AUTHOR AND EDITOR INFORMATION

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• Follow-up
• Miscellaneous
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INTRODUCTION

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Background

Atrioventricular block (AVB) refers to a condition of delayed or interrupted sinus or atrial impulses through the atrioventricular node (AVN) to the ventricles. This may be transient or permanent, and the anatomic level at which it occurs can vary. These 2 features determine the clinical significance of AVB, which may range from minimal to severely symptomatic, including Stokes-Adams syncope, congestive failure, or sudden death.

Depending on the anatomic level at which it occurs, AVB may be classified as first-, second-, or third-degree. In addition, second-degree AVB may be described as a transmission ratio of impulses from the atrium to the ventricle (eg, 2:1, 3:2). If this conduction ratio is higher than 2:1, the term advanced, or high-grade, second-degree AVB may be used.

First-degree AVB is a misnomer because nothing is actually blocked. However, the term is used when prolongation occurs in the conduction time of impulses from the atrium to the ventricle. On the surface ECG, the observable feature is a lengthened PR interval compared to the reference range for age and heart rate; however, this does not distinguish the anatomic level of delay. Anatomic locations of delay can be intra-atrial, atrioventricular (AV) nodal, intra-His bundle, or infra-His bundle. Intracardiac electrograms can be obtained to determine the level of delay, such as prolongation of low septal right atrial–to–His (LSRA-H) interval, split His potentials (H-H'), or prolongation of His-ventricle (H-V) conduction. Prolongation of the H-V interval may predispose individuals to complete (ie, third-degree) AVB.

Second-degree AVB occurs either when successive impulses from the atrium are progressively delayed to the ventricles until failure of conduction occurs (ie, Mobitz type I second-degree AVB, Wenckebach periodicity) or when an atrial impulse fails to conduct to the ventricles without any measurable lengthening of preceding PR intervals (ie, Mobitz type II second-degree AVB). With classic Wenckebach, a progressive prolongation of the PR interval before the nonconducted P wave occurs. This is associated with a maximum increase in the PR interval between the first and second conducted impulses, then decreasing increments of conduction delay, a decremental shortening of the PR interval before the pause, and finally a pause shorter than twice the PR interval between 2 successive conducted impulses. Multiple atrial impulses may be blocked for each that is conducted. An intracardiac ECG can show variable prolongation of the atrium-His (A-H) interval.

In type II second-degree AVB, the surface ECG demonstrates normal PR intervals preceding the QRS with abrupt failure of a P wave to conduct. The intracardiac electrograms may show sudden prolongation of A-H or H-V intervals. If block occurs in the more distal regions of the His-Purkinje system, the QRS of conducted impulses may be abnormal, demonstrating a bundle branch configuration or intraventricular conduction delay.

In complete (ie, third-degree) AVB (also termed complete heart block), the conduction of sinus or atrial impulses is interrupted entirely at the level of the AVN, in the bundle of His, or is associated with an aberrantly located anatomical AVN that precludes normal anatomical communication to the distal conduction tissue. The atria and ventricles depolarize independently of each other, and the ventricular rate is typically slower than the atrial rate. Surface ECG may demonstrate either narrow or wide QRS configurations, depending on the location of the interruption and the foci of escape pacemaker tissue (ie, narrow if supra-His, wide if intra- or infra-His bundle). Escape ventricular rhythms with a narrow QRS may be more stable than those with wide QRS morphologies.

Pathophysiology

Acquired AVB has multiple etiologies, including normal variations in vagal tone, associated congenital cardiac defects, direct injury to the conduction system, infections, genetic causes, and metabolic disorders. In each instance, first-, second-, or third-degree block may result.

Increased vagal tone causing first- or second-degree AVB can commonly be observed during sleep with vagal stimulation, such as carotid or ocular massage, or following exercise. Typically, such a finding requires no therapy. Surgical repair of certain congenital cardiac defects in which suture or incisional damage occurs to the healthy conduction tissue may transiently or permanently damage the conduction tissue. Certain congenital defects may be inherently associated with AVB because of anatomical alteration in the arrangement of the AVN to the proximal His-Purkinje conduction tissue. These defects include double-outlet right ventricle {S,L,L} or L-transposition of the great arteries {S,L,L}, AV canal defects in the setting of heterotaxy syndrome, and other congenital heart lesions.

