AUTHOR AND EDITOR INFORMATION

Section 1 of 11  

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• Follow-up
• Miscellaneous
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INTRODUCTION

Section 2 of 11  

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Background

In the heart, normal impulse initiation begins in the sinoatrial node. The excitation wave then travels through the working atrial myocardium. During this time, surface ECG recordings show the P wave. Following intramyocardial conduction to the area of the lower intra-atrial septum, this impulse reaches the atrioventricular (AV) node. The AV node then conducts the impulse to the His bundle. The his bundle divides into the right and left bundles, which distribute this impulse to the ventricles. During AV node and His-Purkinje conduction, the PR segment is observed. Heart block occurs when slowing or complete block of this conduction occurs. Traditionally, heart block can be divided into first-, second-, and third-degree block.

First-degree AV block is a condition in which a one-to-one relationship exists between P waves and QRS complexes, but the PR interval is longer than 200 millisecond (ms). Thus, first-degree AV block is a misnomer because it is a delay or slowing of conduction rather than a block, per se. Occasionally, first-degree AV block may be associated with other conduction disturbances, including bundle-branch block and fascicular blocks (bifascicular or trifascicular block), and becomes an indication for permanent pacing when patients develop symptoms. In addition, permanent pacing is indicated when the PR interval is markedly prolonged (>300 ms) and the patient has documented left ventricular systolic dysfunction and symptoms of heart failure. In patients with neuromuscular diseases and any degree of AV block, implantation of a pacemaker should be prompted.

Second-degree AV block exists when more P waves than QRS complexes are seen on the ECG, but a relationship between P waves and QRS complexes still exists. In other words, not all P waves are followed by QRS complexes (conducted). Traditionally, this type of AV block is divided into subcategories as type I and type II.

Type I second-degree AV block is also known as Wenckebach block. In type I block, the PR interval is changing (most frequently prolonging) with each P wave to the point when the P wave is no longer conducted (followed by QRS complexes). In a typical Wenckebach block, the degree of PR prolongation is greatest in the first interval and progressively less with subsequent intervals. This results in shortening of the R-R interval. Also, the R-R interval enveloping the pause is less than twice the duration of the first R-R interval following the pause. Upon examination of the ECG tracing, this block results with the appearance of grouping beats. This type of AV block is not infrequently seen in athletes, most likely resulting from increased vagal tone. It resolves spontaneously after physical deconditioning.

Type II second-degree AV block is also known as Mobitz II block. In type II block, the PR interval is constant, but occasionally P waves are not followed by the QRS complexes (nonconducted). Occasionally, the first PR interval following nonconducted P waves may be shorter by 20 ms.

To differentiate between type I block and type II block, at least 3 consecutive P waves must be present in the tracing. If only every other P wave is conducted (2:1), this block cannot be classified into either of these categories. Frequently, Wenckebach block is related to a disease process within the proximal portion of the AV node, tends to respond to atropine, and may resolve. On the other hand, Mobitz II block usually results from the infrahisian block (process involving the His-Purkinje system), does not respond to atropine, and has a poor prognosis. Although the morphological criteria are somewhat helpful in ascertaining the location of the block in the conduction system, only an invasive electrophysiologic study is able to precisely determine the location of the block. All blocks resulting from prolongation of the HV interval on intracardiac tracing carry a poor prognosis and are an indication for permanent pacing.

An AV block resembling second-degree AV block has been reported with sudden surges of vagal tone associated with cough, hiccups, swallowing, micturition, or airway manipulation in otherwise healthy subjects. The distinguishing feature is simultaneous slowing of the sinus rate. This condition is paroxysmal and benign but must be carefully differentiated from a true second-degree AV block because the prognosis is very different.

Third-degree AV block (also called complete heart block) exists when more P waves than the QRS complexes exist and no relationship exists between them (no conduction). Typically, the escape rhythm is wide complex and the heart rate varies from 30-40 bpm. Characteristically in this block, the atrial rate is 110 bpm. In most cases of third-degree AV block, permanent pacing is required.

