Atrioventricular (AV) block is a cardiac disorder in which conduction from the atria to the ventricles is delayed in the AV node. Normally, all impulses that travel to the ventricles from the sinoatrial node and atria travel through the AV node, however, reduced AV nodal conductional affects synchronization between atrial depolarizations and ventricular depolarizations Therefore, atrioventricular (AV) block is a conduction disturbance in the AV node or His-Purkinje system in which impulses from the atria are excessively delayed in traveling to the ventricles, or fail to reach the ventricles at all.
Classification
Atrioventricular (AV) block is classified as first degree, second degree, and third degree. First degree AV block is characterized by a delay in conduction, resulting in an abnormally long (>0.20 s) PR interval on the
Atrioventricular Block☆
FJ Dowd, Creighton University School of Medicine, Omaha, NE, USA
2014 Elsevier Inc. All rights reserved.
Discussion
Atrioventricular (AV) block is a cardiac disorder in which conduction from the atria to the ventricles is delayed in the AV node. Normally, all impulses that travel to the ventricles from the sinoatrial node and atria travel through the AV node, however, reduced AV nodal conductional affects synchronization between atrial depolarizations and ventricular depolarizations (Das and Zipes, 2012).
Therefore, atrioventricular (AV) block is a conduction disturbance in the AV node or His-Purkinje system in which impulses from the atria are excessively delayed in traveling to the ventricles, or fail to reach the ventricles at all.
Classification
Atrioventricular (AV) block is classified as first degree, second degree, and third degree. First degree AV block is characterized by a delay in conduction, resulting in an abnormally long (>0.20 s) PR interval on the electrocardiogram (ECG). In first degree AV block, all impulses from the atria reach the ventricles. Therefore, the PR intervals in first degree AV block usually range from 0.21 to 0.40 s. PR intervals longer than 0.40 s usually lead to impulses that fail to reach the ventricle, a situation that leads to a higher degree of block (Saoudi et al., 2001). In second degree AV block, some, but not all, of the impulses from the atria reach the ventricles. Second degree AV block is subdivided into Mobitz I (Wenchebach block) and Mobitz II block. Mobitz I AV block is characterized by a progressive increase in the PR interval over time until an impulse fails to conduct through the AV node. After this pause in conduction, the process repeats itself.
Atrioventricular Block☆
FJ Dowd, Creighton University School of Medicine, Omaha, NE, USA
2014 Elsevier Inc. All rights reserved.
Discussion
Atrioventricular (AV) block is a cardiac disorder in which conduction from the atria to the ventricles is delayed in the AV node. Normally, all impulses that travel to the ventricles from the sinoatrial node and atria travel through the AV node, however, reduced AV nodal conductional affects synchronization between atrial depolarizations and ventricular depolarizations (Das and Zipes, 2012).
Therefore, atrioventricular (AV) block is a conduction disturbance in the AV node or His-Purkinje system in which impulses from the atria are excessively delayed in traveling to the ventricles, or fail to reach the ventricles at all.
Classification
Atrioventricular (AV) block is classified as first degree, second degree, and third degree. First degree AV block is characterized by a delay in conduction, resulting in an abnormally long (>0.20 s) PR interval on the electrocardiogram (ECG). In first degree AV block, all impulses from the atria reach the ventricles. Therefore, the PR intervals in first degree AV block usually range from 0.21 to 0.40 s. PR intervals longer than 0.40 s usually lead to impulses that fail to reach the ventricle, a situation that leads to a higher degree of block (Saoudi et al., 2001). In second degree AV block, some, but not all, of the impulses from the atria reach the ventricles. Second degree AV block is subdivided into Mobitz I (Wenchebach block) and Mobitz II block. Mobitz I AV block is characterized by a progressive increase in the PR interval over time until an impulse fails to conduct through the AV node. After this pause in conduction, the process repeats itself.
In Mobitz II AV block, the PR interval is constant and then suddenly a nonconducted impulse follows. The dropped impulse may or may not be at regular intervals. Mobitz type II AV blocks are characterized by their unpredictability (Barold and Hayes, 2001).
