Atrioventricular Block 

Updated: Nov 14, 2017
Author: Chirag M Sandesara, MD, FACC, FHRS; Chief Editor: Jose M Dizon, MD 

Overview

Practice Essentials

Atrioventricular (AV) block is an interruption or delay of electrical conduction from the atria to the ventricles due to conduction system abnormalities in the AV node or the His-Purkinje system. Conduction delay or block can be physiologic if the atrial rate is abnormally fast or pathologic at normal atrial rates. AV block is generally defined based on a regular atrial rhythm.

Signs and symptoms

First-degree AV block

  • Generally asymptomatic
  • Excessive delay causes dyspnea, weakness, or dizziness

Second-degree AV block

  • Can be asymptomatic
  • Palpitations, weakness, lightheadedness, or syncope
  • Manifests on physical examination as bradycardia (especially Mobitz II) and/or irregularity of heart rate (during Mobitz I AV block)

Third-degree AV block

  • Fatigue, dizziness, and light-headedness are common and, with concomitant structural heart disease, heart failure, weakness, chest pain, confusion and syncope may occur
  • Associated with profound bradycardia unless the site of the block is located in the AV node
  • Can cause asystole leading to cardiac arrest and/or death

See Clinical Presentation for more specific information.

Diagnosis

Laboratory studies

  • Electrolyte levels (hyperkalemia) and drug levels (eg, digitalis), if elevated potassium level or drug toxicity is suspected
  • Troponin levels: If AV block is suspected due to ischemia/infarction
  • Infection (Lyme titers), myxedema (thyroid stimulating hormone [TSH] levels), or connective tissue disease (ANA) studies in cases of systemic illness

Electrocardiography

  • 12-Lead ECG recordings and/or rhythm/monitor strips
  • 24-Hour and/or prolonged monitoring
  • Implantable loop recorder

Additional modalities

  • Electrophysiologic test: To determine the location of the block and other arrhythmias
  • Echocardiography: To assess ventricular function (especially if an implantable device is required)
  • Exercise test: To assess if AV block worsens or improves with exercise

See Clinical Presentation and Workup for more specific information on the diagnosis of AV block.

Management

Pacemaker implantation

A pacemaker is indicated when there is irreversible and longstanding asymptomatic bradycardia due to AV block:[1, 2] :

See Treatment: Pacemaker Implantation for more specific information on this topic.

Pharmacologic therapy

Considerations regarding the administration of anticholinergic agents include the following:

  • Atropine and isoproterenol infusion can improve AV conduction in emergencies in which bradycardia is caused by AV block in the AV node
  • Dopamine and dobutamine can occasionally be helpful

See Medication for more specific information on the treatment of atrioventricular block.

Background

Atrioventricular (AV) block is an interruption or delay of electrical conduction from the atria to the ventricles due to conduction system abnormalities in the AV node or the His-Purkinje system. Conduction delay or block can be physiologic if the atrial rate is abnormally fast or pathologic at normal atrial rates. AV block is generally defined based on a regular atrial rhythm.

AV block is categorized as first-, second-, and third-degree AV block. First-degree AV block is defined as AV conduction slowing; on the electrocardiogram (ECG), the PR interval exceeds 0.20 seconds (sec). In second-degree AV block, some P waves conduct while others do not. This type is subdivided into Mobitz I (Wenckebach), Mobitz II, 2:1, paroxysmal, and high-grade AV block. During third-degree (“complete”) AV block, no AV conduction occurs when it should be able to occur.

Symptoms of AV block range from no symptoms to weakness, fatigue, shortness of breath, exercise intolerance, or syncope. Low cardiac output can cause hypotension and end-organ hypoperfusion. Death due to asystole is possible in complete heart block. Treatment involves correction or resolution of underlying causes and, if AV block is progressive, symptomatic, and/or persistent, a pacemaker is required.[3]

First-degree AV block

First-degree AV block consists of the following:

  • PR prolongation (PR interval >0.20 sec in adults and >0.160 sec in young children) with 1:1 AV conduction (see the image below)
  • A narrow QRS is associated with conduction slowing in the AV node, but it may be in the atria, and with a wide QRS, conduction slowing is in the His-Purkinje system
  • Marked PR interval prolongation (>0.30 sec) is usually associated with slowing in the AV node 
This rhythm strip shows first-degree atrioventricu This rhythm strip shows first-degree atrioventricular block block with a PR interval of 0.360 sec. Note the fixed prolonged PR interval.