Atrioventricular dissociation is present when atrial and ventricular activation are independent of each other. It can result from complete heart block or physiological refractoriness of conduction tissue. It can occur in a situation when the atrial/sinus rate is slower than the ventricular rate, namely with accelerated junctional tachycardia and ventricular tachycardia. Occasionally, the atrial and ventricular rates are so close that the tracing would suggest normal AV conduction; only careful examination of the long rhythm strip may reveal a variation in PR interval. This form of AV dissociation is called isorhythmic AV dissociation. Maneuvers or medications resulting in acceleration of atrial/sinus rate will result in restoration of normal conduction.

Pathophysiology

Heart block results from various pathological states causing infiltration, fibrosis, or loss of connection in portions of the healthy conduction system. Acute myocardial infarction (MI) can cause complete AV block. Common causes of heart block are listed in Causes.

Frequency

United States

The prevalence of third-degree AV block is 0.02%.

International

The prevalence of third-degree AV block is 0.04%.

Mortality/Morbidity

Third-degree AV block may be an underlying condition in patients who present with sudden cardiac death.

Age

The incidence of AV conduction abnormalities increases with advancing age, resembling the age-related incidence of ischemic heart disease. An early peak in incidence occurs in infancy and early childhood due to congenital complete AV block, which is sometimes not recognized until childhood or even adolescence.

CLINICAL

Section 3 of 11  

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• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• Multimedia
• References

History

• Symptoms attributable to complete AV block include syncope, near-syncope, lightheadedness, fatigue, dyspnea, and angina.
• Some patients are asymptomatic.
• Third-degree AV block may be an underlying condition in patients who present with sudden cardiac death.

Physical

• Initial triage of patients with complete heart block consists of determining symptoms, assessing vitals signs, and looking for evidence of compromised peripheral perfusion.
• Careful examination of the neck veins can often show evidence of cannon a waves. A variable intensity S₁ may be heard. In addition, the pulse rate may be slow.
• If the slow rate or loss of atrial contraction prior to ventricular contraction has caused heart failure, then venous pressures will be elevated, including the jugular venous pressure.
• Any new murmurs or gallops should be noted because strong associations exist between cardiomyopathies, mitral calcification, aortic calcification, or endocarditis and complete AV block.
• If heart failure is present as evidenced by rales, an S₃ gallop, peripheral edema, or hepatomegaly, then a more compelling need for immediate pacing exists.
• Because endocarditis and Lyme disease cause heart block, pay attention to any signs of infection during the general examination. This is particularly true in endemic areas for Lyme disease.
• Neurologic examination may provide clues to the etiology of AV block because neuromuscular disease, especially myotonic dystrophy and Duchenne muscular dystrophy, can cause AV block.

Causes

• A phenomenon known as ventriculophasic sinus arrhythmia (P-P intervals) refers to the P-P interval surrounding a QRS complex being slightly shorter than the other P-P intervals.
• Lev disease (calcification of the conduction system and valves) has been found to have an inherited basis.
• The common causes of AV heart block are as follows:
• • Drugs - Calcium channel blockers, beta-blockers, quinidine, procainamide
  • Degenerative diseases – Lenègre disease (sclerodegenerative process involving only the conduction system) and Lev disease
  • Infectious causes - Lyme borreliosis (particularly in endemic areas), rheumatic fever, myocarditis, Chagas disease (Central America and South America), Aspergillus myocarditis
  • Rheumatic diseases - Ankylosing spondylitis, Reiter syndrome, relapsing polychondritis, rheumatoid arthritis, scleroderma
  • Infiltrative processes - Amyloidosis, sarcoidosis, tumors, Hodgkin disease, multiple myeloma
  • Neuromuscular disorders - Becker muscular dystrophy, myotonic muscular dystrophy
  • Ischemic or infarctive causes - AV nodal block associated with interior wall MI, His-Purkinje block associated with anterior wall MI
  • Metabolic causes - Hypoxia, hyperkalemia

DIFFERENTIALS

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Other Problems to be Considered

Ischemia should always be in the differential for a patient with new-onset, high-degree AV block. Take simple measures to rule out ischemia, such as 12-lead ECG and measurement of cardiac enzyme levels. If warranted, a more in-depth evaluation, including perfusion imaging, may be needed.

