First degree AV block overview

# Overview
First-degree AV block is a disease of the electrical conduction system of the heart in which the PR interval is prolonged. It is defined as PR prolongation of more than 200 milliseconds (normal PR interval is between 120 and 200 msec). First-degree AV block was first described by Dr. Engelmann in 1984. Dr. Ashmar further studied the blocked impulses and their impact on the conduction in the myocardium. The atrioventricular node  is a normal electrical pathway between the atria and ventricles and it is located in the right atrium. First-degree AV block pathogenesis can be attributed to an electrical conduction delay in the AV node or His-Purkinje system. First-degree AV block can be associated with normal QRS complex or wide QRS complex on the ECG. It usually involves the atrioventricular node, but it can involve other structures. In first-degree AV block, all atrial impulses are conducted to the ventricles; however, there is a delay in conduction within the AV node resulting in a prolonged PR interval on ECG (>200 msec or >5 small blocks).  In other words, a first-degree AV block is a slowed conduction without loss of atrioventricular synchrony. Common causes of first-degree AV block include ischemic heart disease, congenital heart disease, electrolyte  abnormalities (particularly hypokalemia and hypomagnesemia), inflammation, infections (endocarditis, rheumatic fever, Chagas disease, Lyme disease, diphtheria), drugs (antiarrhythmic  Ia, Ic, II, III, IV and digoxin, -blockers, calcium channel blockers ), infiltrative diseases (sarcoidosis), collagen vascular diseases (SLE, rheumatoid arthritis, scleroderma), idiopathic degenerative diseases (Lenegre and Lev diseases) and neuromuscular disorders  and increased vagal tone in younger patients. First-degree AV block should be differentiated from third-degree AV block, second degree AV block, supraventricular tachycardia  with long PR. The prevalence of First-degree AV block is approximately 1000-2000 per 100,000 individuals in developed countries. The incidence of First-degree AV block was estimated to be 1000 cases per 100,000 in children and adolescent athletes  and significantly lower than adults due to lower vagal tone in children. First-degree AV block is more commonly observed among elderly patients. Men are more commonly affected with first-degree AV block than women. The male to female ratio is approximately 2 to 1. First-degree AV block was more commonly observed among African-American subjects compared with Caucasian subjects. Common risk factors associated with 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. Ambulatory electrocardiographic monitoring is useful for screening of intermittent  atrioventricular block, LBBB and bifascicular block in asymptomatic patients. In patients with symptomatic atrioventricular block or bradycardia during sleep, screening about sleep apnea is recommended. Screening for congenital heart block is recommended in pregnant women with Ro/SSA antibodies. Women with history of neonatal lupus, fetal echos are recommended weekly or every other week from week 18 to 28. It is unclear how often first degree heart block progresses to complete heart block, some cases my revert to normal sinus rhythm or complete heart block.First-degree atrioventricular block may be due to conduction delay in the atrium, atrioventricular node, and/or His-Purkinje system. The atrioventricular node is the site most commonly involved in adults. However, more than 1 site of conduction  delay is often present. Isolated First-degree atrioventricular has few clinical consequences. There are no symptoms or signs  associated with it. First-degree AV block  rarely progresses to  more severe form of conduction abnormalities. In the setting of neuromuscular diseases such as myotonic dystrophy 1  with conduction abnormalities in the heart, First-degree AV block may progress to complete heart block during variable period of time. Common complications associated with first-degree heart block may include increased risk of atrial fibrillation, increased risk of pacemaker implantation. Prognosis of First degree AV block is generally good. However, some studies showed  worse prognosis with PR prolongation. Presence of First degree AV block is shown to be associated with a higher risk of cardiovascular and all-cause mortality as well as higher risk of heart failure, left ventricular dysfunction, and atrial fibrillation.
Symptoms related to atrioventricular block vary and related to the degree of atrioventricular block, the ventricular rate, and the frequency of its occurrence. 
Patients presented with First-degree AV block are usually asymptomatic. However, severe first-degree AV block may cause  symptoms similar to pace maker syndrome including heart failure symptoms, exertional intolerance. Pseudo pacemaker syndrome is defined when the PR interval is >300ms leading to atrial contraction during the closed atrioventricular valves, loss of atrioventricular synchrony and decrease in cardiac output and an increase pulmonary capillary wedge pressure.First degree AV block may be an incidental finding on an routine ECG. General assessment about signs of underlying cardiac disease including auscultation for murmurs or additional heart sounds, assessment of JVD, peripheral edema, evaluation of skin about cyanosis, clubbing, or other signs of  cardiac disease is warranted. There are no specific laboratory findings associated with First-degree AV block. However, in suspicion of underlying causes of atrioventricular block, laboratory testing about metabolic disorder, infectious disease, rheumatology disorder is reasonable. Echocardiography is useful for finding structural heart disease in patients with First-degree AV  block while symptoms suspected to be cardiac in origin including syncope, presyncope, lightheadedness.In the presence of atrioventricular block and evidence of structural heart disease, cardiac imaging is considered. However, routine cardiac imaging is not recommended in patients with  asymptomatic first-degree atrioventricular block and no clinical evidence of structural heart disease.The presence of severe first-degree atrioventricular block (PR >0.30 s) and a narrow QRS usually indicates atrioventricular node delay. However, ambulatory ECG monitoring is useful for finding the alternative changes in QRS morphology. In addition, exercise stress test can be used to identify the ischemia as the precursor of the development of atrioventricular block.Commonly, there is no need for treating first-degree AV block. Permanent pacemaker indicates only for symptomatic first-degree AV block with PR>300 ms,neuromuscular disease, or in  presence of wide QRS compelex. First-degree AV block in the setting of acute myocardial infarction usually reverses after recovery from acute phase of myocardial infarction. Antiarrhythmic medications should be avoided in first-degreeAV block.Common factors associated with placement of permanent pacemaker include presence or absence of symptoms, level of atrioventricular block, unstable scaped ventricular rhythm with rapid progression to complete heart block. First-degree AV block is typically a benign condition that do not progress suddenly to complete heart block. Placement of permanent pacemaker is reserved for the condition that symptomatic First-degree AV block affects quality of life as well as evidence of atrioventricular block in neurologic conditions with Lamin A/C mutation ( limb girdle, emery dreifuss, muscular dystrophies). Effective measures for primary prevention  of atrioventricular  block include treatment of hypertension and maintenance of normal blood glucose levels. There are no established measures for the secondary prevention of first-degree heart block.

