TheHonesty

First Degree Atrioventricular Block As a rule, the first degree AV block, that is, a prolongation of the PQ interval >0.22s (Figure 21(a)), means a delayed supraventricular spreading of excitation. From: Comprehensive Biomedical Physics, 2014 Related terms: Beta Adrenergic Receptor Blocking Agent Electrocardiogram P Wave PR Interval Heart Failure Bradycardia Atrioventricular Block Complete Heart Block Second Degree Atrioventricular Block View all Topics First-Degree Atrioventricular Block Fred F. Ferri MD, FACP, in Ferri’s Clinical Advisor 2022, 2022 Treatment Treatment (Fig. 2) is indicated in patients with symptoms that correlate with the first-degree AV block or in patients without symptoms, but who are at risk for progression to higher degrees of AV block. The management of first-degree AV block is summarized inTable 1. Nonpharmacologic Therapy In asymptomatic patients without high risk of progression to advanced AV block, treatment includes observation, reassurance, and education. Cardiac pacing is a nonpharmacologic treatment modality for appropriately selected patients (see below). Acute General Rx Patients with symptoms due to reversible causes of AV conduction delay should receive appropriate treatment (e.g., correction of electrolyte abnormalities, antimicrobial therapy of infections associated with AV block, withdrawal of offending medications) of underlying conditions responsible for AV block. Chronic Rx Permanent cardiac pacing may be indicated in patients with symptoms that correlate with the first-degree AV block and persist after correction of reversible causes (class IIA). Patients without symptoms but at risk for progression to higher degrees of AV block also benefit from permanent cardiac pacing (Lamin A/C mutations with PR >240 msec and left bundle branch block class IIA), certain neurodegenerative disorders (including myotonic dystrophy type 1) with PR >240 msec and QRS>120 msec, or fascicular block (class IIB). Disposition AV conduction delay has been shown to be associated with increased risk of all-cause mortality, heart failure, and atrial fibrillation. Referral Patients who are candidates for cardiac pacing should be referred to a cardiologist or cardiac electrophysiologist. View chapter on ClinicalKey Arrhythmias Warren Smith, Margaret Hood, in Cardiothoracic Critical Care, 2007 First-Degree AV Block First-degree AV block is diagnosed when the PR interval exceeds 0.2 seconds (5 small squares on the ECG; Fig. 21-18). It occurs as a consequence of disease in the AV node and is common in older patients. In the context of an aortic root abscess, first-degree AV block with a rapidly lengthening PR interval (over days) may signal the imminent development of complete AV block. In general, no treatment is required for first-degree AV block unless prolongation of the PR interval is extreme (>400 ms) or rapidly evolving, in which case pacing is indicated. Prophylactic antiarrhythmic drug therapy is best avoided in patients with marked first-degree AV block. View chapterPurchase book Bradyarrhythmias and Atrioventricular Block Douglas P. Zipes MD, in Braunwald’s Heart Disease: A Textbook of Cardiovascular Medicine, 2019 First-Degree Atrioventricular Block During first-degree AV block, every atrial impulse is conducted to the ventricles and a regular ventricular rate is produced, but the PR interval exceeds 0.20 second in adults. PR intervals as long as 1.0 second have been noted and can at times exceed the P-P interval, a phenomenon known asskipped P waves. Clinically important PR interval prolongation can result from a conduction delay in the AV node (A-H interval), in the His-Purkinje system (H-V interval), or at both sites. Equally delayed conduction over both bundle branches can infrequently produce PR prolongation without significant QRS complex aberration. On occasion, an intra-atrial conduction delay can result in PR prolongation. If the QRS complex on the scalar ECG is normal in contour and duration, the AV delay almost always resides in the AV node and rarely within the His bundle itself. If the QRS complex shows a BBB pattern, the conduction delay may be within the AV node or the His-Purkinje system (Fig. 40.5). In the latter case, a His bundle ECG is necessary to localize the site of conduction delay. Acceleration of the atrial rate or enhancement of vagal tone by carotid massage can cause first-degree AV nodal block to progress to type I second-degree AV block. Conversely, type I second-degree AV nodal block can revert to a first-degree block with deceleration of the sinus rate. View chapter on ClinicalKey Bradyarrhythmias—Conduction