Iatrogenic heart block due to medications is not uncommon and should always be considered.

WORKUP

Section 5 of 11  

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Lab Studies

• Routine laboratory testing should include a measure of the serum potassium level, prothrombin time and activated partial thromboplastin time, and CBC count. If the patient is taking a drug (eg, digoxin) potentially responsible for the condition, the drug level should be measured.
• The presence of fever or an elevated WBC count should be evaluated by using blood cultures because endocarditis can be complicated by heart block.
• The decision to perform serologic test for Lyme disease or any of the collagen vascular diseases depends on other associated history and findings.

Imaging Studies

• If examination findings or history suggests cardiomyopathy or valvular disease, then a transthoracic echocardiogram should be performed; a routine echocardiogram in all patients is not inappropriate.

Other Tests

• If history or 12-lead ECG findings suggest active coronary artery disease, then cardiac enzyme levels measurements and an evaluation of ischemia, including either cardiac catheterization or stress testing, are needed.
• The most important study is the 12-lead ECG.
• • Complete lack of conduction (no P waves cause a QRS complex) characterizes third-degree heart block.
  • If complete AV block exists, then the R-R interval is very regular; therefore, before diagnosing third-degree AV block, the R-R interval should be either marched out or measured.
  • If high-grade AV block exists without complete heart block, then some irregularity may occur during intervals following conducted P waves.
  • The various pathologies causing conduction system disease and heart block are listed in Causes. These systemic or myocardial diseases rarely present as conduction block, with the exception of Lyme disease, inferior MI, and some of the neuromuscular diseases. Unless suggested by history, examination findings, family history, risk factors, or 12-lead ECG findings, the authors do not screen for underlying pathology.
  • Surface ECG and review of prior ECG data can provide important clues to the level of third-degree AV block. The physician can begin with a review of the current QRS width and morphology, comparing QRS during heart block to that when conducted (see Image 1). If the QRS is narrow (<120 ms) during conducted beats and narrow with the same morphology during escape beats, then the block is in the AV junction. If the conducted QRS was narrow at baseline and is wide during the escape rhythm, then this is likely a distal level of block located anatomically in the His bundle or both the right and left bundles.

Procedures

• Ambulatory monitoring may be performed to document heart-transient heart block or other bradyarrhythmias in patients presenting with symptoms suggestive of bradycardia.
• Diagnostic electrophysiologic studies can be performed to assess AV conduction and to discern the level of block (AV nodal or infranodal) when necessary.

TREATMENT

Section 6 of 11  

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Medical Care

New-onset third-degree AV block is a medical emergency. The treatment is based on the level of the block. A common misconception of an inexperienced physician is to gauge a patient's stability based on the heart rate and blood pressure rather than the symptoms and level of the block. An asymptomatic patient with inferior wall MI causing complete heart block at the AV nodal level and a heart rate of 35 bpm is at very little immediate risk. A patient in the acute phase of an anterior wall MI with intermittent distal high-grade block is at immediate danger of impending asystole and requires immediate preparation for pacing of some kind, even though the heart rate between asystolic episodes might be 90 bpm.

The first, and sometimes most important, medical treatment for heart block is the withdrawal of any potentially aggravating or causative medications. Many antihypertensive, antianginal, antiarrhythmic, and heart failure medications cause AV block that resolves after withdrawal of the offending agent. Medical treatment of complete heart block is limited to patients with conduction disease in the AV node.

• Initial efforts should focus on assessing the need for temporary pacing and initiating the pacing.
• Once the patient has been stabilized, a decision must be made regarding permanent pacemaker implantation.
• Unless the heart block is due to a medication that can be discontinued or an infectious process that can be effectively treated, most patients with acquired complete heart block should receive permanent pacemakers or implantable cardioverter/defibrillators (ie, if a high risk of sudden cardiac death exists).

Surgical Care

Complete heart block associated with repeated pauses, inadequate escape rhythm, or block below the AV node should be stabilized with temporary pacing. Although the transcutaneous pacer should be placed on all patients, this mode of pacing is not highly reliable and is extremely uncomfortable for the patient. When assessing capture with transcutaneous pacing, the common mistake of looking for electrical capture on the monitor should be avoided. The pacing artifact is usually large and that QRS complex can rarely be seen reliably. Instead, palpation for the pulse is the best indication of capture. The decision to place a transvenous pacing wire depends on the availability of fully trained personnel and equipment for placing a transvenous wire. All patients with persistent block below the AV node should be prepared for temporary wire placement.