Infectious agents causing myocardial inflammation and infiltration (eg, viral, bacterial, spirochete), ischemia from acquired coronary disease (eg, Kawasaki), or congenital abnormalities causing inferior wall infarcts can transiently or permanently damage the conduction tissue, leading to variable degrees of AVB.

Myotonic muscular dystrophy, an autosomal dominant disorder of skeletal muscle and conduction abnormalities due to trinucleotide repeat gene mutations, is a genetic disorder associated with acquired heart block. Additionally, one of the mitochondrial DNA mutations leads to Kearns-Sayre syndrome, a disorder clinically characterized by findings of progressive oculomotor weakness, progressive AV conduction block, retinal abnormalities, and multiple endocrinopathies.

Frequency

United States

The overall prevalence of postsurgical complete AVB has been reduced to 5%.

Age

In children, the most common cause of permanent acquired complete AVB is postsurgical, followed by congenital heart disease associated with complete AVB. Other etiologies of acquired AVB are often reversible and include digitalis and other drug intoxications, viral myocarditis, acute rheumatic fever, Lyme disease, and infectious mononucleosis.

In adults, other causes of complete AVB include myocardial infarction (especially inferior), coronary spasm (usually of the right coronary artery), neoplasms (eg, cardiac mesotheliomas), and the etiologies mentioned for the pediatric group. Hypertension and aortic and/or mitral stenosis are believed to accelerate the degeneration of the conducting system through calcification and fibrosis. Almost exclusive to adult patients are 2 degenerative diseases of the specialized conducting system. In Lev disease, calcification and sclerosis of the cardiac skeleton frequently involve the mitral and aortic valves, the central fibrous body, and the summit of the ventricular septum. Lenègre disease is thought to represent a primary sclerodegenerative disease of the conducting system with no involvement of the myocardium or fibrous skeleton of the heart.

CLINICAL

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History

• Most pediatric patients with postoperative complete AVB have had intracardiac surgery for atrial septal defect (ASD) repair, ventricular septal defect (VSD) repair, valve repair or replacements, or other complex congenital heart surgery.
• • Some patients with surgical complete AVB develop weakness, syncope, or congestive heart failure (CHF); others may be asymptomatic. Most of these patients exhibit complete AVB by the time cardiopulmonary bypass ends, and a few develop block in the first weeks after surgery. Fortunately, many recover AV conduction within the first 7-10 postoperative days.
  • Rarely, postsurgical patients develop complete AVB months or years after surgery. Late recovery of AV conduction is less common.
• Symptoms consist of dizziness, exercise intolerance, syncope, failure to thrive (in infants), and CHF. Patients with infranodal block tend to be more symptomatic than those with higher block sites because low intrinsic escape pacemakers are slower and less reliable than higher-level intrinsic pacemakers.

Physical

Patients are bradycardic for their age. However, if the escape heart rate is rapid enough to maintain adequate cardiac output and cerebral perfusion, patients are asymptomatic. These individuals usually present with heart rates of 50-60 beats per minute (bpm).

Conversely, if the heart rate is inadequate to maintain blood flow to the brain (usually <50 bpm in children), patients can have syncopal episodes. If the escape pacemaker is inadequate and asystole is prolonged (eg, 10 s), sudden death may ensue.

Newborns with complete congenital AVB may have presentations ranging from asymptomatic with heart rates increasing to 100 bpm to being hydropic because of CHF, or they may even be stillborn with hydrops.

