be  observed in patients with infranodal atrioventricular block.

# Treatment

## Medical Therapy
Treatment for a Mobitz type I second-degree AV block (Wenckebach) is often not necessary. Occasionally Mobitz type 1 second degree AV blocks may result in bradycardia leading to hypotension and responds well to medications. If unresponsive to atropine or beta-adrenergic agonists, pacing (transcutaneous or transvenous) should be initiated for stabilization. If the patient is on any beta-blockers, calcium channel blockers or digoxin, the medications should be discontinued. All patients with Mobitz 1 block should be admitted and monitored. Treatment for a Mobitz type II involves initiating pacing as soon as this rhythm is identified. Mobitz type II second-degree AV blocks may imply structural damage to the AV conduction system. This rhythm often deteriorates into a complete heart block. These patients require transvenous pacing until a permanent pacemaker is placed. Unlike Mobitz type I second degree AV block (Wenckebach), Mobitz type II AV block often do not respond to atropine or beta-adrenergic agonists.

## Surgery
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 candidates for pacemaker insertion on initial presentation. So, 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. If no reversible causes are present, definitive treatment of Mobitz type II second degree AV block involves permanent pacemaker placement in most patients. There is no benefit of implantation of permanent pacacemaker in patients with long-standing asymptomatic persistent or permanent atrial fibrillation  with a low heart rate and appropriate chronotropic response.

## Primary Prevention
Effective measures for primary prevention  of atrioventricular  block include treatment of hypertension and maintenance of normal blood glucose levels. 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 without primary prevention strategy.

## Secondary Prevention
Secondary prevention of atrioventricular block may include correction of electrolytes disturbance, ischemia, and treating decompensated heart failure.