System Abnormalities Brian Olshansky MD, … Nora Goldschlager MD, in Arrhythmia Essentials (Second Edition), 2017 First-Degree Block Description First-degree AVB (Fig. 2.1) represents delay in conduction from the atria to the ventricles and manifests as a prolonged PR interval of more than 0.20 seconds, but all impulses are conducted. The PR interval represents the time from the onset of atrial depolarization due to sinus node activation to the onset of ventricular repolarization (i.e., conduction time from the atrium → AV node [AVN] → His bundle → Purkinje system → ventricles). However, it does not reflect conduction from the sinus node to the atrial tissue. The conduction delay may occur in the atria, AVN, His bundle, and His-Purkinje system. If the QRS complex is narrow and normal appearing, the greatest AV delay usually occurs in the AVN. If the QRS is wide, the conduction delay or block is more likely to occur in the His-Purkinje system than it would be if the QRS were narrow. However, block in the AVN can manifest as a prolonged PR and wide QRS if preexisting bundle branch block (BBB) or rate-dependent aberrancy is present. Diagnosis is usually easy, based on the surface electrocardiogram (ECG) showing a prolonged PR interval. On the ECG during first-degree AVB, the P wave may be buried in the previous QRS complex (especially during sinus rhythm) or the previous T wave and can be difficult to distinguish from a junctional rhythm. If block is in the His-Purkinje system, first-degree AVB is usually associated with BBB. Associated Conditions Spontaneous causes of first-degree AVB include intrinsic disease of the AVN and/or His-Purkinje system), high vagal tone, or dual AV nodal pathways (in which two separate populations of PR intervals may be seen). Other causes include drugs that slow AVN conduction, such as calcium channel blockers, β-adrenergic blockers, or digoxin. Isolated first-degree AVB can occur with anterior or inferior myocardial infarctions (MIs). First-degree AVB is generally self-limited and not associated with progression to complete heart block (CHB), but it may be associated with poorer prognosis in those with heart disease. Exceptions include rheumatic fever (for which it is a sign of acute carditis) and endocarditis (for which it may suggest the presence of a valve ring abscess, especially involving the aortic valve), in which first-degree AVB may presage the development of higher levels of block. Clinical Symptoms and Presentation First-degree AVB does not usually cause symptoms; however, if the PR interval is markedly prolonged (more than 300 to 600 ms), a loss of optimal AV synchrony may reduce the atrial contribution to ventricular filling, therefore decreasing cardiac output, especially in patients with diastolic dysfunction, and lead to symptoms of heart failure. Rarely the PR interval may be sufficiently long to cause nearly simultaneous contraction of the atria and ventricles, resulting in pacemaker syndrome–like symptoms (fatigue, shortness of breath, near syncope, neck fullness, chest pain). Approach to Management There is no specific therapy for first-degree AVB, unless the PR interval is so prolonged as to lead to symptoms. If permanent pacing is considered for symptomatic, very prolonged first-degree AVB, a dual-chamber pacemaker programmed to DDD mode of function but with or without rate response, is the recommended pacing system. His bundle pacing is an option if the level of block is at the AVN. Right ventricular pacing should be minimized because it can cause left ventricular (LV) dyssynchrony; however, His bundle pacing may not be achievable in some patients because it is technically more difficult to achieve than right ventricular apical or outflow tract pacing and is not available in all centers. If the level of block is infra-Hisian, His bundle pacing is not indicated. Another option, not well tested for isolated first-degree AVB, is cardiac resynchronization therapy (CRT) pacing, in which both right and left ventricles are paced. This approach is preferable especially if there is a longstanding need for ventricular pacing for hemodynamic purposes and there is ventricular dysfunction (Table 2.1, Algorithm 2.1). View chapterPurchase book Disorders of Cardiac Rhythm and Conduction in Newborns Richard J. Martin MBBS, FRACP, in Fanaroff and Martin’s Neonatal-Perinatal Medicine, 2020 First-Degree Atrioventricular Block First-degree AV block (FAV) is manifested by prolongation of the PR interval on the ECG while each sinus or atrial beat reaches the ventricles (Fig. 77.13). To make the diagnosis of FAV in the newborn or neonate, the