AUTHOR AND EDITOR INFORMATION

Section 1 of 10  

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

INTRODUCTION

Section 2 of 10  

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

Background

Second-degree heart block, or second-degree atrioventricular (AV) block, refers to a disorder of the cardiac conduction system in which some atrial impulses are not conducted to the ventricles. Electrocardiographically, some P waves are not followed by a QRS complex. Second-degree AV block is composed of 2 types: Mobitz I or Wenckebach block, and Mobitz II.

The Mobitz I second-degree AV block is characterized by a progressive prolongation of the PR interval, which results in a progressive shortening of the R-R interval. Ultimately, the atrial impulse fails to conduct, a QRS complex is not generated, and there is no ventricular contraction. The PR interval is the shortest in the first beat in the cycle, while the R-R interval is the longest in the first beat in the cycle.

The Mobitz II second-degree AV block is characterized by an unexpected nonconducted atrial impulse. Thus, the PR and R-R intervals between conducted beats are constant.

Pathophysiology

Mobitz type I block is caused by conduction delay in the AV node in 72% of patients and by conduction delay in the His-Purkinje system in the remaining 28%. The presence of a narrow QRS complex suggests the site of the delay is more likely to be in the AV node. However, a wide QRS complex may be observed with either AV nodal or infranodal conduction delay.

In Mobitz type II block, the conduction delay occurs infranodally. The QRS complex is likely to be wide, except in patients where the delay is localized to the bundle of His.

Frequency

United States

In the United States, the prevalence of second-degree heart block in young adults is reported to be 0.003%. However, the rate is significantly higher among trained athletes, occurring in 2.4% of athletes undergoing routine ECGs.

Mortality/Morbidity

Mobitz type I second-degree AV block is localized to the AV node, and thus is not associated with any increased risk of morbidity or death, in the absence of organic heart disease. In addition, when the block is localized to the AV node, no risk of progression to a type II second-degree block or complete heart block exists. However, when a Mobitz type I block occurs during an acute myocardial infarction, mortality is increased. Mobitz type II blocks do carry a risk of progressing to complete heart block, and thus are associated with an increased risk of mortality. Mobitz I blocks localized to the His-Purkinje system are associated with the same risks as type II blocks.

CLINICAL

Section 3 of 10  

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

History

• Mobitz I (Wenckebach) block
• • Most patients are asymptomatic.
  • Patients may experience light-headedness, dizziness, or syncope, but these symptoms are uncommon.
  • Patients may have chest pain if the heart block is related to myocarditis or ischemia.
  • Patients may have a history of structural heart disease.
• Mobitz II block
• • Unlike Mobitz I block, patients with type II block are more likely to experience light-headedness, dizziness, or syncope, although they may be asymptomatic as well.
  • Patients may have chest pain if the heart block is related to myocarditis or ischemia.

Physical

• Patients often have a regularly irregular heartbeat.
• Bradycardia may be present.
• Symptomatic patients may have signs of hypoperfusion, including hypotension.

Causes

• Mobitz I block can occur in individuals with high vagal tone, such as athletes or young children.
• Mobitz I block can occur in infants and young children with structural heart disease (eg, tetralogy of Fallot) and in individuals of any age following valvular surgery (especially mitral valve).
• Other causes of type I block include myocardial infarction (especially inferior wall), and drug induced (including beta-blockers, calcium channel blockers, amiodarone, digoxin, and possibly pentamidine).
• Mobitz II block most commonly is caused by an acute myocardial infarction (anterior or inferior). Drug-induced etiologies can also occur.

DIFFERENTIALS

Section 4 of 10  

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

Other Problems to be Considered

Heart block, congenital

WORKUP

Section 5 of 10  

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

Lab Studies

• Serum electrolytes, calcium, and magnesium levels should be checked.
• A digoxin level should be obtained for patients on digoxin.
• Cardiac enzymes tests are indicated for any patient with suspected myocardial ischemia.
• Myocarditis-related laboratory studies (eg, Lyme titers, HIV serologies, enterovirus polymerase chain reaction [PCR], adenovirus PCR, Chagas titers), if clinically relevant.

Imaging Studies

• Routine imaging studies are not required. However, if myocarditis is a concern, an ECHO may be indicated. If myocardial ischemia is a concern, a chest radiograph may be indicated.

Other Tests

• Follow-up ECGs and cardiac monitoring are appropriate.

TREATMENT

Section 6 of 10  

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

Prehospital Care

• Second-degree heart block in the asymptomatic patient does not require any specific therapy in the prehospital setting.
• If the patient is symptomatic, standard advanced cardiac life support (ACLS) guidelines for bradycardia, including the use of atropine and transcutaneous pacing, is indicated.