Second-degree AV block

Second-degree AV block is characterized by the following:

  • Some, but not all, atrial impulses (generally occurring at a regular rate) failing to conduct to the ventricles
  • Categorized as Mobitz I, II, 2:1, paroxysmal, or high-degree AV block

Mobitz I second-degree AV block

Mobitz I second-degree AV block is associated with constant PP intervals, progressive PR prolongation, followed by one nonconducted P wave. Often, there is progressive shortening of the RR intervals during the Wenckebach cycle and a pause due to a blocked P wave that is less than the sum of two PP intervals. This form of AV block occurs in less then 50% of all types of AV block and is generally due to a block in the AV node.[4] Rarely, it can occur with a block within or below the His Bundle; this is intra or infra-Hisian Wenckebach block, respectively.[5]

Mobitz I block rarely progresses to third-degree block. Distinguishing a block in the AV node versus infra-Hisian block can be challenging. Carotid sinus massage will worsen AV node conduction during Wenckebach block, but it may improve conduction if the block occurs below the His bundle. Exercise will worsen a block in the His-Purkinje system but improve it if the block occurs in the AV node. Mobitz I AV block usually consists of a ratio of nonconducted to conducted beats of 4:3, 3:2, and so on (see the image below).

Second-degree Mobitz type I atrioventricular block Second-degree Mobitz type I atrioventricular block. Note the prolongation of the PR interval preceding the dropped beat and the shortened PR interval following the dropped beat.

Mobitz II second-degree AV block

Mobitz II second-degree AV block is characterized by constant PP and RR intervals (see the image below). There is a constant PR interval before failure of an atrial impulse to conduct to the ventricle (small changes can occur in the PR interval after the blocked beat). The pause—including the blocked P waves—equals two PP cycles (unless an early junctional escape beat occurs). Mobitz II block occurs in the His-Purkinje system and may be associated with a narrow QRS but, as there often is in His-Purkinje disease, a wide complex QRS is commonly seen (bundle branch block) in the conducted beats. Mobitz II AV block may progress to complete AV block. Carotid sinus massage improves AV conduction in the His-Purkinje system, as it causes the sinus rate to slow; however, walking, exercise, or atropine use may lead to progressively worse AV block, as impulses may impinge upon an already damaged conduction system.

Second-degree Mobitz II atrioventricular block. No Second-degree Mobitz II atrioventricular block. Note the fixed PR interval, but after the third beat, an atrial impulse fails to conduct to the ventricle. Courtesy of Wikimedia Commons (https://commons.wikimedia.org/wiki/File:Second_degree_heart_block.png).

2:1 second-degree AV block

A 2:1 AV block is noted when every other P wave (with a regular PP interval) is conducted to the ventricle (see the image below). The block may be at the level of AV node or below. Close analysis of a long rhythm strip may help define the location of 2:1 AV block, especially during particular maneuvers. For example, if the block is in the His bundle or below, exercise, walking, or even standing may improve the AV node conduction; however, it will slow the ventricular rate and increase the level of the block. Carotid massage or atropine use may improve conduction if the block is in the His-Purkinje system, but it will worsen the block if it is at the level of the AV node. 

Look for the presence of a bundle branch block. If present, a 2:1 AV block is more likely occurring in the His-Purkinje system.

A constant PP interval and normal PR interval in c A constant PP interval and normal PR interval in conducted beats is present. This progresses to 2:1 atrioventricular (AV) block. A 2:1 AV block can be present with conduction delay in the AV node or His-Purkinje system, but it is more likely to be in the AV node for all patients (with a greater chance of AV block in the His-Purkinje system if there is a bundle branch block). Review extended monitoring strips because Mobitz I or Mobitz II may be present at other times, and this might help to determine the level of the block.

Paroxysmal AV block

Paroxysmal AV block is a form of second-degree AV block that is not persistent or repetitive, as there is an abrupt block in AV conduction. It may occur due to paroxysmal augmentation in vagal activation.