PR interval needs to measure longer than 160 msec. The PR interval reflects the time from initiation of atrial activation, travel time through the atrial tissue and the AV node. Any delay of impulse propagation at any level during this activation will result in prolongation of the PR interval and the diagnosis of FAV. Although this is referred to asfirst-degree heart block, it is not true block within the conduction system, and a better terminology is “prolonged AV conduction.” The AV node, similar to the sinus node, receives innervation from both parasympathetic and sympathetic nervous systems. Increased parasympathetic or vagal tone often leads to delay in impulse conduction through the AV node. This increase in vagal tone is the most common cause of first-degree heart block. A first-degree block frequently resolves when a catecholamine or sympathetic surge is present. The abnormal or nonphysiologic reasons for PR prolongation in a neonate are cardiomyopathies, congenital heart disease (Ebstein anomaly, primum atrial septal defect, and AV canal), and medications like digoxin. No treatment is indicated for first-degree AV block. In cases in which a significant PR prolongation is seen, periodic follow-up may be indicated to ensure that a more advanced block does not develop. An increase in vagal tone that can be seen with reflux or apnea can result in transient prolongation of the PR interval with rapid normalization after the episode. Secondary causes such as this should be evaluated for in patients that experience episodic PR prolongation. In the neonatal period, persistent PR prolongation is uncommon and should warrant evaluation, including an echocardiogram to evaluate for structural abnormalities or myocarditis. View chapter on ClinicalKey Atrioventricular Septal Defects Anisa Chaudhry, … Gruschen R. Veldtman, in Diagnosis and Management of Adult Congenital Heart Disease (Third Edition), 2018 Electrocardiography First-degree AV block is common and thought to be attributable to abnormal internodal conduction time rather than abnormal AV nodal His-Purkinje conduction.22 Leftward axis deviation is the most typical ECG finding. This can be a frank left axis deviation or an extreme leftward axis deviation in the form of superior right axis deviation (common in Down syndrome patients). The superior QRS axis reflects the inferiorly and posteriorly displaced conduction system. A study by Hakkacova et al. demonstrated that imbalance in papillary muscle positioning in primum ASD patients (with the anterior papillary muscle closer to the septum and posterior papillary muscle further from the septum compared with healthy patients) correlated with leftward deviation of the QRS axis (p <0.0007).1,23 Voltage evidence of right ventricular hypertrophy and incomplete right bundle branch block are also commonly seen.21 View chapterPurchase book Disorders of Cardiac Rhythm Frank A. Fish, … James A. Johns, in Pediatric Critical Care (Third Edition), 2006 Conduction Abnormalities First-degree atrioventricular (AV) block is characterized by a prolonged PR interval for age and rate while maintaining 1:1 AV conduction. First-degree AV block usually is exaggerated with increasing heart rate except when related to high vagal tone, in which case it resolves as vagal tone is blocked or diminished. Second-degree block occurs when atrial depolarizations intermittently fail to conduct to the ventricle. Second-degree block can be further characterized as Mobitz type I or Mobitz type II. Mobitz type I, also called Wenckebach conduction, displays progressively prolonged PR intervals (and correspondingly shorter RR intervals) prior to a single nonconducted atrial complex. Wenckebach conduction usually represents block in the AV node, is unlikely to progress suddenly to high-grade block, and in some settings may be benign. Mobitz type II, characterized by abrupt failure to conduct without prior lengthening of the PR interval, usually is attributed to block within the His conduction system and may indicate greater potential for sudden progression to complete AV block. As such, type II block may be more ominous and may require more aggressive intervention (i.e., pacing). In attempting to distinguish type I from type II block, it is useful to compare the PR interval of the first conducted beat following block to the last conducted beat prior to block to best appreciate whether PR prolongation preceded block. Higher grades of second-degree AV block are best characterized by the ratio of atrial to ventricular depolarizations (2:1, 3:1, 4:1, etc.). This ratio of conduction does not imply the level of