Emergency Department Care

• Mobitz I (Wenckebach) block
• • No specific therapy is required in the ED, unless the patient is symptomatic.
  • Patients with suspected myocardial ischemia should be treated with an appropriate anti-ischemic regimen.
  • AV nodal blocking agents (including beta-blockade) should be avoided.
  • Symptomatic patients should be treated with atropine and transcutaneous pacing. However, atropine should be administered with caution in patients with suspected myocardial ischemia, as ventricular dysrhythmias can occur in this situation.
• Mobitz II block
• • As with type I block, AV nodal agents should be avoided, and an anti-ischemic regimen should be instituted if ischemia is suspected.
  • Transcutaneous pacing pads should be applied to all patients, including asymptomatic patients, as patients with Mobitz II second-degree AV block have a propensity to progress to complete heart block. The transcutaneous pacemaker should be tested to ensure capture. If capture is not able to be achieved, then insertion of a transvenous pacemaker is indicated, even in asymptomatic patients.
  • Urgent cardiology consult is indicated for patients who have symptomatic type II block and for those asymptomatic patients who are unable to achieve capture with transcutaneous pacing.
  • Some institutions recommend insertion of a transvenous pacemaker for all new Mobitz type II blocks, although this practice varies greatly from institution to institution.
  • Patients who are hemodynamically unstable for whom an emergent cardiology consult is not available should undergo placement of a temporary transvenous pacing wire in the ED. A chest radiograph is required to confirm position of the wire and to exclude complications, including hemothorax or pneumothorax.

Consultations

• For symptomatic patients with Mobitz I block, a cardiology consultation is indicated. Asymptomatic patients with a Mobitz I block can be referred to a cardiologist on an outpatient basis.
• For any patient with a new Mobitz II block, cardiology consultation is indicated, regardless of symptoms.

MEDICATION

Section 7 of 10  

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

The goal of therapy is to improve conduction through the AV node by reducing vagal tone via atropine-induced receptor blockade. However, this goal will only be effective if the level of the blockade is at the site of the AV node. Patients with infranodal second-degree heart block are unlikely to benefit from atropine. In addition, in patients who have deinervated hearts (eg, patients who have a cardiac transplant), atropine is also not likely to be effective.

Drug Category: Anticholinergic

Drug therapy in second-degree heart block is aimed at vagolysis; atropine is the only currently recommended agent.

FOLLOW-UP

Section 8 of 10  

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

Further Inpatient Care

• Mobitz I (Wenckebach) block
• • Admit the patient with symptoms or with concomitant acute myocardial ischemia/infarction.
  • Admission should be to a unit with telemetry monitoring, which has transcutaneous pacing capabilities.
• Mobitz II block
• • Admit all patients to a unit with monitored beds, where transcutaneous and transvenous pacing capabilities are available.
  • The admitting cardiologist should determine whether permanent pacemaker implantation is indicated.

Further Outpatient Care

• Patients who are discharged from the ED with Wenckebach block should have prompt follow-up arranged with a cardiologist.

Prognosis

• Wenckebach block carries a good prognosis with appropriate follow-up and treatment.
• Mobitz II block is more dangerous, as it progresses more frequently to complete heart block with hemodynamic instability. In addition, it is associated with myocardial infarction, and all its attendant risks.

MULTIMEDIA

Section 9 of 10  

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

Media file 1:  An electrocardiogram of a patient with Mobitz I (Wenckebach) second-degree AV block.
	View Full Size Image
Media type:  ECG

Media file 2:  An electrocardiogram of a patient with Mobitz II second-degree AV block.
	View Full Size Image
Media type:  ECG

REFERENCES

Section 10 of 10

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

• American Heart Association. Guidelines for cardiopulmonary resuscitation and emergency cardiac care. Emergency Cardiac Care Committee and Subcommittees, American Heart Association. Part III. Adult advanced cardiac life support. JAMA. Oct 28 1992;268(16):2199-241. [Medline].
• Antoniou T, Gough KA. Early-onset pentamidine-associated second-degree heart block and sinus bradycardia: case report and review of the literature. Pharmacotherapy. Jun 2005;25(6):899-903. [Medline].
• Ariyarajah V, Asad N, Tandar A, Spodick DH. Interatrial block: pandemic prevalence, significance, and diagnosis. Chest. Aug 2005;128(2):970-5. [Medline].
• Meimoun P, Zeghdi R, D'Attelis N, Berrebi A, Braunberger E, Deloche A. Frequency, predictors, and consequences of atrioventricular block after mitral valve repair. Am J Cardiol. May 1 2002;89(9):1062-6. [Medline].
• Stein R, Medeiros CM, Rosito GA, Zimerman LI, Ribeiro JP. Intrinsic sinus and atrioventricular node electrophysiologic adaptations in endurance athletes. J Am Coll Cardiol. Mar 20 2002;39(6):1033-8. [Medline].
• Strasberg B, Amat-Y-Leon F, Dhingra RC, Palileo E, Swiryn S, Bauernfeind R. Natural history of chronic second-degree atrioventricular nodal block. Circulation. May 1981;63(5):1043-9. [Medline].
• Syverud S. Cardiac pacing. Emerg Med Clin North Am. May 1988;6(2):197-215. [Medline].
• Zehender M, Meinertz T, Keul J, Just H. ECG variants and cardiac arrhythmias in athletes: clinical relevance and prognostic importance. Am Heart J. Jun 1990;119(6):1378-91. [Medline].

Article Last Updated: Sep 5, 2006