First degree AV block overview
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First degree AV block is a disease of the electrical conduction system of the heart in which the PR interval is lengthened or prolonged. In first degree heart block, the disease is almost always at the level of the atrioventricular node (AV node). It is defined as PR prolongation of more than 200 milli seconds (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 its impact on the conduction in the myocardium.
There is no established system for the classification of First degree AV block.
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 an 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, first degree block is a slowed conduction without loss of atrioventricular synchrony.
Differentiating First Degree AV block from Other Diseases
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.
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.
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.
Diagnostic Study of Choice
History and Symptoms
First degree AV block is an incidental finding on an EKG and is not associated with specific physical examination findings.
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.
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.
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.
There are no established measures for the secondary prevention of first degree heart block.