High-degree AV block

High-degree AV block consists of multiple constant P waves in a row that do not conduct. The conduction ratio can be 3:1 or higher; the PR interval of conducted beats can be constant (with His-Purkinje disease) or variable (with AV nodal block). This is distinct from complete AV block, because some P waves conduct to the ventricle. Atrial fibrillation with pauses greater than 5 seconds is due to a high-degree AV block. It can be associated with a narrow or wide QRS complex. 

High-degree atrioventricular block is demonstrated High-degree atrioventricular block is demonstrated with a 4:1 atrial-to-ventricular conduction ratio. Note the P wave prior to the QRS conducts whereas the others do not. Courtesy of Life in the Fast Lane (https://lifeinthefastlane.com/ecg-library/basics/high-grade-block/), Edward J Burns, MD, Sydney, Australia.

Third-degree AV block

Third-degree AV block consists of the following features:

  • No atrial impulses conduct to the ventricle when they should conduct (ie, complete AV block)
  • The P waves, generally regular, reflect a sinus node rhythm independent from a junctional (narrow) or ventricular (wide) escape rhythm (see the image below), and they may be regular or irregular
  • The junctional escape rate is 40-60 bpm, and the ventricular escape rate is 20-40 bpm
  • Non-sinus atrial rhythms such as atrial fibrillation, atrial flutter, or atrial tachycardia can be associated with third-degree AV block 
This rhythm strip shows third-degree atrioventricu This rhythm strip shows third-degree atrioventricular block (complete heart block). The atrial rate is faster than the ventricular rate, and no association exists between the atrial and ventricular activity. Courtesy of Life in the Fast Lane (https://lifeinthefastlane.com/ecg-library/basics/complete-heart-block/), Edward J Burns, MD, Sydney, Australia.

AV block mimickers

Blocked premature atrial complexes can be a physiologic process. These are not considered a form of AV block but are due to ectopic beats that occur when the AV node is refractory.

Paroxysmal vagal activation can occur during sleep and may be associated with sleep apnea. Other causes include cough, vomiting, urination, eating, and defecation. When AV block is present, the PR interval slows around the time of the AV block. With high vagal tone, the PR intervals will lengthen, whereas in 2:1 AV block, for example, the PR intervals are constant.

Atrial tachycardia/atrial flutter with 2:1 AV conduction is not a form of AV block. It is due physiologic impingement on AV nodal refractoriness.

AV dissociation

During AV dissociation, the atria and ventricles beat independently of each other. AV dissociation occurs when a subsidiary pacemaker in the AV node or the ventricle overtakes the sinus node for impulse initiation due to slowing of the sinus node, or it may occur when a subsidiary site beats faster than the sinus node. Complete AV block can occur with AV dissociation, but AV dissociation alone does not indicate AV block.

See the Medscape Drugs & Diseases article Atrioventricular Dissociation for more detailed information on this topic.

Pathophysiology

Atrioventricular (AV) block is due to the following conditions:

  • Ischemia
  • Degenerative changes
  • Infection (myocarditis) and infiltrative processes such as Lyme disease and sarcoidosis
  • Drugs (beta blockers, calcium antagonists) that slow AV nodal or block His-Purkinje (eg, procainamide, flecainide) conduction or lengthen AV node refractoriness
  • Bezold-Jarisch reflex: An inferior myocardial infarction may cause a temporarily increased vagal tone leading to transient Mobitz I or complete AV block 

Etiology

Delay or lack of conduction through the atrioventricular (AV) node and below has multiple causes. Degenerative changes (eg, fibrosis, calcification, or infiltration) are the most common cause of non-ischemic AV block. Idiopathic fibrosis or calcification of the AV conduction system, commonly seen in the elderly, can cause complete AV block.

First-degree AV block 

Causes of first-degree AV block include the following:

  • Delayed conduction within the AV node or His-Purkinje system
  • Intrinsic disease of the AV node, high vagal tone, or dual AV nodal pathways (two separate PR intervals)
  • Medications that block the AV node (beta-blockers, calcium channel antagonists, digitalis, and select antiarrhythmic medications) may prolong the PR interval

First-degree AV block is also associated with a greater risk of atrial fibrillation and a poorer prognosis in the general population and in patients with heart failure.