block or even whether any conduction abnormality exists. Atrial tachyarrhythmias, including atrial flutter, often result in second-degree AV block in those with normal AV nodal function. Likewise, vagally mediated AV block may result in transient high-grade block (although the sinus rate usually slows concurrently). Complete or third-degree AV block represents complete failure of the atrial depolarizations to propagate to the ventricle. As with sinus bradycardias, there may be a junctional or idioventricular rhythm escape rhythm that typically is regular but may be quite slow. Periodic shortening of the RR interval may be the only clue to distinguish complete AV block from high-grade second-degree block with intermittent conduction (Figure 26-1). In complete AV block, ventricular systole may alter the sinus rate (ventriculo-phasic variation). Bundle branch block patterns occur when impaired conduction in the specialized intraventricular conduction system results in delayed right or left ventricular depolarization, resulting in an aberrant QRS complex. Bundle branch block and AV block sometimes represent normal physiologic responses to shortening of the cycle length (as with premature atrial systoles or tachycardia initiation) or may result from increased vagal tone, pharmacologic effects, or primary disease within the specialized conduction tissue. View chapterPurchase book Electrophysiology, Pacing, and Devices Elizabeth A. Stephenson, Andrew M. Davis, in Paediatric Cardiology (Third Edition), 2010 Atrioventricular Block: Definitions First-degree atrioventricular block is PR interval prolongation greater than the upper limit of normal for age. This can be gleaned from normative paediatric electrocardiographic data.262 The nomenclature of second-degree atrioventricular block has a fascinating history and often causes confusion.263 Mobitz type I second-degree atrioventricular block (Wenckebach) is usually associated with a narrow QRS complex and is generally considered benign. However, Mobitz type II second-degree block is not benign, as it is always anatomically infra-nodal and can be associated with less stable escape rhythms. Type I block is classically characterised by progressive prolongation of the PR interval before a single non-conducted P-wave (Fig. 19-13). Type I block, however, is frequently atypical (especially in children) and can be diagnosed when the post-block PR interval is shorter than the pre-block PR interval (Fig. 19-14). In contrast, type II second-degree atrioventricular block is characterised by fixed PR intervals before and after blocked beats and is usually associated with a wide QRS complex (Fig. 19-15). When atrioventricular conduction occurs in a 2:1 pattern, block cannot be classified as type I or type II, although the width of the QRS may be suggestive (Fig. 19-16).264 Advanced second-degree atrioventricular block refers to non-conduction of two or more consecutive P waves but with some conducted beats, indicating some preservation of atrioventricular conduction. Third-degree atrioventricular block (complete heart block) is absence of atrioventricular conduction and characteristically has a regular escape rhythm (Fig. 19-17). View chapterPurchase book Cardiac Arrhythmias and Sleep☆ I. Cakulev, R. Mehra, in Reference Module in Neuroscience and Biobehavioral Psychology, 2017 Atrioventricular Conduction Delay Prolonged atrioventricular (AV) conduction arrhythmias are well documented in the normal population during sleep. Increased parasympathetic activity on the AV node conduction is responsible for various degrees of AV conduction delays. Various degrees of PR prolongation even on a beat-to-beat basis are commonly seen when the conduction varies in the AV node region. Hence, episodes of AV conduction delays that occur at sleep but are not present while awake are almost always due to changes in the conduction properties of the AV node. First-degree block consists of a prolonged PR interval on the ECG of greater than 200 ms (Fig. 5). Second-degree AV block (Wenckebach Mobitz I block) consists of gradual prolongation of the PR interval leading up to a nonconducted P wave (Fig. 6). Both are commonly seen in healthy normal population during sleep and are secondary to elevated parasympathetic activity. This phenomenon is especially prevalent in young and physically fit individuals. Second-degree AV block (Mobitz II) consists of a prolonged PR interval with a dropped QRS complex without the gradual PR prolongation seen in Mobitz I (Fig. 7). Type III AV block occurs when there is no consistent relationship between the P wave and QRS complex (Fig. 8).