Causes

• Postsurgical complete AVB is the most common cause for acquired AVB in children, resulting from trauma to the AVN at the time of surgery (ie, hemorrhage, ischemia, necrosis, inflammation, traumatic disruption). It may occur following repairs, including those for VSD, L-transposition of the great arteries (especially), tetralogy of Fallot (TOF), ASD (especially ostium primum ASD), and atrial baffle procedures for complete transposition of the great arteries (although rare). These patients not only have very slow heart rates, but they are also prone to asystole, with a high risk for sudden death.
• Complete AVB may occur with systemic infections, such as diphtheria, Borrelia burgdorferi infection (Lyme disease), Chagas disease, Rocky Mountain spotted fever, Yersinia enterocolitica infection, infectious mononucleosis, bacterial endocarditis, and viral myocarditis.
• • Diphtheria is an infection of mucous membranes or skin caused by Corynebacterium diphtheriae, of which certain strains produce the diphtheria toxin. This protein can cause myocarditis, polyneuritis, and other systemic effects. Myocarditis occurs in 10-25% of patients with diphtheria and may develop during the acute phase or after several weeks. This consists of varying degrees of heart block, including complete AVB and arrhythmias, such as atrial fibrillation, premature ventricular beats, ventricular tachycardia, and ventricular fibrillation.
  • Lyme disease is caused by the tick-transmitted spirochete B burgdorferi. It begins with an expanding skin lesion known as erythema migrans. Within several days or weeks, the spirochete spreads hematogenously, and approximately 8% of patients develop cardiac involvement. The most common abnormality is fluctuating degrees of AVB (ie, first-, second-, and third-degree AVB). Other more diffuse forms of cardiac involvement include left ventricular dysfunction, cardiomegaly, or pancarditis. Cardiac involvement usually lasts for a few weeks, although it may recur.
  • Chagas disease is an endemic zoonosis in certain regions of Central and South America. It is caused by the protozoan Trypanosoma cruzi and is transmitted by hematophagous triatomae insects (ie, reduviids). These insects become infected after sucking blood that contains parasites from vertebrate hosts (eg, birds, mammals). Parasites reproduce in the guts of reduviids, and infective forms are discharged with feces at the time of subsequent blood meals. Therefore, the second host becomes infected with parasites because of skin abrasions that have been contaminated with infected feces. Symptomatic chronic Chagas disease becomes apparent years or even decades after infection. Right bundle branch block is the most common ECG abnormality, but other types of AVB, including complete AVB, are frequently observed. Other forms of rhythm disturbances include premature ventricular contractions, tachyarrhythmias, and bradyarrhythmias. Cardiomyopathy results in right-sided or biventricular heart failure.
• Rheumatic fever is an inflammatory disease that occurs from pharyngeal infection with group A streptococci, with multisystemic involvement. In acute rheumatic carditis, all the layers of the heart may be compromised. Even though most patients with carditis are asymptomatic, some have valvular damage of the mitral and/or aortic valves that may produce regurgitation and even CHF. In terms of rhythm disturbance in patients with carditis, tachycardia disproportionate to the degree of fever and varying degrees of heart block may occur. The most common type of AVB is first degree.
• Reiter syndrome is a seronegative arthropathy that may be accompanied by pancarditis with involvement of the AVN (ie, varying degrees of AVB) and proximal aortitis with valve regurgitation. Both of these features are more common in patients with long-standing disease and peripheral joint involvement.
• Myotonic dystrophy forms 1 and 2 are neuromuscular diseases inherited in an autosomal dominant fashion, with genetic anticipation, in which cardiac involvement consists of disorders of impulse formation and, especially, AV conduction. Caused by an expanded CTG repeat in the 3-prime untranslated region of the DMPK gene and by an expanded CCTG repeat in intron 1 of the ZNF9 gene, respectively, they are typically progressive, beginning as asymptomatic PR prolongation and leading to complete AVB. Syncope and sudden death may occur; therefore, symptomatic patients require insertion of a permanent pacemaker. Diagnosis is mainly clinical.
• Kearns-Sayre syndrome is caused by a mitochondrial DNA deletion. Its characteristic triad includes progressive external ophthalmoplegia, pigmentary degeneration of the retina, and progressive cardiac conduction defects, including complete heart block. Other features may include ataxia, hearing loss, dementia, short stature, delayed secondary sexual characteristics, hypoparathyroidism, hypothyroidism, and peripheral neuropathy. The diagnosis is usually made clinically and by demonstration of ragged red fibers on skeletal muscle biopsy.
• Tuberous sclerosis is a neurocutaneous disorder caused by mutations in any of 4 separate genetic loci in which cutaneous lesions of multiple types are associated with tumors and malformations of the CNS. Mental retardation may be severe, and patients develop intractable seizures. Cardiac rhabdomyomas within the AVN may cause complete AVB and different types of rhythm disturbances, such as ventricular tachycardia when localized in the ventricles. Diagnosis is clinical. The earliest lesions are leaf-shaped hypopigmented spots scattered over the trunk and limbs; they are observed better using a Wood lamp than with the naked eye.
• The term cardiac mesothelioma refers to heterotopic epithelial replacement of the AV node. It is a rare entity and, thus, a rare cause for complete AVB. Cases of congenital complete AVB and complete AVB in adults secondary to cardiac mesotheliomas have been reported. Definitive diagnosis is made only at autopsy, where necropsy may show extensive infiltration of the AV node and proximal bundle by mesothelioma tissue. Therefore, mesothelioma of the AV node is a rare cause of supra-His AV block, arrhythmias, and sudden death.
• Amyloidosis is a systemic disease in which abnormal production of immunoglobulins leads to their deposit in various tissues such as tongue, intestines, skeletal and smooth muscles, nerves, skin, ligaments, heart, liver, spleen, and kidneys. When the heart is affected, cardiomyopathy with marked left ventricular wall thickening, severely decreased systolic and diastolic function, and complete AVB may ensue.
• Sarcoidosis is an inflammatory systemic disease characterized by formation of granulomas that most frequently affect the lungs, lymph nodes, skin, eyes, and liver. However, it may affect any organ. When it affects the heart, it may cause different types of conduction disturbance, from bundle branch block to complete AVB. Interestingly, AVB may be reversible. Sudden death has been described.
• Primary cardiac lymphoma has been described to cause complete AVB. Case reports of sinus rhythm restoration after tumor remission have been described.
• Drugs such as digoxin, beta-blockers, and calcium channel blockers may cause transient complete AVB, which can be fatal if the ventricular escape rhythm is inadequate to maintain brain blood flow or if asystole is prolonged.
• Clonidine, tricyclic antidepressants, penetrating chest trauma, and radiation may cause complete AVB.
• Mediastinal radiation for lymphomas such as Hodgkin has been described to cause complete AVB.

