First 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], Ahmed Elsaiey, MBBCH [3]
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. An atrioventricular block (or AV block) is a type of heart block involving impairment of the conduction between the atria and the ventricles of the heart. It usually involves the atrioventricular node, but it can involve other structures too. AV block is categorized according to the degree and the site of conduction block. 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, β-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 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.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 casese of atrioventricular block, laboratory testing about metabolic disorder, infectious disease, rheumatology disorder is reasonable.
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, β-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 [[third-degree AV block], 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
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 and Ultrasound
Ultrasound can be used in cases of first degree AV block in order to follow the improvement in the cardiac output when the dual chamber pacing used.
CT scan
MRI
Other Imaging Findings
There are no other imaging findings associated with first degree AV block.
Other Diagnostic Studies
There are no other diagnostic studies for first degree AV block.
Treatment
Medical Therapy
Surgery
Primary Prevention
Secondary Prevention
There are no established measures for the secondary prevention of first degree heart block.