First Degree Atrioventricular Block

As a rule, the first degree AV block, that is, a prolongation of the PQ interval >0.22s (Figure 21(a)), means a delayed supraventricular spreading of excitation.

From: Comprehensive Biomedical Physics, 2014

Related terms:

Beta Adrenergic Receptor Blocking Agent

Electrocardiogram

P Wave

PR Interval

Heart Failure

Bradycardia

Atrioventricular Block

Complete Heart Block

Second Degree Atrioventricular Block

View all Topics

First-Degree Atrioventricular Block

Fred F. Ferri MD, FACP, in Ferri’s Clinical Advisor 2022, 2022

Treatment

Treatment (Fig. 2) is indicated in patients with symptoms that correlate with the first-degree AV block or in patients without symptoms, but who are at risk for progression to higher degrees of AV block. The management of first-degree AV block is summarized inTable 1.

Nonpharmacologic Therapy

In asymptomatic patients without high risk of progression to advanced AV block, treatment includes observation, reassurance, and education. Cardiac pacing is a nonpharmacologic treatment modality for appropriately selected patients (see below).

Acute General Rx

Patients with symptoms due to reversible causes of AV conduction delay should receive appropriate treatment (e.g., correction of electrolyte abnormalities, antimicrobial therapy of infections associated with AV block, withdrawal of offending medications) of underlying conditions responsible for AV block.

Chronic Rx

Permanent cardiac pacing may be indicated in patients with symptoms that correlate with the first-degree AV block and persist after correction of reversible causes (class IIA). Patients without symptoms but at risk for progression to higher degrees of AV block also benefit from permanent cardiac pacing (Lamin A/C mutations with PR >240 msec and left bundle branch block class IIA), certain neurodegenerative disorders (including myotonic dystrophy type 1) with PR >240 msec and QRS>120 msec, or fascicular block (class IIB).

Disposition

AV conduction delay has been shown to be associated with increased risk of all-cause mortality, heart failure, and atrial fibrillation.

Referral

Patients who are candidates for cardiac pacing should be referred to a cardiologist or cardiac electrophysiologist.

View chapter on ClinicalKey

Arrhythmias

Warren Smith, Margaret Hood, in Cardiothoracic Critical Care, 2007

First-Degree AV Block

First-degree AV block is diagnosed when the PR interval exceeds 0.2 seconds (5 small squares on the ECG; Fig. 21-18). It occurs as a consequence of disease in the AV node and is common in older patients. In the context of an aortic root abscess, first-degree AV block with a rapidly lengthening PR interval (over days) may signal the imminent development of complete AV block. In general, no treatment is required for first-degree AV block unless prolongation of the PR interval is extreme (>400 ms) or rapidly evolving, in which case pacing is indicated. Prophylactic antiarrhythmic drug therapy is best avoided in patients with marked first-degree AV block.

View chapterPurchase book

Bradyarrhythmias and Atrioventricular Block

Douglas P. Zipes MD, in Braunwald’s Heart Disease: A Textbook of Cardiovascular Medicine, 2019

First-Degree Atrioventricular Block

During first-degree AV block, every atrial impulse is conducted to the ventricles and a regular ventricular rate is produced, but the PR interval exceeds 0.20 second in adults. PR intervals as long as 1.0 second have been noted and can at times exceed the P-P interval, a phenomenon known asskipped P waves. Clinically important PR interval prolongation can result from a conduction delay in the AV node (A-H interval), in the His-Purkinje system (H-V interval), or at both sites. Equally delayed conduction over both bundle branches can infrequently produce PR prolongation without significant QRS complex aberration. On occasion, an intra-atrial conduction delay can result in PR prolongation. If the QRS complex on the scalar ECG is normal in contour and duration, the AV delay almost always resides in the AV node and rarely within the His bundle itself. If the QRS complex shows a BBB pattern, the conduction delay may be within the AV node or the His-Purkinje system (Fig. 40.5). In the latter case, a His bundle ECG is necessary to localize the site of conduction delay. Acceleration of the atrial rate or enhancement of vagal tone by carotid massage can cause first-degree AV nodal block to progress to type I second-degree AV block. Conversely, type I second-degree AV nodal block can revert to a first-degree block with deceleration of the sinus rate.