The ultimate decision whether to place a permanent pacemaker in patients with persistent heart block without a reversible cause depends on many factors. A clinical statement from the American College of Cardiology, the American Heart Association, and the Heart Rhythm Society was published in 2002. This paper outlined the indications for permanent pacing.

Summary of the indications for permanent pacing (ie, pacing for acquired AV block in adults)

• Class I - Conditions for which evidence and/or general agreement exists that a given procedure or treatment is beneficial, useful, and effective
• • Third-degree AV block and advanced second-degree AV block at any anatomic level associated with any one of the following conditions:
  • • Bradycardia with symptoms (including heart failure) presumed to be due to AV block
    • Arrhythmias and other medical conditions that require drugs resulting in symptomatic bradycardia
    • Documented periods of asystole greater than or equal to 3 seconds or any escape rate less than 40 bpm in awake symptom-free patients
    • After catheter ablation of the AV junction (Pacing is virtually always planned in this situation unless the operative procedure is AV junction modification.)
    • Postoperative AV block that is not expected to resolve after cardiac surgery
    • Neuromuscular diseases with AV block, such as myotonic muscular dystrophy, Kearns-Sayre syndrome, Erb dystrophy (limb-girdle), and peroneal muscular atrophy, with or without symptoms, because unpredictable progression of AV conduction disease may occur
  • Second-degree AV block, regardless of type or site of block, with associated symptomatic bradycardia
• Class II - Conditions for which conflicting evidence and/or a divergence of opinion about the usefulness or efficacy of a procedure or treatment exists
• • Class IIa - Weight of evidence or opinion is in favor of usefulness or efficacy
  • • Asymptomatic third-degree AV block at any anatomic site with average awake ventricular rates of 40 bpm or faster, especially if cardiomegaly or left ventricular (LV) dysfunction is present
    • Asymptomatic type II second-degree AV block with a narrow QRS (When type II second-degree AV block occurs with a wide QRS, pacing becomes a class I recommendation.)
    • Asymptomatic type I second-degree AV block at intra- or infra-His levels found at electrophysiologic study performed for other indications
    • First- or second-degree AV block with symptoms similar to those of pacemaker syndrome
  • Class IIb - Usefulness and efficacy are less well established by evidence or opinion
  • • Marked first-degree AV block (>0.3 s) in patients with LV dysfunction and symptoms of congestive heart failure in whom a shorter AV interval results in hemodynamic improvement, presumably by decreasing left atrial filling pressure
    • Neuromuscular diseases, such as myotonic muscular dystrophy, Kearns-Sayre syndrome, Erb dystrophy (limb-girdle), and peroneal muscular atrophy, with any degree of AV block (including first-degree AV block), with or without symptoms, because unpredictable progression of AV conduction disease may occur
• Class III - Conditions for which evidence and/or general agreement exists that a procedure or treatment is not useful or effective and, in some cases, may be harmful
• • Asymptomatic first-degree AV block
  • Asymptomatic type I second-degree AV block at the supra-His (AV node) level or not known to be intra- or infra-Hisian
  • AV block expected to resolve and/or unlikely to recur (eg, drug toxicity, Lyme disease, during hypoxia in sleep apnea syndrome in absence of symptoms)

Recommendations for permanent pacing in chronic bifascicular and trifascicular block

• Class I
• • Intermittent third-degree AV block
  • Type II second-degree AV block
  • Alternating bundle-branch block
• Class IIa
• • Syncope not demonstrated to be due to AV block when other likely causes have been excluded, specifically ventricular tachycardia
  • Upon electrophysiologic study, incidental finding of markedly prolonged HV interval (>100 ms) in asymptomatic patients
  • Upon electrophysiologic study, incidental finding of pacing-induced infra-His block that is not physiologic

• Class IIb - Neuromuscular diseases, such as myotonic muscular dystrophy, Kearns-Sayre syndrome, Erb dystrophy (limb-girdle), and peroneal muscular atrophy, with any degree of fascicular block, with or without symptoms, because unpredictable progression of AV conduction disease may occur.
• Class III
• • Fascicular block without AV block or symptoms
  • Fascicular block with first-degree AV block without symptoms

Consultations

• A cardiologist or electrophysiologist should be involved in the treatment of all patients with complete heart block.

