Third degree AV block causes

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Cafer Zorkun, M.D., Ph.D. [2]

Overview

Many conditions can cause third degree heart block, but the most common cause is coronary ischemia. Progressive degeneration of the electrical conduction system of the heart can lead to third degree heart block. This may be preceded by first degree AV block, second degree AV block, bundle branch block, or bifascicular block. In addition, acute myocardial infarction may present with third degree AV block.

Third degree heart block may also be congenital and has been linked to the presence of lupus in the mother. It is thought that maternal antibodies may cross the placenta and attack the heart tissue during gestation. The cause of congenital third degree heart block in many patients is unknown.

Causes

Common Causes

• Acute ST elevation MI - Complete heart block occurs in 2.5% to 8% of patients.
  • Inferior ST elevation MI: AV block is more common in patients with inferior MIs (1/3rd of patients).
    • In 90% of patients the inferior wall is supplied by the RCA which gives off a branch to the AV node.
    • As a rule the AV block is transient and normal function returns within a week of the acute episode.
  • Anterior ST elevation MI: AV block may be seen in up to 21%.
    • Incidence of second degree AV block and third degree AV block is 5 to 7%.
    • Block is the result of damage to the interventricular septum supplied by the LAD.
    • There is damage to the bundle branches either in the form of bilateral bundle branch block or trifascicular block.
    • RBBB, RBBB + LAHB, RBBB + LPHB or LBBB often appear before the development of AV block.
    • The PR is normal or minimally prolonged before the onset of second degree AV block or third degree AV block.
    • Although the AV block is usually transient, there is a relatively high incidence of recurrence or high-degree AV block after the acute event.
    • 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.
    • 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 was 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
    • Usually transient, disappears when the patient recovers
  • Dilated cardiomyopathy results in various degrees of heart block are seen in 15% of patients
  • HCM: 3% of patients with HCM will develop heart block

• Valvular heart disease
  • 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
    • 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

Causes by Organ System

Cardiovascular	AV nodal disease, Myocarditis, Acute myocardial infarction (especially acute inferior MI), Hypertension, Acute rheumatic fever, Dilated cardiomyopathy, HCM, Myocarditis, Valvular heart disease, Transposition of the great vessels, ASDs, VSD, Tetralogy of Fallot, Endocardial cushion defect
Chemical / poisoning	No underlying causes
Dermatologic	No underlying causes
Drug Side Effect	Calcium channel blockers, Beta-blockers, Digitalis, Cardiac glycosides, Cholinesterase inhibitors, Quinidine,Procainamide
Ear Nose Throat	No underlying causes
Endocrine	No underlying causes
Environmental	No underlying causes
Gastroenterologic	Hemochromatosis
Genetic	No underlying causes
Hematologic	Thalassemia major
Iatrogenic	No underlying causes
Infectious Disease	Acute rheumatic fever, Chagas disease, Diphtheria, Lyme disease, Myocarditis
Musculoskeletal / Ortho	Ankylosing spondylitis, Muscular dystrophy
Neurologic	No underlying causes
Nutritional / Metabolic	No underlying causes
Obstetric/Gynecologic	No underlying causes
Oncologic	No underlying causes
Opthalmologic	No underlying causes
Overdose / Toxicity	No underlying causes
Psychiatric	No underlying causes
Pulmonary	Sarcoidosis
Renal / Electrolyte	No underlying causes
Rheum / Immune / Allergy	Ankylosing spondylitis, Dermatomyositis, Scleroderma, SLE
Sexual	No underlying causes
Trauma	No underlying causes
Urologic	No underlying causes
Dental	No underlying causes
Miscellaneous	Amyloidosis, Enhanced vagal tone (for example in athletes), Normal variants

Causes in Alphabetical Order

• Acute myocardial infarction (especially acute inferior MI)
• Acute rheumatic fever
• Amyloidosis
• Ankylosing spondylitis
• ASD
• AV nodal disease
• Beta-blockers
• Calcium channel blockers
• Cardiac glycosides
• Chagas disease
• Cholinesterase inhibitors
• Dermatomyositis
• Digitalis
• Dilated cardiomyopathy
• Diphtheria
• Ebstein's anomaly
• Endocardial cushion defect
• Enhanced vagal tone (for example in athletes)
• HCM
• Hemochromatosis
• Hypertension
• Lyme disease
• Muscular dystrophy
• Myocarditis
• Normal variants
• Procainamide
• Quinidine
• Sarcoidosis
• Scleroderma
• SLE
• Tetralogy of Fallot
• Thalassemia major
• Transposition of the great vessels
• Valvular heart disease
• VSD

References

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