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Revision as of 18:29, 4 February 2013

Second degree AV block Microchapters

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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]

Causes

Common Causes

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.

  • Acute ST elevation MI - Second degree block in 3.5% to 10%.
    • 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%.
      • 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.
  • 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 byhypertension
  • 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
    • Ebstein's anomaly may be associated with first-degree AV block.
  • 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, Ebstein's anomaly, 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 No underlying causes
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

References


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