Second-degree AV block

Causes of Mobitz I second-degree AV block include the following:

  • Increased vagal tone (functional block)
  • Degenerative disease of the AV conduction system (commonly seen in the elderly)
  • Occurs physiologically at high heart rates (especially with pacing) as a result of increased refractoriness of the AV node
  • AV node–blocking medications
  • Lyme disease, myocarditis, and radiofrequency ablation of the slow pathway

Causes of Mobitz II second-degree AV block include the following:

  • Degenerative disease of the His-Purkinje system
  • Damage of the conduction system from coronary artery disease, valve surgery, myocardial infarction, myocarditis, infiltrative cardiomyopathies (sarcoidosis, hemochromatosis), myxedema, Lyme disease, neuromuscular disease, and AV junction ablation [6]
  • Systemic diseases (eg, ankylosing spondylitis, Reiter syndrome)

Third-degree AV block

Causes of third-degree AV block include the following::

  • Reversible causes: AV node–blocking drugs and ischemia
  • Pathologic causes: Include cardiomyopathy (infiltrative, idiopathic, and infarction), myocarditis (Lyme disease), endocarditis with abscess formation, and hyperkalemia
  • Anterior myocardial infarction causing ischemia or infarction of the bundle branches
  • Cardiac surgery, catheter ablation of the AV node, alcohol septal ablation, or neuromuscular disease

Epidemiology

Epidemiology of specific AV blocks in the United States

All types of AV blocks occur more frequently in people older than 70 years, especially in those who have structural heart disease.

First-degree AV block

  • Can be found in healthy adults but is associated with an increased risk of atrial fibrillation and adverse outcomes in some studies
  • Incidence increases with age
  • At age 20 years: PR interval may exceed 0.20 sec in 0.5-2% of healthy people
  • At age 60 years: Over 5% of healthy individuals have PR intervals exceeding 0.20 seconds

Mobitz type II second-degree AV block

  • Rare in healthy individuals
  • Wenckebach observed in 1-2% of healthy young people, especially during sleep

Congenital third-degree AV block

  • About 1 case per 15,000-20,000 births, primarily the result of AV node injury from maternal antibodies in women with lupus or Sjogren syndrome [7]
  • Most commonly diagnosed between 18-24 weeks’ gestation; may cause first-, second-, or third-degree AV block
  • Approximately 20% mortality; most surviving children require pacemakers [7, 8]

International data

The international incidence is similar to that of the United States.

Age-, sex-, and race-related demographics

A 60% female preponderance exists in congenital third-degree AV block; for acquired third-degree AV block, a 60% male preponderance exists. No racial proclivity exists in AV blocks.

Prognosis

The prognosis in atrioventricular (AV) block is directly related to its degree and the patient's underlying medical problems. Those with advanced AV block who are not treated with permanent pacing remain at risk for syncope and sudden cardiac death, especially individuals with underlying structural heart disease.

In a prospective study of patients with first-degree AV block and the outcomes associated with it using data from 7,575 individuals from the Framingham Heart Study (mean age, 47 years; 54% women) who underwent a 12-lead electrocardiography, Cheng et al found 124 individuals had PR intervals above 0.2 sec on the baseline examination.[9] On follow-up, first-degree AV block was associated with an increased incidence of atrial fibrillation, pacemaker implantation, and all-cause mortality.[9]  Other studies have also demonstrated similar findings.[10, 11, 12]

Symptomatic Mobitz type I AV block is associated with increased mortality if left untreated. A retrospective study that evaluated 299 patients (average age 75 years) with Mobitz I AV block found that device implantation was associated with a 46% reduction in mortality (hazard ratio [HR] 0.54; 95% confidence interval [CI] 0.35 to 0.82; P = 0.004).[9]  Other studies have confirmed these findings.[13, 14]

