First degree AV block

Associate Editor-In-Chief:

Assistant Editor-In-Chief: Sandeep Krishnan, University of Missouri-Kansas City Medical School

Overview
First degree AV block or PR prolongation 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 has a prevalence in the normal (young adult) population of 0.65-1.1% and the incidence is 0.13 per 1000 persons.

Causes
First degree AV block may be due to conduction delay in the AV node, in the His-Purkinje system (made up by the bundle of His and the Purkinje fibers), or a combination of the two. The majority of cases are due to a dysfuction of the AV node; however, when first degree heart block coexists with a bundle-branch block, the cause is more likely to be a conduction delay in the His-Purkinje system.

Differential Diagnosis of Causes of First Degree AV Block
The most common causes of first degree heart block are an AV nodal disease, enhanced vagal tone (for example in athletes), myocarditis, acute myocardial infarction (especially acute inferior MI), electrolyte disturbances and drugs. The drugs that most commonly cause first degree heart block are those that increase the refractory time of the AV node, thereby slowing AV conduction. These include calcium channel blockers, beta-blockers, digitalis cardiac glycosides and anything that increases cholinergic activity such as cholinesterase inhibitors.

Diagnosis
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.

Normal Variants

 * 1) PR prolongation can be found in 0.5% of healthy patients
 * 2) Second degree block type I may be seen in healthy patients during sleep
 * 3) Transient AV block can occur with vagal maneuvers

ST Elevation MI
In acute ST elevation MI:
 * First degree block occurs in 8% to 13%
 * Second degree block in 3.5% to 10%
 * Complete heart block in 2.5% to 8%

Inferior ST Elevation MI

 * Inferior ST elevation MI: AV block is more common in patients with inferior MIs (1/3rd of patients)
 * 1) In 90% of patients the inferior wall is supplied by the RCA which gives off a branch to the AV node
 * 2) As a rule the AV block is transient and normal function returns within a week of the acute episode

Anterior ST Elevation MI

 * Anterior ST elevation MI: AV block may be seen in up to 21%
 * 1) Incidence of second degree AV block and third degree AV block is 5 to 7%
 * 2) Block is the result of damage to the interventricular septum supplied by the LAD
 * 3) There is damage to the bundle branches either in the form of bilateral bundle branch block or trifascicular block
 * 4) RBBB, RBBB + LAHB, RBBB + LPHB or LBBB often appear before the development of AV block
 * 5) The PR is normal or minimally prolonged before the onset of second degree AV block or third degree AV block
 * 6) Although the AV block is usually transient, there is a relatively high incidence of recurrence or high-degree AV block after the acute event
 * 7) In addition to ischemia, fibrosis and calcification of the summit of the ventricular septum that involve the branching part of the bundle branches, may play a role in the genesis of the conduction defect.
 * 8) It used to be thought that CAD was the most frequent cause of chronic complete AV block, but it actually causes only 15% of cases

Degenerative Diseases

 * Sclerodegenerative disease of the bundle branches first described by Lenegre
 * The pathologic process is called idiopathic bilateral bundle branch fibrosis and the heart block is called primary heart block
 * This is the most common cause of chronic AV block (46%)
 * Lev described similar degenerative lesions, which he referred to as sclerosis of the left side of the cardiac skeleton. There is progressive fibrosis and calcification of the mitral annulus, the central fibrous body, the pars membranacea, the base of the aorta, and the summit of the muscular ventricular septum. Various portions of the His bundle or the bundle branches may be involved, resulting in AV block.

Hypertension

 * Chronic AV block in patients with HTN is thought to be due to CAD or sclerosis of the left side of the cardiac skeleton exacerbated by hypertension

Diseases of the Myocardium

 * Acute rheumatic fever: PR prolongation is a common (25 to 95% of cases) sign in patients with acute rheumatic fever
 * Type I second degree AV block may occur, but complete AV block is uncommon
 * usually transient, disappears when the patient recovers


 * Amyloidosis
 * Ankylosing spondylitis
 * Chagas disease
 * Dermatomyositis
 * Dilated cardiomyopathy results in various degrees of heart block are seen in 15% of patients
 * Diphtheria
 * HCM: 3% of patients with HCM will develop heart block
 * Hemochromatosis
 * Lyme disease
 * Muscular dystrophy
 * Myocarditis
 * Sarcoid
 * Scleroderma
 * SLE
 * Tumors, primary and secondary

Valvular Heart Disease
Valvular Diseases
 * Calcific aortic stenosis may be accompanied by chronic partial or complete AV block
 * There is an extension of the calcification to involve the main bundle or its bifurcation, resulting in degeneration and necrosis of the conduction tissue
 * May also occur in rheumatic mitral valve disease, but is less common
 * Occasionally, massive calcification of the mitral annulus as an aging process may cause AV block
 * May also be seen in bacterial endocarditis, especially of the aortic valve

Drugs

 * Digoxin is one of the most common causes of reversible AV block
 * When second degree AV block is induced, it is always of the Type I variety
 * When complete block occurs, the QRS complexes are narrow because the block is of the AV node
 * The ventricular response rate is more rapid than that due to organic lesions, and increased automaticity of the AV junctional pacemaker may be responsible.


 * Quinidine and Procainamide may produce slight prolongation of the PR
 * β blockers may cause AV block
 * Diltiazem and verapamil may cause AV conduction delay and PR interval prolongation

Congenital

 * Occurs in the absence of other evidence of organic heart disease
 * Site is usually proximal to the bifurcation of the His bundle, most often in the AV node
 * Narrow QRS with a rate > 40 beats per minute
 * Frequently seen in those with corrected transposition of the great vessels, and occasionally in ASDs and Ebstein's anomaly

Trauma

 * May be induced during open heart surgery in the area of AV conduction tissue
 * Seen in patients operated on for the correction of VSD, tetralogy of Fallot, and endocardial cushion defect.
 * May be due to edema, transient ischemia, or actual disruption of the conduction tissue. The block may therefore be permanent or transient.
 * Also reported with both penetrating and non-penetrating trauma of the chest

Electrocardiographic Findings

 * 1) PR interval is greater than 0.20 seconds = 200 miliseconds
 * 2) Each P wave is followed by a QRS
 * 3) Range of PR interval is between 0.21 and 0.40 seconds
 * 4) P wave may be mistaken for a T wave or a U wave
 * 5) The PR interval is more variable in those without heart disease
 * 6) In patients with a narrow QRS, His-Bundle recordings show that the conduction delay is in the AV node, with prolongation of the atrial His (AH) time, rarely is a prolonged His ventricular (HV) time responsible.
 * 7) In patients with PR prolongation and QRS prolongation, then the conduction delay may occur in various regions of the conduction system

Treatment
The management includes identifying and correcting electrolyte imbalances and withholding any medications that are associated with PR prolongation. This condition does not require admission unless there is an associated myocardial infarction. Even though it usually does not progress to higher forms of heart block, it may require outpatient follow up and monitoring of the EKG especially if there is an associated bundle branch block. If there is a need for treatment of an unrelated condition care should be taken not to introduce any medication that may slow AV conduction. If this is not feasible, clinicians should be very cautious when introducing any drug that may slow conduction and if drugs are introduced, regular monitoring of the EKG is indicated.

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.

Individuals who have first degree block as part of a triad of first degree heart block, right bundle branch block, and either left anterior fascicular block or left posterior fascicular block (known as trifascicular block) may be at an increased risk of progression to complete heart block.