In third degree AV block, no impulses reach the ventricles from the atria and there is a complete dissociation between atrial depolarizations and depolarizations in the ventricles (Boutjdir, 2000).
Conduction defects can also exist in the His-Purkinje system, such as a bundle branch block.
Consequences
While first degree atrioventricular (AV) block usually has no immediate consequences, its long-term presence could contribute to some progressive loss of cardiac function Cheng et al., 2009. Second degree Mobitz I AV block has a better prognosis than second degree Mobitz II AV block in that symptoms of the former are generally mild and mortality is usually not increased. For second degree Mobitz II AV block, syncope, heart failure, and decreased cardiac output are definite possibilities. Moreover, second degree Mobitz II block can lead to third degree AV block, which can cause a reduced cardiac output due to bradycardia. With third degree AV block, heart failure and reduced cardiac output are likely, and asystole and sudden death a definite risk.
Atrioventricular Block☆
FJ Dowd, Creighton University School of Medicine, Omaha, NE, USA
2014 Elsevier Inc. All rights reserved.
Discussion
Atrioventricular (AV) block is a cardiac disorder in which conduction from the atria to the ventricles is delayed in the AV node. Normally, all impulses that travel to the ventricles from the sinoatrial node and atria travel through the AV node, however, reduced AV nodal conductional affects synchronization between atrial depolarizations and ventricular depolarizations (Das and Zipes, 2012).
Therefore, atrioventricular (AV) block is a conduction disturbance in the AV node or His-Purkinje system in which impulses from the atria are excessively delayed in traveling to the ventricles, or fail to reach the ventricles at all.
Classification
Atrioventricular (AV) block is classified as first degree, second degree, and third degree. First degree AV block is characterized by a delay in conduction, resulting in an abnormally long (>0.20 s) PR interval on the electrocardiogram (ECG). In first degree AV block, all impulses from the atria reach the ventricles. Therefore, the PR intervals in first degree AV block usually range from 0.21 to 0.40 s. PR intervals longer than 0.40 s usually lead to impulses that fail to reach the ventricle, a situation that leads to a higher degree of block (Saoudi et al., 2001). In second degree AV block, some, but not all, of the impulses from the atria reach the ventricles. Second degree AV block is subdivided into Mobitz I (Wenchebach block) and Mobitz II block. Mobitz I AV block is characterized by a progressive increase in the PR interval over time until an impulse fails to conduct through the AV node. After this pause in conduction, the process repeats itself.
In Mobitz II AV block, the PR interval is constant and then suddenly a nonconducted impulse follows. The dropped impulse may or may not be at regular intervals. Mobitz type II AV blocks are characterized by their unpredictability (Barold and Hayes, 2001).
In third degree AV block, no impulses reach the ventricles from the atria and there is a complete dissociation between atrial depolarizations and depolarizations in the ventricles (Boutjdir, 2000).
Conduction defects can also exist in the His-Purkinje system, such as a bundle branch block.
Consequences
While first degree atrioventricular (AV) block usually has no immediate consequences, its long-term presence could contribute to some progressive loss of cardiac function Cheng et al., 2009. Second degree Mobitz I AV block has a better prognosis than second degree Mobitz II AV block in that symptoms of the former are generally mild and mortality is usually not increased. For second degree Mobitz II AV block, syncope, heart failure, and decreased cardiac output are definite possibilities. Moreover, second degree Mobitz II block can lead to third degree AV block, which can cause a reduced cardiac output due to bradycardia. With third degree AV block, heart failure and reduced cardiac output are likely, and asystole and sudden death a definite risk.
Associated Disorders and Causes
Cardiac disease, such as myocardial infarction or inflammatory disease, can be associated with various types of AV block. Coronary artery disease and degenerative diseases of the AV node are also sometimes associated with AV block (Saoudi et al., 2001). Autoantibodies have been implicated in congenital heart block (Wahren-Herlenius and Sonesson, 2006).