Higher degrees of AV block are associated with poor outcomes. Kosmidou et al examined 3,115 patients undergoing percutaneous coronary intervention for ST-segment elevation myocardial infarction and found high-degree AV block (Mobitz type II or third-degree AV block) in 1.5% of the cohort.[15] Patients with high-degree AV block had increased mortality at 30-day, 1-year, and 3-year follow-ups (unadjusted HR 3.83, 95% CI 1.40 to 10.48; unadjusted HR 4.37, 95% CI 2.09 to 9.38; and unadjusted HR 2.78, 95% CI 1.31 to 5.91, respectively).[15]

Fetuses with second- or third-degree AV block, associated with congenital heart disease, have increased mortality, and surviving infants may require pacemaker therapy.[16]

Complications

Complications include the following:

  • Death due to asystole or Torsade de pointes
  • Cardiovascular collapse with syncope, aggravation of ischemic heart disease, congestive heart failure, and exacerbation of renal disease

Patient Education

Patients need to be aware of the reversible causes of atrioventricular (AV) block. Avoiding AV node–blocking antihypertensive agents could eliminate symptoms.

Inform patients with known first-degree AV block who are asymptomatic that block may increase over time, and they should quickly seek medical attention if syncope, lightheadedness, chest pain, or shortness of breath occurs.

 

Presentation

History

The degree atrioventricular (AV) block is generally associated with the severity of symptoms and clinical history. Common presentations and symptoms are outlined below for the different types of AV block.

Any level of AV block leading to profound bradycardia may also lead to life-threatening torsade de pointes (TdP). This is due to the inverse relationship between bradycardia and repolarization time; it may prolong the QT interval and predispose a patient to TdP especially if there is a long short interval. 

First-degree AV block

History

  • High vagal tone
  • Acute inferior myocardial infarction (MI)
  • Use of antiarrhythmic medication(s)
  • Being a conditioned athlete
  • Hypokalemia, hypomagnesemia
  • Degeneration of the AV node

Symptoms

  • None
  • Shortness of breath
  • Exercise intolerance
  • Fatigue

Second-degree AV Block

History

  • Inflammatory disease
  • Infiltrative disease
  • Hyperkalemia
  • Catheter ablation of the slow pathway
  • Valve replacement surgery
  • Acute MI

Symptoms

  • Fatigue
  • Shortness of breath
  • Exercise intolerance
  • Dizziness
  • Syncope
  • Chest pain

Third-degree AV Block

History

  • Degenerative disease
  • Infiltrative disease
  • Myocarditis
  • Neuromuscular disease
  • Ischemia/infarction
  • Hypoxia
  • Hyperkalemia
  • Use of antiarrhythmic medication(s)

Symptoms

  • Fatigue
  • Shortness of breath
  • Exercise intolerance
  • Lightheadedness, dizziness
  • Syncope
  • Chest pain
  • Neck palpitations

Physical Examination

Routine physical examination does not lead to the diagnosis of first-degree atrioventricular (AV) block. Second-degree AV block may manifest as bradycardia (Mobitz II), irregularity of the heart rate (Mobitz I), or a variable pulse and variable neck vein distention.

Third-degree AV block may be associated with profound bradycardia. In cases of concomitant structural heart disease, pulmonary edema and jugular venous distention may be noted. Cannon A-waves may be observed intermittently due to right atrium contraction against a closed tricuspid valve. A variable S2 and variable strength of the pulse is noted. 

 

DDx

Diagnostic Considerations

Mobitz I and Mobitz II atrioventricular (AV) block must be differentiated from sinus node dysfunction and increased vagal tone. AV block generally occurs with a constant PP interval. Blocked premature atrial beats must be differentiated from Mobitz II AV block. Blocked atrial beats will occur earlier than expected compared with the fixed PP interval in Mobitz II AV block. Atrial fibrillation with a slow heart rate may indicate second-degree AV block. If the ventricular rate is slow and regular during atrial fibrillation, third-degree AV block is likely present.

Important considerations in the differential diagnosis

AV dissociation due to sinus bradycardia with a junctional escape rhythm is not AV block. Severely prolonged first-degree AV block may appear to be a junctional rhythm. The P waves may be buried in the preceding QRS complex and suggest retrograde activation. However, in junctional rhythm, the retrograde P waves are inverted and they are upright in sinus rhythm with first-degree AV block. Variations in rate start with changes in timing of the PP interval rather than the RR interval.