DIFFERENTIALS

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Other Problems to be Considered

AV dissociation is secondary to sinus or atrial bradycardia with a faster escape rhythm, in which appropriately timed atrial impulses conduct to the ventricles. The R-R interval changes when a sinus/atrial beat is conducted, as opposed to complete AVB where R-R intervals do not vary.

AV dissociation with second-degree AVB varies from occasionally dropped sinus/atrial beats to occasionally conducted beats. In this last example, it may be hard to distinguish from complete AVB.

Transient postsurgical complete AVB is caused by edema of adjacent tissues. This resolves after 1-2 weeks.

WORKUP

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Lab Studies

• Lyme disease is diagnosed by its clinical picture and serologic confirmation. Serologic test findings may be negative during the first several weeks, but most patients have a positive antibody response to B burgdorferi by enzyme-linked immunoabsorbent assay (ELISA). In addition, B burgdorferi may be cultured from skin lesions during the acute phase or from skin lavage as it has been recently described.
• Diagnosis of acute Chagas disease is made by microscopic examination of fresh anticoagulated blood to visualize the parasites. Whenever this technique is unsuccessful, xenodiagnosis can yield positive test results in virtually all patients with acute Chagas disease and in half of those with chronic disease. In this technique, uninfected reduviids feed on the patient's blood and, 30 days later, their intestinal contents are examined for the presence of parasites. In case of chronic Chagas disease, serologic tests to detect antibodies against T cruzi antigens, such as complement fixation and ELISA, are used.
• Definitive diagnosis of diphtheria is made by isolation of C diphtheriae from local lesions.
• The diagnosis of Reiter syndrome is mainly clinical because no specific laboratory tests are used for this entity. However, nonspecific findings include elevated sedimentation rate and C-reactive protein, elevated immunoglobulin A (IgA), mild normochromic and normocytic anemia, and absent rheumatoid factor and antinuclear antibodies.
• Perform laboratory testing for lymphoma, amyloidosis, sarcoidosis, and digoxin as indicated.