Atrioventricular (AV) block can be caused by drugs that reduce AV nodal conduction. This includes digitalis, beta-adrenoceptor antagonists, and calcium channel blockers. Various degenerative diseases affecting the AV node can also cause AV block. These may involve fibrosis, calcification, and other processes. Various infections, as well as inflammatory and connective tissue disorders of the heart, can lead to AV block. These include Lyme disease (Pinto, 2002), sarcoidosis, hemochromatosis, and ankylosing spondylitis. Cardiac surgery can affect the AV node and lead to AV block, and hypothyroidism may also be associated with it as well
Epidemiology
Atrioventricular (AV) block occurs more often in the elderly, with more than 5% of otherwise healthy patients over 60 years of age having first degree AV block. Second degree Mobitz I AV block is more common than second degree Mobitz II AV block. Although Mobitz I AV block may occur in about 1–2% of healthy adults, Mobitz type II and third degree AV block are rare in otherwise healthy patients.
Atrioventricular Block☆
FJ Dowd, Creighton University School of Medicine, Omaha, NE, USA
2014 Elsevier Inc. All rights reserved.
Discussion
Atrioventricular (AV) block is a cardiac disorder in which conduction from the atria to the ventricles is delayed in the AV node. Normally, all impulses that travel to the ventricles from the sinoatrial node and atria travel through the AV node, however, reduced AV nodal conductional affects synchronization between atrial depolarizations and ventricular depolarizations (Das and Zipes, 2012).
Therefore, atrioventricular (AV) block is a conduction disturbance in the AV node or His-Purkinje system in which impulses from the atria are excessively delayed in traveling to the ventricles, or fail to reach the ventricles at all.
Classification
Atrioventricular (AV) block is classified as first degree, second degree, and third degree. First degree AV block is characterized by a delay in conduction, resulting in an abnormally long (>0.20 s) PR interval on the electrocardiogram (ECG). In first degree AV block, all impulses from the atria reach the ventricles. Therefore, the PR intervals in first degree AV block usually range from 0.21 to 0.40 s. PR intervals longer than 0.40 s usually lead to impulses that fail to reach the ventricle, a situation that leads to a higher degree of block (Saoudi et al., 2001). In second degree AV block, some, but not all, of the impulses from the atria reach the ventricles. Second degree AV block is subdivided into Mobitz I (Wenchebach block) and Mobitz II block. Mobitz I AV block is characterized by a progressive increase in the PR interval over time until an impulse fails to conduct through the AV node. After this pause in conduction, the process repeats itself.
In Mobitz II AV block, the PR interval is constant and then suddenly a nonconducted impulse follows. The dropped impulse may or may not be at regular intervals. Mobitz type II AV blocks are characterized by their unpredictability (Barold and Hayes, 2001).
In third degree AV block, no impulses reach the ventricles from the atria and there is a complete dissociation between atrial depolarizations and depolarizations in the ventricles (Boutjdir, 2000).
Conduction defects can also exist in the His-Purkinje system, such as a bundle branch block.
Consequences
While first degree atrioventricular (AV) block usually has no immediate consequences, its long-term presence could contribute to some progressive loss of cardiac function Cheng et al., 2009. Second degree Mobitz I AV block has a better prognosis than second degree Mobitz II AV block in that symptoms of the former are generally mild and mortality is usually not increased. For second degree Mobitz II AV block, syncope, heart failure, and decreased cardiac output are definite possibilities. Moreover, second degree Mobitz II block can lead to third degree AV block, which can cause a reduced cardiac output due to bradycardia. With third degree AV block, heart failure and reduced cardiac output are likely, and asystole and sudden death a definite risk.
Associated Disorders and Causes
Cardiac disease, such as myocardial infarction or inflammatory disease, can be associated with various types of AV block. Coronary artery disease and degenerative diseases of the AV node are also sometimes associated with AV block (Saoudi et al., 2001). Autoantibodies have been implicated in congenital heart block (Wahren-Herlenius and Sonesson, 2006).