The irregularity of the RR intervals in Mobitz I AV block may appear to be sinus rhythm with premature atrial complexes or multifocal atrial tachycardia. The presence of distinct, similar P waves and the grouped beating of the QRS complexes can help to distinguish Mobitz type I AV block from the aforementioned arrhythmias.

Differential Diagnoses

 

Workup

Approach Considerations

A review of the patient's comorbidities and medications is essential, as oftentimes these individuals have underlying structural heart disease or coronary artery disease, or they may be taking atrioventricular (AV) node–blocking medications that may be causing or exacerbating the block.

Inspection of the rhythm strip or electrocardiogram (ECG) can help clinicians identify the type and level of block. 

Laboratory Studies

Laboratory studies may be helpful in patients with second- or third-degree atrioventricular (AV) block. Blood tests may indicate hyperkalemia, acidosis, or drug toxicity as causes for AV block. In cases when AV block is suspected due to an acute myocardial infarction, obtain measures of cardiac enzymes.

If clinical evaluation suggests systemic illness, laboratory studies for infection, Lyme disease, myxedema, or connective tissue disease should be performed.

Electrocardiography

Routine electrocardiographic (ECG) recording and cardiac monitoring with careful evaluation of the relationship between the P waves and QRS complexes will lead to the diagnosis of the type of atrioventricular (AV) block.

Identifying transient AV block with sudden pauses and/or low heart rate causing syncope requires 24-hour Holter monitoring, multiple ECG recordings, event (loop) ECG recordings or, in selected cases, monitoring with implantable loop recorders.

See Background for more information regarding the ECG characteristics of each type of AV block.

See the images below.

This rhythm strip shows first-degree atrioventricu This rhythm strip shows first-degree atrioventricular block block with a PR interval of 0.360 sec. Note the fixed prolonged PR interval.
Second-degree Mobitz type I atrioventricular block Second-degree Mobitz type I atrioventricular block. Note the prolongation of the PR interval preceding the dropped beat and the shortened PR interval following the dropped beat.
Second-degree Mobitz II atrioventricular block. No Second-degree Mobitz II atrioventricular block. Note the fixed PR interval, but after the third beat, an atrial impulse fails to conduct to the ventricle. Courtesy of Wikimedia Commons (https://commons.wikimedia.org/wiki/File:Second_degree_heart_block.png).
A constant PP interval and normal PR interval in c A constant PP interval and normal PR interval in conducted beats is present. This progresses to 2:1 atrioventricular (AV) block. A 2:1 AV block can be present with conduction delay in the AV node or His-Purkinje system, but it is more likely to be in the AV node for all patients (with a greater chance of AV block in the His-Purkinje system if there is a bundle branch block). Review extended monitoring strips because Mobitz I or Mobitz II may be present at other times, and this might help to determine the level of the block.
High-degree atrioventricular block is demonstrated High-degree atrioventricular block is demonstrated with a 4:1 atrial-to-ventricular conduction ratio. Note the P wave prior to the QRS conducts whereas the others do not. Courtesy of Life in the Fast Lane (https://lifeinthefastlane.com/ecg-library/basics/high-grade-block/), Edward J Burns, MD, Sydney, Australia.
This rhythm strip shows third-degree atrioventricu This rhythm strip shows third-degree atrioventricular block (complete heart block). The atrial rate is faster than the ventricular rate, and no association exists between the atrial and ventricular activity. Courtesy of Life in the Fast Lane (https://lifeinthefastlane.com/ecg-library/basics/complete-heart-block/), Edward J Burns, MD, Sydney, Australia.

Electrophysiologic Testing

An electrophysiology study (EPS) is indicated when atrioventricular (AV) block is the suspected cause of syncope and the diagnosis cannot be made by electrocardiography alone to identify the level of the block.

