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.
First-degree AV block
Second-degree AV block
Third-degree AV block
See Clinical Presentation for more specific information.
Laboratory studies
Electrocardiography
Additional modalities
See Clinical Presentation and Workup for more specific information on the diagnosis of AV block.
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:
See Medication for more specific information on the treatment of atrioventricular block.
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 consists of the following:
View Image | 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 is characterized by the following:
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).
View Image | Second-degree Mobitz type I atrioventricular block. Note the prolongation of the PR interval preceding the dropped beat and the shortened PR interval .... |
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.
View Image | Second-degree Mobitz II atrioventricular block. Note the fixed PR interval, but after the third beat, an atrial impulse fails to conduct to the ventri.... |
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.
View Image | 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.... |
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.
View Image | High-degree atrioventricular block is demonstrated with a 4:1 atrial-to-ventricular conduction ratio. Note the P wave prior to the QRS conducts wherea.... |
Third-degree AV block consists of the following features:
View Image | This rhythm strip shows third-degree atrioventricular block (complete heart block). The atrial rate is faster than the ventricular rate, and no associ.... |
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.
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.
Atrioventricular (AV) block is due to the following conditions:
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.
Causes of first-degree AV block include the following:
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.
Causes of Mobitz I second-degree AV block include the following:
Causes of Mobitz II second-degree AV block include the following:
Causes of third-degree AV block include the following::
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
Mobitz type II second-degree AV block
Congenital third-degree AV block
The international incidence is similar to that of the United States.
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.
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 include the following:
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.
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.
History
Symptoms
History
Symptoms
History
Symptoms
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.
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 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.
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.
View Image | This rhythm strip shows first-degree atrioventricular block block with a PR interval of 0.360 sec. Note the fixed prolonged PR interval. |
View Image | Second-degree Mobitz type I atrioventricular block. Note the prolongation of the PR interval preceding the dropped beat and the shortened PR interval .... |
View Image | Second-degree Mobitz II atrioventricular block. Note the fixed PR interval, but after the third beat, an atrial impulse fails to conduct to the ventri.... |
View Image | 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.... |
View Image | High-degree atrioventricular block is demonstrated with a 4:1 atrial-to-ventricular conduction ratio. Note the P wave prior to the QRS conducts wherea.... |
View Image | This rhythm strip shows third-degree atrioventricular block (complete heart block). The atrial rate is faster than the ventricular rate, and no associ.... |
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] :
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 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.
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.
Patients with AV block should have their activities restricted to reduce the risk of injury until they have been properly evaluated and treated.
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 (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.
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.
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
In addition, permanent pacemaker implantation is recommended after acute myocardial infarction for the following:
Class II indications for implantation of a permanent pacemaker in AV block
Class III (pacing not indicated) conditions in patients with AV block
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.
Clinical Context: 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.
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.
Clinical Context: Isoproterenol binds and stimulates beta-receptors of the heart, smooth and skeletal muscle, vasculature, and alimentary tract. It has positive inotropic and chronotropic actions.
Clinical Context: 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.
Clinical Context: Intravenous dobutamine infusion typically begins at a dose of 5 mcg/kg/minute and can be titrated up to 20 mcg/kg/minute.
For hemodynamically unstable patients with AV block, intravenous dopamine or dobutamine should be started to improve chronotropic and inotropic cardiac function..
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 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 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.
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 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 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.
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 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 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.