Atrioventricular (AV) block can be caused by drugs that reduce AV nodal conduction. This includes digitalis, beta-adrenoceptor antagonists, and calcium channel blockers. Various degenerative diseases affecting the AV node can also cause AV block. These may involve fibrosis, calcification, and other processes. Various infections, as well as inflammatory and connective tissue disorders of the heart, can lead to AV block. These include Lyme disease (Pinto, 2002), sarcoidosis, hemochromatosis, and ankylosing spondylitis. Cardiac surgery can affect the AV node and lead to AV block, and hypothyroidism may also be associated with it as well (Saoudi et al., 2001).
Epidemiology
Atrioventricular (AV) block occurs more often in the elderly, with more than 5% of otherwise healthy patients over 60 years of age having first degree AV block. Second degree Mobitz I AV block is more common than second degree Mobitz II AV block. Although Mobitz I AV block may occur in about 1–2% of healthy adults, Mobitz type II and third degree AV block are rare in otherwise healthy patients.
Signs and Symptoms
In first degree atrioventricular (AV) block, there are usually few symptoms. In second and third degree AV block, symptoms may range from giddiness to syncope and, in the case of third degree AV block, asystole and sudden death. An increase in the PR interval or a loss of AV conduction is observed on the ECG. Also on the ECG, second degree Mobitz I AV block is associated with partial dissociation of atrial and ventricular depolarizations and usually a narrow QRS complex. For second degree Mobitz II AV block, the QRS complex is usually wide because the block is usually located in the His-Purkinje system. Third degree AV block can show either a narrow or wide QRS complex, depending on the site of the block.
Standard Therapies
If treatment is required for atrioventricular (AV) block, temporary pacing or a permanent pacemaker is used to control ventricle rate (Curtis et al., 2013; Kusumoto and Goldschlager, 1996). Third degree AV block, and some second degree Mobitz II AV block, are usually treated by pacemaker therapy. Pacemakers are usually not used for second degree Mobitz I AV block and first degree AV block, unless significant symptoms are present. While drug therapy is not often used to treat this condition, there may be reasons to administer drugs to increase AV conduction rates on a temporary basis. Isoproterenol is used to overcome bradycardia, as is atropine if the block is restricted to the AV node.
Atrioventricular Block☆
FJ Dowd, Creighton University School of Medicine, Omaha, NE, USA
2014 Elsevier Inc. All rights reserved.
Discussion
Atrioventricular (AV) block is a cardiac disorder in which conduction from the atria to the ventricles is delayed in the AV node. Normally, all impulses that travel to the ventricles from the sinoatrial node and atria travel through the AV node, however, reduced AV nodal conductional affects synchronization between atrial depolarizations and ventricular depolarizations (Das and Zipes, 2012).
Therefore, atrioventricular (AV) block is a conduction disturbance in the AV node or His-Purkinje system in which impulses from the atria are excessively delayed in traveling to the ventricles, or fail to reach the ventricles at all.
Classification
Atrioventricular (AV) block is classified as first degree, second degree, and third degree. First degree AV block is characterized by a delay in conduction, resulting in an abnormally long (>0.20 s) PR interval on the electrocardiogram (ECG). In first degree AV block, all impulses from the atria reach the ventricles. Therefore, the PR intervals in first degree AV block usually range from 0.21 to 0.40 s. PR intervals longer than 0.40 s usually lead to impulses that fail to reach the ventricle, a situation that leads to a higher degree of block (Saoudi et al., 2001). In second degree AV block, some, but not all, of the impulses from the atria reach the ventricles. Second degree AV block is subdivided into Mobitz I (Wenchebach block) and Mobitz II block. Mobitz I AV block is characterized by a progressive increase in the PR interval over time until an impulse fails to conduct through the AV node. After this pause in conduction, the process repeats itself.
In Mobitz II AV block, the PR interval is constant and then suddenly a nonconducted impulse follows. The dropped impulse may or may not be at regular intervals. Mobitz type II AV blocks are characterized by their unpredictability (Barold and Hayes, 2001).