For patients with AV block regardless of syncope, EPS may have value in the management of the following conditions, as it will determine the level of AV block, predict progression of the disease, and identify the need for pacemaker therapy[17, 18] :

  • Asymptomatic second-degree Mobitz I AV block with bundle branch block, class II
  • Questionable second-degree Mobitz II AV block with a narrow QRS complex, class II
  • Transient second-degree AV block, bundle branch block, and an inferior myocardial infarction where the site of the block is suspected to be in the His-Purkinje system rather than in the AV node, class I
  • Third-degree AV block with an accelerated ventricular rate, class II
  • Progressive conduction disease due to neuromuscular disorders or suspected SCN5A mutations [19]
  • A prolonged HV interval (conduction time from the His bundle to the ventricular myocardium) (>0.100 sec) is a highly predictive but insensitive marker of high-degree AV block

Imaging Studies

Imaging studies are not generally indicated in diagnosing atrioventricular (AV) block. However, echocardiography can assess for aortic valve stenosis with calcification, wall-motion abnormalities in acute ischemia, cardiomyopathy, and congenital heart disease (eg, congenitally corrected transposition of the great vessels).

Coronary angiography or myocardial perfusion imaging may be necessary in evaluating for ischemia.

Fluorine-18 fluoro-2-deoxyglucose positron emission computed tomography (18-FDG PET) scanning has the potential to be prognostic in patients with unexplained AV block; higher adverse cardiac events and worse outcomes appear to be more likely in patients with high-degree AV block and abnomal 18-FDG-PET scan findings.[20]

Exercise

Exercise may shorten the PR interval in first-degree atrioventricular (AV) block if it is related to an increased vagal tone, but the interval will not change if the block is in the His-Purkinje system. A severely prolonged PR interval may cause exercise intolerance and pacemaker syndrome.

Exercise may be used to evaluate 2:1 heart block and to differentiate Mobitz I second-degree AV block (where the conducted rate increases) from Mobitz type II AV block (where the block becomes more significant and, often, symptomatic).

Exercise is not recommended in patients with third-degree AV block.

 

Treatment

Approach Considerations

Permanent pacing is the therapy of choice in patients with symptomatic atrioventricular (AV) block with bradycardia.

Temporary transcutaneous or transvenous pacing is required if a slow heart rate (or asystole) caused by AV block requires correction and permanent pacing is not immediately indicated or not available.

Activity restriction

Patients with AV block should have their activities restricted to reduce the risk of injury until they have been properly evaluated and treated.

Consultations

Consultation with a cardiologist and/or cardiac electrophysiologist is indicated in the case of advanced heart block or unexplained syncope. An electrophysiologist must be consulted when invasive electrophysiology testing is needed to determine the level and/or magnitude of the AV block.

 

Pacemaker Implantation

Pacemaker implantation (or a pacemaker defibrillator if there is ventricular dysfunction) is indicated for symptomatic, irreversible atrioventricular (AV) block. Pacemaker implantation may include ventricular (VVI) or dual chamber (DDD) modes of pacing. Chronic right ventricular (RV) pacing may worsen left ventricular (LV) function. Cardiac resynchronization therapy (CRT) may be necessary in certain cases in which long-term pacing is required with reduced LV function. 

The BLOCK-HF (Biventricular Versus Right Ventricular Pacing in Patients With Left Ventricular Dysfunction and Atrioventricular Block) randomized trial evaluated the effect of CRT in heart failure patients with AV block and found that CRT caused LV reverse remodeling and improved ejection fraction.[21] In addition, CRT pacing reduced mortality and heart failure admissions compared to RV pacing.[21] Based on these findings, for patients with high-degree AV block, CRT pacing therapy could be considered for patients with New York Heart Association (NYHA) functional class I, II, or III symptoms, an ejection fraction below 50%, and when chronic RV pacing is required. However, this study was published after the 2008 pacemaker guidelines[1, 2] and is not currently incorporated into any guidelines.

An electrophysiologist should determine the pacing type and the optimal pacing mode based on the level of block.

 

Long-Term Monitoring

Patients with asymptomatic first-degree or Mobitz I atrioventricular (AV) block do not require long-term monitoring with repeated rhythm strips/electrocardiograms or an event monitor.

Patients with asymptomatic, intermittent second-degree Mobitz II, high-degree, or third-degree AV block benefit from long-term event monitoring to assess for symptoms, bradycardia, or periods of asystole, as this would affect the timing of pacemaker implantation.