View chapter on ClinicalKey

Bradyarrhythmias—Conduction System Abnormalities

Brian Olshansky MD, … Nora Goldschlager MD, in Arrhythmia Essentials (Second Edition), 2017

First-Degree Block

Description

First-degree AVB (Fig. 2.1) represents delay in conduction from the atria to the ventricles and manifests as a prolonged PR interval of more than 0.20 seconds, but all impulses are conducted. The PR interval represents the time from the onset of atrial depolarization due to sinus node activation to the onset of ventricular repolarization (i.e., conduction time from the atrium → AV node [AVN] → His bundle → Purkinje system → ventricles). However, it does not reflect conduction from the sinus node to the atrial tissue. The conduction delay may occur in the atria, AVN, His bundle, and His-Purkinje system. If the QRS complex is narrow and normal appearing, the greatest AV delay usually occurs in the AVN. If the QRS is wide, the conduction delay or block is more likely to occur in the His-Purkinje system than it would be if the QRS were narrow. However, block in the AVN can manifest as a prolonged PR and wide QRS if preexisting bundle branch block (BBB) or rate-dependent aberrancy is present. Diagnosis is usually easy, based on the surface electrocardiogram (ECG) showing a prolonged PR interval. On the ECG during first-degree AVB, the P wave may be buried in the previous QRS complex (especially during sinus rhythm) or the previous T wave and can be difficult to distinguish from a junctional rhythm. If block is in the His-Purkinje system, first-degree AVB is usually associated with BBB.

Associated Conditions

Spontaneous causes of first-degree AVB include intrinsic disease of the AVN and/or His-Purkinje system), high vagal tone, or dual AV nodal pathways (in which two separate populations of PR intervals may be seen). Other causes include drugs that slow AVN conduction, such as calcium channel blockers, β-adrenergic blockers, or digoxin. Isolated first-degree AVB can occur with anterior or inferior myocardial infarctions (MIs). First-degree AVB is generally self-limited and not associated with progression to complete heart block (CHB), but it may be associated with poorer prognosis in those with heart disease. Exceptions include rheumatic fever (for which it is a sign of acute carditis) and endocarditis (for which it may suggest the presence of a valve ring abscess, especially involving the aortic valve), in which first-degree AVB may presage the development of higher levels of block.

Clinical Symptoms and Presentation

First-degree AVB does not usually cause symptoms; however, if the PR interval is markedly prolonged (more than 300 to 600 ms), a loss of optimal AV synchrony may reduce the atrial contribution to ventricular filling, therefore decreasing cardiac output, especially in patients with diastolic dysfunction, and lead to symptoms of heart failure. Rarely the PR interval may be sufficiently long to cause nearly simultaneous contraction of the atria and ventricles, resulting in pacemaker syndrome–like symptoms (fatigue, shortness of breath, near syncope, neck fullness, chest pain).

Approach to Management

There is no specific therapy for first-degree AVB, unless the PR interval is so prolonged as to lead to symptoms. If permanent pacing is considered for symptomatic, very prolonged first-degree AVB, a dual-chamber pacemaker programmed to DDD mode of function but with or without rate response, is the recommended pacing system. His bundle pacing is an option if the level of block is at the AVN. Right ventricular pacing should be minimized because it can cause left ventricular (LV) dyssynchrony; however, His bundle pacing may not be achievable in some patients because it is technically more difficult to achieve than right ventricular apical or outflow tract pacing and is not available in all centers. If the level of block is infra-Hisian, His bundle pacing is not indicated. Another option, not well tested for isolated first-degree AVB, is cardiac resynchronization therapy (CRT) pacing, in which both right and left ventricles are paced. This approach is preferable especially if there is a longstanding need for ventricular pacing for hemodynamic purposes and there is ventricular dysfunction (Table 2.1, Algorithm 2.1).