In third degree AV block, no impulses reach the ventricles from the atria and there is a complete dissociation between atrial depolarizations and depolarizations in the ventricles (Boutjdir, 2000).
Conduction defects can also exist in the His-Purkinje system, such as a bundle branch block.
Consequences
While first degree atrioventricular (AV) block usually has no immediate consequences, its long-term presence could contribute to some progressive loss of cardiac function Cheng et al., 2009. Second degree Mobitz I AV block has a better prognosis than second degree Mobitz II AV block in that symptoms of the former are generally mild and mortality is usually not increased. For second degree Mobitz II AV block, syncope, heart failure, and decreased cardiac output are definite possibilities. Moreover, second degree Mobitz II block can lead to third degree AV block, which can cause a reduced cardiac output due to bradycardia. With third degree AV block, heart failure and reduced cardiac output are likely, and asystole and sudden death a definite risk.
Associated Disorders and Causes
Cardiac disease, such as myocardial infarction or inflammatory disease, can be associated with various types of AV block. Coronary artery disease and degenerative diseases of the AV node are also sometimes associated with AV block (Saoudi et al., 2001). Autoantibodies have been implicated in congenital heart block (Wahren-Herlenius and Sonesson, 2006).
Atrioventricular (AV) block can be caused by drugs that reduce AV nodal conduction. This includes digitalis, beta-adrenoceptor antagonists, and calcium channel blockers. Various degenerative diseases affecting the AV node can also cause AV block. These may involve fibrosis, calcification, and other processes. Various infections, as well as inflammatory and connective tissue disorders of the heart, can lead to AV block. These include Lyme disease (Pinto, 2002), sarcoidosis, hemochromatosis, and ankylosing spondylitis. Cardiac surgery can affect the AV node and lead to AV block, and hypothyroidism may also be associated with it as well (Saoudi et al., 2001).
Epidemiology
Atrioventricular (AV) block occurs more often in the elderly, with more than 5% of otherwise healthy patients over 60 years of age having first degree AV block. Second degree Mobitz I AV block is more common than second degree Mobitz II AV block. Although Mobitz I AV block may occur in about 1–2% of healthy adults, Mobitz type II and third degree AV block are rare in otherwise healthy patients.
Signs and Symptoms
In first degree atrioventricular (AV) block, there are usually few symptoms. In second and third degree AV block, symptoms may range from giddiness to syncope and, in the case of third degree AV block, asystole and sudden death. An increase in the PR interval or a loss of AV conduction is observed on the ECG. Also on the ECG, second degree Mobitz I AV block is associated with partial dissociation of atrial and ventricular depolarizations and usually a narrow QRS complex. For second degree Mobitz II AV block, the QRS complex is usually wide because the block is usually located in the His-Purkinje system. Third degree AV block can show either a narrow or wide QRS complex, depending on the site of the block.
Standard Therapies
If treatment is required for atrioventricular (AV) block, temporary pacing or a permanent pacemaker is used to control ventricle rate (Curtis et al., 2013; Kusumoto and Goldschlager, 1996). Third degree AV block, and some second degree Mobitz II AV block, are usually treated by pacemaker therapy. Pacemakers are usually not used for second degree Mobitz I AV block and first degree AV block, unless significant symptoms are present. While drug therapy is not often used to treat this condition, there may be reasons to administer drugs to increase AV conduction rates on a temporary basis. Isoproterenol is used to overcome bradycardia, as is atropine if the block is restricted to the AV node.
Animal Models
A dog model is used to test electrophysiological aspects of heart block (atrioventricular block) (van-Opstal et al., 2001). Chronic complete heart block in dogs has been shown to lead to myocardial hypertrophy and polymorphic ventricular tachycardia (Schreiner et al., 2004). A porcine model for complete heart block has been used to test gene transfer to create a biological pacemaker in vivo (Hu et al., 2014). In an ovine model, heart block was induced using radio-frequency ablation. This model can be used to test therapies for heart block (Sill et al., 2011).