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
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 estimated to be 650-1600 per 100,000 individuals in the united states. First degree AV block is associated with advanced age and is more prevalent in men older than 60 years.
Risk Factors
Common risk factors of congenital heart block includes pregnant woman with lupus and congenital heart defects. Common risk factors of acquired heart block include patients with history of heart diseases, patients with sarcoidosis, and exposure to toxic dose of digitalis increase the risk of heart block.
Screening
There is insufficient evidence to recommend routine screening for first degree AV block. However, screening for congenital AV block is recommended.
Natural History, Complications, and Prognosis
Isolated first degree heart block has few if any clinical consequences. There are no symptoms or signs associated with it, and there is little danger of progression to complete heart block.
Diagnosis
Diagnostic Study of Choice
History and Symptoms
First degree AV block patients are usually asymptomatic at rest. In the setting of left ventricular dysfunction markedly prolonged PR interval can causeexercise intolerance and syncope.
Physical Examination
First degree AV block is an incidental finding on an EKG and is not associated with specific physical examination findings.
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.