# Historical Perspective
First-degree AV block was first described by Dr. Engelmann in 1984. Dr. Ashmar further studied the blocked impulses and their impact on the conduction in the myocardium.

# Classification
There is no established system for the classification of First degree AV block.

# Pathophysiology
The atrioventricular node  is a normal electrical pathway between the atria and ventricles and it is located in the right atrium. First-degree AV block pathogenesis can be attributed to an electrical conduction delay in the AV node or His-Purkinje system. First-degree AV block can be associated with normal QRS complex or wide QRS complex on the ECG.

# Causes
Common causes of first-degree AV block include ischemic heart disease, congenital heart disease, electrolyte  abnormalities (particularly hypokalemia and hypomagnesemia), inflammation, infections (endocarditis, rheumatic fever, Chagas disease, Lyme disease, diphtheria), drugs (antiarrhythmic  Ia, Ic, II, III, IV and digoxin, -blockers, calcium channel blockers ), infiltrative diseases (sarcoidosis), collagen vascular diseases (SLE, rheumatoid arthritis, scleroderma), idiopathic degenerative diseases (Lenegre and Lev diseases) and neuromuscular disorders  and increased vagal tone in younger patients.

# Differentiating First Degree AV block from Other Diseases
First-degree AV block should be differentiated from , second degree AV block, supraventricular tachycardia  with long PR.

# Epidemiology and Demographics
The prevalence of First-degree AV block is approximately 1000-2000 per 100,000 individuals in developed countries. The incidence of First-degree AV block was estimated to be 1000 cases per 100,000 in children and adolescent athletes  and significantly lower than adults due to lower vagal tone in children. First-degree AV block is more commonly observed among elderly patients. Men are more commonly affected with first-degree AV block than women. The male to female ratio is approximately 2 to 1. First-degree AV block was more commonly observed among African-American subjects compared with Caucasian subjects.

# Risk Factors
Common risk factors associated with 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
Ambulatory electrocardiographic monitoring is useful for screening of intermittent  atrioventricular block, LBBB and bifascicular block in asymptomatic patients. In patients with symptomatic atrioventricular block or bradycardia during sleep, screening about sleep apnea is recommended. Screening for congenital complete heart block is recommended in pregnant women with Ro/SSA antibodies. Women with history of neonatal lupus, fetal echos are recommended weekly or every other week from week 18 to 28. It is unclear how often first degree heart block progresses to complete heart block, some cases my revert to normal sinus rhythm or complete heart block.