Other Tests

• Electrophysiology study
• • Intracardiac electrophysiologic (EP) studies are usually not necessary for many patients with AVB, but, when performed, the EP results can be used to determine whether the AVB is above, within, or below the bundle of His. Patients with infra-His AVB have a much slower ventricular rate (ie, heart rate) and require permanent pacemaker therapy to avoid asystole.
    • The block is located in the AVN above the His bundle when the ventricular depolarization is preceded by a His depolarization, which, in turn, is not coupled to the atrial depolarization.
    • The block is within the His bundle if the His bundle depolarization is preceded by an atrial depolarization and a second His bundle depolarization precedes a ventricular depolarization.
    • The block is below the His bundle when the His depolarization follows an atrial depolarization but does not precede a ventricular depolarization.
  • Adequacy of the junctional or ventricular pacemaker can be evaluated by pacing the ventricles at different rates for 30-60 seconds, stopping abruptly, and measuring their recovery time (ie, from the last paced ventricular beat to the first spontaneous ventricular beat). A junctional or ventricular recovery time exceeding 3 seconds in children is abnormal and has been associated with risk for sudden death.
  • EP studies are not required for patients with complete AVB and symptoms such as dizziness, syncope, or CHF. These patients require permanent pacemaker therapy.

TREATMENT

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Medical Care

Treatment is focused on restoring AV sequential activation and/or maintaining a heart rate tolerated by the patient, which is assessed by the absence of symptoms.

• In symptomatic patients, perform cardiac compressions and administer adrenergic agonists (to accelerate the escape rhythm) while preparing for temporary cardiac pacing. Temporary cardiac pacing can be transcutaneous, transesophageal, or transvenous. The ultimate goal is permanent cardiac pacing.
• In postoperative patients with intermittent AVB, externalized temporary cardiac pacing wires that can be attached to an external temporary pulse generator set at a predetermined rate to maintain adequate cardiac output is typically required.
• In postoperative patients with persistent complete AVB lasting more than 7 days within or below the bundle of His, permanent pacemaker therapy is currently indicated.
• If the escape rhythm is less than 50 bpm in infants or 45 bpm in adolescents, permanent pacemaker therapy may be indicated to prevent symptoms of CHF.
• • Pacing is indicated in patients with complete AVB who have exercise intolerance.
  • Patients with postoperative complete AVB and those with Kearns-Sayre syndrome require prophylactic pacemaker therapy before symptoms develop because of the high risk for sudden death from asystole (>60% in some series).
  • Prophylactic pacemaker therapy is indicated for any patient with complete AVB with a wide QRS escape rhythm.
• Asymptomatic patients require no immediate pacemaker treatment, although these individuals should be monitored closely. If the escape rhythm slows, they may become symptomatic and require permanent pacemaker therapy.
• Acquired AVB from myocarditis, Lyme disease, and surgically induced trauma caused by adjacent tissue edema in patients with structurally normal hearts is usually transient and may not require therapy or may require only temporary pacing. However, in 40-56% of postsurgical patients, complete AVB persists beyond 10-14 days, and pacemaker therapy is indicated. Recognizing Lyme disease is important because appropriate antibiotic therapy for 10-20 days with tetracyclines, erythromycin, IV penicillin, or ceftriaxone can revert the complete AVB (in addition to preventing rheumatologic and neurologic symptoms).

Activity

Patient activity should continue as tolerated.

MEDICATION

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Drug therapy is currently not a component of the standard of care for this condition.

FOLLOW-UP

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Further Outpatient Care

• Asymptomatic patients require no immediate pacemaker treatment, although these individuals should be monitored closely.

Prognosis

• Most patients with postoperative complete AVB recover AV conduction within the first 7-10 postoperative days.

MISCELLANEOUS

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Medical/Legal Pitfalls

• Postoperative complete AVB has a high risk of sudden death. Regardless of symptoms or underlying escape rate, these patients should always receive a permanent pacemaker system if the AVB persists more than 8-14 days and if no contraindications to pacemaker implantation exist.