View chapterPurchase book

Disorders of Cardiac Rhythm and Conduction in Newborns

Richard J. Martin MBBS, FRACP, in Fanaroff and Martin’s Neonatal-Perinatal Medicine, 2020

First-Degree Atrioventricular Block

First-degree AV block (FAV) is manifested by prolongation of the PR interval on the ECG while each sinus or atrial beat reaches the ventricles (Fig. 77.13). To make the diagnosis of FAV in the newborn or neonate, the PR interval needs to measure longer than 160 msec. The PR interval reflects the time from initiation of atrial activation, travel time through the atrial tissue and the AV node. Any delay of impulse propagation at any level during this activation will result in prolongation of the PR interval and the diagnosis of FAV. Although this is referred to asfirst-degree heart block, it is not true block within the conduction system, and a better terminology is “prolonged AV conduction.”

The AV node, similar to the sinus node, receives innervation from both parasympathetic and sympathetic nervous systems. Increased parasympathetic or vagal tone often leads to delay in impulse conduction through the AV node. This increase in vagal tone is the most common cause of first-degree heart block. A first-degree block frequently resolves when a catecholamine or sympathetic surge is present. The abnormal or nonphysiologic reasons for PR prolongation in a neonate are cardiomyopathies, congenital heart disease (Ebstein anomaly, primum atrial septal defect, and AV canal), and medications like digoxin.

No treatment is indicated for first-degree AV block. In cases in which a significant PR prolongation is seen, periodic follow-up may be indicated to ensure that a more advanced block does not develop. An increase in vagal tone that can be seen with reflux or apnea can result in transient prolongation of the PR interval with rapid normalization after the episode. Secondary causes such as this should be evaluated for in patients that experience episodic PR prolongation. In the neonatal period, persistent PR prolongation is uncommon and should warrant evaluation, including an echocardiogram to evaluate for structural abnormalities or myocarditis.

View chapter on ClinicalKey

Atrioventricular Septal Defects

Anisa Chaudhry, … Gruschen R. Veldtman, in Diagnosis and Management of Adult Congenital Heart Disease (Third Edition), 2018

Electrocardiography

First-degree AV block is common and thought to be attributable to abnormal internodal conduction time rather than abnormal AV nodal His-Purkinje conduction.22

Leftward axis deviation is the most typical ECG finding. This can be a frank left axis deviation or an extreme leftward axis deviation in the form of superior right axis deviation (common in Down syndrome patients). The superior QRS axis reflects the inferiorly and posteriorly displaced conduction system. A study by Hakkacova et al. demonstrated that imbalance in papillary muscle positioning in primum ASD patients (with the anterior papillary muscle closer to the septum and posterior papillary muscle further from the septum compared with healthy patients) correlated with leftward deviation of the QRS axis (p <0.0007).1,23

Voltage evidence of right ventricular hypertrophy and incomplete right bundle branch block are also commonly seen.21

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Disorders of Cardiac Rhythm

Frank A. Fish, … James A. Johns, in Pediatric Critical Care (Third Edition), 2006

Conduction Abnormalities

First-degree atrioventricular (AV) block is characterized by a prolonged PR interval for age and rate while maintaining 1:1 AV conduction. First-degree AV block usually is exaggerated with increasing heart rate except when related to high vagal tone, in which case it resolves as vagal tone is blocked or diminished.

Second-degree block occurs when atrial depolarizations intermittently fail to conduct to the ventricle. Second-degree block can be further characterized as Mobitz type I or Mobitz type II. Mobitz type I, also called Wenckebach conduction, displays progressively prolonged PR intervals (and correspondingly shorter RR intervals) prior to a single nonconducted atrial complex. Wenckebach conduction usually represents block in the AV node, is unlikely to progress suddenly to high-grade block, and in some settings may be benign. Mobitz type II, characterized by abrupt failure to conduct without prior lengthening of the PR interval, usually is attributed to block within the His conduction system and may indicate greater potential for sudden progression to complete AV block. As such, type II block may be more ominous and may require more aggressive intervention (i.e., pacing). In attempting to distinguish type I from type II block, it is useful to compare the PR interval of the first conducted beat following block to the last conducted beat prior to block to best appreciate whether PR prolongation preceded block.