# Natural History, Complications, and Prognosis
First-degree atrioventricular block may be due to conduction delay in the atrium, atrioventricular node, and/or His-Purkinje system. The atrioventricular node is the site most commonly involved in adults. However, more than 1 site of conduction  delay is often present. Isolated First-degree atrioventricular has few clinical consequences. There are no symptoms or signs  associated with it. First-degree AV block  rarely progresses to  more severe form of conduction abnormalities. In the setting of neuromuscular diseases such as myotonic dystrophy 1  with conduction abnormalities in the heart, First-degree AV block may progress to complete heart block during variable period of time. Common complications associated with first-degree heart block may include increased risk of atrial fibrillation, increased risk of pacemaker implantation. Prognosis of First degree AV block is generally good. However, some studies showed  worse prognosis with PR prolongation. Presence of First degree AV block is shown to be associated with a higher risk of cardiovascular and all-cause mortality as well as higher risk of heart failure, left ventricular dysfunction, and atrial fibrillation.

# Diagnosis

## Diagnostic Study of Choice

## History and Symptoms
Symptoms related to atrioventricular block vary and related to the degree of atrioventricular block, the ventricular rate, and the frequency of its occurrence. 
Patients presented with First-degree AV block are usually asymptomatic. However, severe first-degree AV block may cause  symptoms similar to pace maker syndrome including heart failure symptoms, exertional intolerance. Pseudo pacemaker syndrome is defined when the PR interval is >300ms leading to atrial contraction during the closed atrioventricular valves, loss of atrioventricular synchrony and decrease in cardiac output and an increase pulmonary capillary wedge pressure.

## Physical Examination
First degree AV block may be an incidental finding on an routine ECG. General assessment about signs of underlying cardiac disease including auscultation for murmurs or additional heart sounds, assessment of JVD, peripheral edema, evaluation of skin about cyanosis, clubbing, or other signs of  cardiac disease is warranted.

## Laboratory Findings
There are no specific laboratory findings associated with First-degree AV block. However, in suspicion of underlying causes of atrioventricular block, laboratory testing about metabolic disorder, infectious disease, rheumatology disorder is reasonable.

## Electrocardiogram
In normal individuals, the AV node slows the conduction of electrical impulse through the heart. This is manifest on a surface EKG as the PR interval. The normal PR interval is from 120 milliseconds (ms) to 200 milliseconds (ms) in duration.  This is measured from the initial deflection of the P wave to the beginning of the QRS complex.
In first degree heart block, the diseased AV node conducts the electrical activity slower.  This is seen as a PR interval greater than 200 milliseconds (ms) in length on the surface EKG. It is usually an incidental finding on a routine EKG.
First degree heart block does not require any particular evaluation except for electrolyte and drug screens especially if an overdose is suspected.

## X-ray
There are no x-ray findings associated with first degree AV block.

## Echocardiography
Echocardiography is useful for finding structural heart disease in patients with First-degree AV  block while symptoms suspected to be cardiac in origin including syncope, presyncope, lightheadedness.

## Other Imaging Findings
In the presence of atrioventricular block and evidence of structural heart disease, cardiac imaging is considered. However, routine cardiac imaging is not recommended in patients with  asymptomatic first-degree atrioventricular block and no clinical evidence of structural heart disease.

## Other Diagnostic Studies
The presence of severe first-degree atrioventricular block (PR >0.30 s) and a narrow QRS usually indicates atrioventricular node delay. However, ambulatory ECG monitoring is useful for finding the alternative changes in QRS morphology. In addition, exercise stress test can be used to identify the ischemia as the precursor of the development of atrioventricular block.

# Treatment

## Medical Therapy
Commonly, there is no need for treating first-degree AV block. Permanent pacemaker indicates only for symptomatic first-degree AV block with PR>300 ms,neuromuscular disease, or in  presence of wide QRS compelex. First-degree AV block in the setting of acute myocardial infarction usually reverses after recovery from acute phase of myocardial infarction. Antiarrhythmic medications should be avoided in first-degreeAV block.

## Surgery
Common factors associated with placement of permanent pacemaker include presence of symptoms, level of atrioventricular block, unstable scaped ventricular rhythm with rapid progression to complete heart block. First-degree AV block is typically a benign condition that do not progress suddenly to complete heart block. Placement of permanent pacemaker is reserved for the condition that symptomatic First-degree AV block affects quality of life as well as evidence of atrioventricular block in neurologic conditions with Lamin A/C mutation ( limb girdle, emery dreifuss, muscular dystrophies).

## Primary Prevention
Effective measures for primary prevention  of atrioventricular  block include treatment of hypertension and maintenance of normal blood glucose levels

## Secondary Prevention
There are no established measures for the secondary prevention of first-degree heart block.