 

Guidelines

Guidelines Summary

The following are recommendations from the 2012 American College of Cardiology Foundation/American Heart Association/Heart Rhythm Society (ACCF/AHA/HRS) focused update of their 2008 guidelines for device-based therapy of cardiac rhythm abnormalities.[2]

Class I indications for implantation of a permanent pacemaker in atrioventricular (AV) block

  • Advanced second-degree AV block (two or more consecutive P waves are blocked)
  • Symptomatic Mobitz I second-degree block
  • Symptomatic Mobitz II second-degree block
  • Mobitz II second-degree block with wide QRS or chronic bifascicular block, with or without symptoms
  • Exercise-induced second- or third-degree AV block in the absence of ischemia
  • Second- or third-degree AV block in asymptomatic awake patients in sinus rhythm, resulting in periods of asystole longer than 3.0 sec or ventricular rates less than 40 beats per minute
  • Second -or third-degree AV block in asymptomatic awake patients in atrial fibrillation, resulting in pauses of at least 5 seconds
  • AV node ablation
  • Advanced second- or third-degree AV block with or without symptoms associated with neuromuscular diseases (eg, myotonic dystrophy, Kearns-Sayer syndrome, Erb dystrophy [limb-girdle muscular dystrophy]), and peroneal muscular atrophy
  • Recurrent syncope due to carotid massage, causing ventricular pauses of longer than 3 seconds
  • AV block caused by a medication that is an essential drug therapy for another medical condition

In addition, permanent pacemaker implantation is recommended after acute myocardial infarction for the following:

  • Symptomatic and persistent second- and third-degree AV block
  • Persistent second-degree AV block in the His-Purkinje system
  • Alternating bundle branch block
  • Third-degree AV block at or below the His bundle
  • Transient infranodal AV block with bundle branch block

Class II indications for implantation of a permanent pacemaker in AV block

  • Alternating bundle branch block
  • Bifascicular or trifascicular block with syncope
  • First-degree AV block in associated neuromuscular diseases
  • First-degree AV block causing bradycardia and hypotension due to very long PR intervals resulting in AV dissociation
  • Asymptomatic Mobitz II AV second-degree AV block with a narrow QRS complex
  • Second-degree AV block found during electrophysiology study to be infra-AV nodal

Class III (pacing not indicated) conditions in patients with AV block

  • Asymptomatic Mobitz I second-degree AV block
  • Reversible AV block (due to electrolyte abnormalities, Lyme disease, sleep apnea, enhanced vagal tone, following surgery, medications) 
 

Medication

Medication Summary

Long-term medical therapy is not indicated in atrioventricular (AV) block. As noted earlier, permanent pacing is the therapy of choice in symptomatic AV block with bradycardia.

Anticholinergic Agents

Class Summary

The goal of administering anticholinergic agents is to improve conduction through the atrioventricular node (AVN) by reducing vagal tone via muscarinic receptor blockade. This is only effective if the site of block is within the AVN. For patients with suspected infranodal block, this therapy is ineffective and may make the level of the block worse if it is in the His bundle or below.

Atropine IV/IM (Atropair, Atropisol)

Atropine increases AV conduction by reducing vagal tone via muscarinic receptor blockade; this may improve AV conduction if the block is in the AV node but atropine is ineffective in infranodal block and may worsen the block. An insufficient dose may cause paradoxical slowing of the heart rate.

Beta1/Beta2 Adrenergic Agonists

Isoproterenol (Isuprel)

Isoproterenol binds and stimulates beta-receptors of the heart, smooth and skeletal muscle, vasculature, and alimentary tract. It has positive inotropic and chronotropic actions.

Vasopressors, Alpha/Beta Agonists

Class Summary

For hemodynamically unstable patients with AV block, intravenous dopamine or dobutamine should be started to improve chronotropic and inotropic cardiac function.. 

Dopamine (Intropin)

Intravenous dopamine infusion typically begins at a dose of 3 mcg/kg/minute and can be titrated up to 20 mcg/kg/minute for heart rate and blood pressure augmentation.

Dobutamine

Intravenous dobutamine infusion typically begins at a dose of 5 mcg/kg/minute and can be titrated up to 20 mcg/kg/minute.