Higher grades of second-degree AV block are best characterized by the ratio of atrial to ventricular depolarizations (2:1, 3:1, 4:1, etc.). This ratio of conduction does not imply the level of block or even whether any conduction abnormality exists. Atrial tachyarrhythmias, including atrial flutter, often result in second-degree AV block in those with normal AV nodal function. Likewise, vagally mediated AV block may result in transient high-grade block (although the sinus rate usually slows concurrently).

Complete or third-degree AV block represents complete failure of the atrial depolarizations to propagate to the ventricle. As with sinus bradycardias, there may be a junctional or idioventricular rhythm escape rhythm that typically is regular but may be quite slow. Periodic shortening of the RR interval may be the only clue to distinguish complete AV block from high-grade second-degree block with intermittent conduction (Figure 26-1). In complete AV block, ventricular systole may alter the sinus rate (ventriculo-phasic variation).

Bundle branch block patterns occur when impaired conduction in the specialized intraventricular conduction system results in delayed right or left ventricular depolarization, resulting in an aberrant QRS complex. Bundle branch block and AV block sometimes represent normal physiologic responses to shortening of the cycle length (as with premature atrial systoles or tachycardia initiation) or may result from increased vagal tone, pharmacologic effects, or primary disease within the specialized conduction tissue.

View chapterPurchase book

Electrophysiology, Pacing, and Devices

Elizabeth A. Stephenson, Andrew M. Davis, in Paediatric Cardiology (Third Edition), 2010

Atrioventricular Block: Definitions

First-degree atrioventricular block is PR interval prolongation greater than the upper limit of normal for age. This can be gleaned from normative paediatric electrocardiographic data.262

The nomenclature of second-degree atrioventricular block has a fascinating history and often causes confusion.263 Mobitz type I second-degree atrioventricular block (Wenckebach) is usually associated with a narrow QRS complex and is generally considered benign. However, Mobitz type II second-degree block is not benign, as it is always anatomically infra-nodal and can be associated with less stable escape rhythms.

Type I block is classically characterised by progressive prolongation of the PR interval before a single non-conducted P-wave (Fig. 19-13). Type I block, however, is frequently atypical (especially in children) and can be diagnosed when the post-block PR interval is shorter than the pre-block PR interval (Fig. 19-14). In contrast, type II second-degree atrioventricular block is characterised by fixed PR intervals before and after blocked beats and is usually associated with a wide QRS complex (Fig. 19-15). When atrioventricular conduction occurs in a 2:1 pattern, block cannot be classified as type I or type II, although the width of the QRS may be suggestive (Fig. 19-16).264

Advanced second-degree atrioventricular block refers to non-conduction of two or more consecutive P waves but with some conducted beats, indicating some preservation of atrioventricular conduction. Third-degree atrioventricular block (complete heart block) is absence of atrioventricular conduction and characteristically has a regular escape rhythm (Fig. 19-17).

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Cardiac Arrhythmias and Sleep☆

I. Cakulev, R. Mehra, in Reference Module in Neuroscience and Biobehavioral Psychology, 2017

Atrioventricular Conduction Delay

Prolonged atrioventricular (AV) conduction arrhythmias are well documented in the normal population during sleep. Increased parasympathetic activity on the AV node conduction is responsible for various degrees of AV conduction delays. Various degrees of PR prolongation even on a beat-to-beat basis are commonly seen when the conduction varies in the AV node region. Hence, episodes of AV conduction delays that occur at sleep but are not present while awake are almost always due to changes in the conduction properties of the AV node. First-degree block consists of a prolonged PR interval on the ECG of greater than 200 ms (Fig. 5). Second-degree AV block (Wenckebach Mobitz I block) consists of gradual prolongation of the PR interval leading up to a nonconducted P wave (Fig. 6). Both are commonly seen in healthy normal population during sleep and are secondary to elevated parasympathetic activity. This phenomenon is especially prevalent in young and physically fit individuals. Second-degree AV block (Mobitz II) consists of a prolonged PR interval with a dropped QRS complex without the gradual PR prolongation seen in Mobitz I (Fig. 7). Type III AV block occurs when there is no consistent relationship between the P wave and QRS complex (Fig. 8).