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===Common 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 inhibitor]]s.
* Acute MI -  
* Normal variants
* [[Acute rheumatic fever]]
** [[PR prolongation]] can be found in 0.5% of healthy patients.
* Anterior ST elevation [[MI]]
** Transient AV block can occur with [[vagal maneuvers]].
* AV nodal disease
 
* [[Bacterial endocarditis]], especially of the [[aortic valve]]
* Acute ST elevation MI - [[Second degree block]] in 3.5% to 10%.
* [[Beta blocker|β blockers]] may cause [[AV block]]
** Inferior ST elevation [[MI]]: [[AV block]] is more common in patients with inferior [[MI]]s (1/3rd of patients).
* Calcific [[aortic stenosis]]
*** 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
* Degenerative diseases
** Sclerodegenerative disease of the bundle branches first described by Lenegre
* [[Digoxin]]
** The pathologic process is called idiopathic bilateral bundle branch fibrosis and the heart block is called primary heart block
* [[Dilated cardiomyopathy]]
** This is the most common cause of chronic [[AV block]] (46%)
* [[Diltiazem]]
** 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]].
* Enhanced vagal tone
 
* [[HCM]]
* Hypertension
* 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]]
* Iatrogenic after correction of [[VSD]], [[tetralogy of Fallot]], and [[endocardial cushion defect]].
 
* Massive calcification of the mitral annulus
* Diseases of the myocardium
* [[myocarditis]]
** [[Acute rheumatic fever]]: PR prolongation is a common (25 to 95% of cases) sign in patients with [[acute rheumatic fever]]
* Normal variants
*** Usually transient, disappears when the patient recovers
* Penetrating and non-penetrating trauma of the chest
** [[Dilated cardiomyopathy]] results in various degrees of heart block are seen in 15% of patients
* Rheumatic mitral valve disease
** [[HCM]]: 3% of patients with [[HCM]] will develop heart block
* Sclerodegenerative disease of the bundle branches first described by Lenegre
 
* [[Transposition of the great vessels]]
* Valvular Heart Disease
* Valvular Heart Disease
** Calcific [[aortic stenosis]] may be accompanied by chronic partial or complete AV block
* [[verapamil]]
** 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.
 
* 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
** [[Beta blocker|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 [[ASD]]s 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===
===Causes by Organ System===

Revision as of 16:56, 22 August 2013

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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] Mahmoud Sakr, M.D. [3]
Synonyms and keywords:; AV block, atrioventricular block, heart block, Mobitz type I AV block, Mobitz type II AV block, Advanced second degree AV block, (Wenckebach) AV block

Overview

Atrioventricular (AV) block is defined as an interruption in the transmission of an impulse, either transient or permanent, from the atria to the ventricles due to an anatomic or functional impairment in the conduction system. In second degree AV block, some atrial impulses fail to reach the ventricles. Wenckebach described progressive delay between atrial and ventricular contraction and the eventual failure of an atrial beat to reach the ventricles. Mobitz subsequently divided second degree AV block into two subtypes, as determined by the findings on the electrocardiogram. In Mobitz type I AV block, progressive PR interval prolongation precedes a non-conducted P wave. While in Mobitz type II AV block, the PR interval remains unchanged prior to a P wave that suddenly fails to conduct to the ventricles.

Causes

Life Threatening Causes

Life-threatening conditions can result in death or permanent disability within 24 hours if left untreated.

Common Causes

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
Ophthalmologic 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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