MULTIMEDIA

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Media file 1:  This is an example of a normal finding on intracardiac electrophysiologic (EP) study. The surface ECG is represented in different colors, with its corresponding intervals (ie, PR, QT) on top. A catheter with several electrodes is placed inside the heart, close to the superior vena cava–right atrial junction. This catheter records the sinoatrial node (SN) activity and is depicted here as the high-right atrial (HRA) deflection. Beneath the HRA intracardiac electrogram is the His-bundle intracardiac electrogram, which is recorded by the electrodes of a second catheter placed across the posterior aspect of the tricuspid valve. The His-bundle electrogram provides the most information about AV conduction. Three main deflections are present, with 2 intervals: the A deflection corresponds to the activation of the low-right atrium, the H deflection corresponds to the activation of the His-bundle prior to its branching into the Purkinje system, and the V deflection corresponds to the activation of theproximal portion of the right ventricle. The atrium-His (A-H) interval represents the conduction time through the AV node. It shows the time elapsed between the activation of the low-right atrium (A) and the activation of the His-bundle (H), ranging normally from 50-120 milliseconds. The His-ventricle (H-V) interval is measured from the beginning of the H deflection to the beginning of the V deflection, and it represents the conduction time through the His-Purkinje system (normally 35-55 ms). Disease in the AV node prolongs the A-H interval, whereas disease in the distal conducting system prolongs the H-V interval.
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Media type:  Graph

Media file 2:  This is a Mobitz type II second-degree atrioventricular (AV) block. The surface ECG shows normal PR intervals and a P wave that is not followed by a QRS (in this graphic, the first P wave does not conduct through the AV node). The intracardiac electrogram shows no His deflection (H) after the blocked A deflection. In this case, the escape rhythm originates higher in the AV node at a rate of 40-50 beats per minute and is fairly reliable. However, patients may report symptoms of bradycardia such as dizziness, fatigue, and syncope. Because this type of AV block may progress to complete or third-degree AV block, patients should be monitored regularly even in the absence of symptoms.
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Media type:  Graph

Media file 3:  This is a Mobitz type II second-degree atrioventricular (AV) block that may likely progress to a third-degree, or complete, AV block. The difference from Image 2 is that, in this case, an H deflection is present after the A deflection (the atrium-His [A-H] interval is maintained); however, no V deflection is present after the first H deflection. Therefore, in this case, the escape rhythm is slower than in the patient in Image 2 (<40/min) and less reliable. This patient is more likely to receive a pacemaker because of the higher incidence of sudden death secondary to prolonged asystole.
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Media type:  Graph

Media file 4:  This is a 12-lead ECG of a 2-year-old girl with first-degree atrioventricular (AV) block that progressed to a complete, or third-degree, AV block (which is shown here). Her mother brought her to the clinic with described symptoms of easy tiredness and refusal to walk more than 1 block, which was a dramatic change for this girl. A normal sinus rhythm is present (shown by upward P waves in leads I, II, and aVF) at a rate of 135 per minute, which is completely dissociated from the QRS at a rate of 67 per minute. The QRS is narrow at 100 milliseconds with a frontal axis of 62°. No ventricular hypertrophy is present by voltage criteria. Because of the narrow QRS and its escape rate, this ECG is interpreted as complete AV block with junctional escape rhythm.
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Media type:  ECG

Media file 5:  This is a 12-lead ECG of a 2-year-old girl with first-degree atrioventricular (AV) block that progressed to a complete, or third-degree, AV block (see Image 4). This is after dual chamber (DDD-R) pacemaker placement. Sinus P waves are present at a rate of 90 per minute, followed by a pacemaker spike that produces a wide QRS of 128 milliseconds. No spike occurs before each P wave because this type of pacemaker senses the patient's own P waves and stimulates the ventricle afterwards. Therefore, the patient's ventricular rate follows her physiologic needs by tracking the patient's own atrial rate. With a DDD-R pacemaker, if the patient develops sinus bradycardia, the pacemaker takes over and paces the right atrium at the programmed rate, which is followed by the ventricular stimulation, maintaining AV synchrony.
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Media type:  ECG

REFERENCES

Section 11 of 11

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
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• Medication
• Follow-up
• Miscellaneous
• Multimedia
• References

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• Kawano H, Okada R, Kawano Y, et al. Mesothelioma in the atrioventricular node. Case report. Jpn Heart J. Mar 1994;35(2):255-61. [Medline].
• Kiviniemi MS, Pirnes MA, Eranen HJ, et al. Complications related to permanent pacemaker therapy. Pacing Clin Electrophysiol. May 1999;22(5):711-20. [Medline].
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Article Last Updated: Aug 7, 2006