Activity

Bed rest is mandatory for patients with complete heart block. Bed rest minimizes the chance of significant injury if syncope occurs due to ventricular asystole.

• A temporary pacemaker may be required.
• Because these patients have decreased cardiac output, assuming upright posture after prolonged periods in the supine position can lead to orthostatic hypotension with syncope or near-syncope.

MEDICATION

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Review patient medication lists upon presentation to help rule out medication-induced or medication-aggravated heart block. Common drugs that induce AV block include beta-blockers, calcium channel blockers, antiarrhythmics, and digoxin. Withdrawal of offending drugs is the first treatment for heart block. Patients with block at the AV nodal level, in absence of ischemia, can benefit from sympathomimetic agents or vagolytic agents.

Drug Category: Sympathomimetic agents or vagolytic agents

Increase heart rate through vagolytic effects, causing an increase in cardiac output.

FOLLOW-UP

Section 8 of 11  

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Further Inpatient Care

• Patients can be discharged from the hospital the following day after pacemaker implantation. Routinely, chest radiography is required before discharge.

Further Outpatient Care

• Routine postpacemaker care is necessary. This includes transtelephonic checks every 2 months and office visits for pacemaker interrogation every 6-12 months. In the initial period post implantation, these visits are more frequent.

Complications

• Some patients may develop polymorphic ventricular tachycardia when significant bradycardia is present. This is related to prolongation of repolarization with extremely slow rates. This mechanism is also mostly responsible for death in these patients.
• The complications related to pacemaker insertion are rare (<1%).

Prognosis

• When treated with permanent pacing, the prognosis is excellent.

MISCELLANEOUS

Section 9 of 11  

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Medical/Legal Pitfalls

• Failure to correctly diagnose heart block and treat appropriately
• Failure to inform patients about regulations regarding driving for patients with syncope, which vary from state to state

MULTIMEDIA

Section 10 of 11  

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• Miscellaneous
• Multimedia
• References

Media file 1:  ECG before and after complete heart block at the AV nodal level.
	View Full Size Image
Media type:  ECG

Media file 2:  Complete heart block with wide complex escape.
	View Full Size Image
Media type:  ECG

REFERENCES

Section 11 of 11

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

• American Heart Association, American Academy of Pediatrics. Emergency Cardiovascular Care Programs: Pediatric Advance Life Support. Dallas, Tex:. American Heart Association;1997.
• Barold SS, Hayes DL. Second-Degree Atrioventricular Block: A Reappraisal. Mayo Clinic Proceedings. 2001;76:44-57. [Medline].
• Gregoratos G, Abrams J, Epstein AE, et al. ACC/AHA/NASPE 2002 guideline update for implantation of cardiac pacemakers and antiarrhythmia devices: summary article: a report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines. Circulation. Oct 15 2002;106(16):2145-61. [Medline].
• Kojic EM, Hardarson T, Sigfusson N, Sigvaldason H. The prevalence and prognosis of third-degree atrioventricular conduction block: the Reykjavik study. J Intern Med. Jul 1999;246(1):81-6. [Medline].
• McEnvoy GK, ed. AHFS Drug Information 2000. Bethesda, Md:. American Society of Health-System Pharmacists;2000:1187-95.
• Ostaner LD, Brandt RL, Kjelsberg MI, et al. Electrocardiographic findings among the adult population of a total natural community. 1965;31:888-98.
• Rardon DA, Miles WM, Mitrani RD, et al. Electrocardiographic Recognition: Atrioventricular Block and Dissociation. In: Zipes DP, Jalife J, eds. Cardiac Electrophysiology From Cell to Bedside, 2nd ed. Philadelphia, Pa:. WB Saunders;1995.

Article Last Updated: Jan 18, 2006