eMedicine Specialties > Emergency Medicine > Cardiovascular

Heart Block, Second Degree

Author: Michael D Levine, MD, Physician, Department of Medical Toxicology, Banner Good Samaritan Medical Center
Coauthor(s): David FM Brown, MD, Assistant Professor, Division of Emergency Medicine, Harvard Medical School; Vice Chair, Department of Emergency Medicine, Massachusetts General Hospital
Contributor Information and Disclosures

Updated: Nov 30, 2009

Introduction

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, as shown in the image below.

An electrocardiogram of a patient with Mobitz I (...

An electrocardiogram of a patient with Mobitz I (Wenckebach) second-degree AV block.

[ CLOSE WINDOW ]
An electrocardiogram of a patient with Mobitz I (...

An electrocardiogram of a patient with Mobitz I (Wenckebach) second-degree AV block.


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, as shown in the image below. Thus, the PR and R-R intervals between conducted beats are constant.1,2

An electrocardiogram of a patient with Mobitz II ...

An electrocardiogram of a patient with Mobitz II second-degree AV block.

[ CLOSE WINDOW ]
An electrocardiogram of a patient with Mobitz II ...

An electrocardiogram of a patient with Mobitz II second-degree AV block.


Pathophysiology

Mobitz type I block is caused by conduction delay in the AV node in 70% of patients and by conduction delay in the His-Purkinje system in the remaining 30%. 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.1

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.3

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.4 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.4,1 Mobitz I blocks localized to the His-Purkinje system are associated with the same risks as type II blocks.

Clinical

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.1
  • 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).5,6
  • Mobitz II block most commonly is caused by an acute myocardial infarction (anterior or inferior). Drug-induced etiologies can also occur.7
  • Both Mobitz I and Mobitz II can be seen with Lyme disease.8

More on Heart Block, Second Degree

Overview: Heart Block, Second Degree
Differential Diagnoses & Workup: Heart Block, Second Degree
Treatment & Medication: Heart Block, Second Degree
Follow-up: Heart Block, Second Degree
Multimedia: Heart Block, Second Degree
References
Further Reading
[ CLOSE WINDOW ]

References

  1. Barold SS, Hayes DL. Second-degree atrioventricular block: a reappraisal. Mayo Clin Proc. Jan 2001;76(1):44-57. [Medline].

  2. Silverman ME, Upshaw CB Jr, Lange HW. Woldemar Mobitz and His 1924 classification of second-degree atrioventricular block. Circulation. Aug 31 2004;110(9):1162-7. [Medline].

  3. 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].

  4. 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].

  5. 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].

  6. [Guideline] 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].

  7. Belem Lde S, Inacio CA. Second degree atrioventricular block Mobitz type I after administration of benzathine penicillin: case report. Rev Bras Anestesiol. Mar-Apr 2009;59(2):219-22. [Medline].

  8. Haddad FA, Nadelman RB. Lyme disease and the heart. Front Biosci. Sep 1 2003;8:s769-82. [Medline].

  9. [Guideline] 2005 American Heart Association Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiovascular Care. Part 7.3: Management of Symptomatic Bradycardia and Tachycardia. Circulation. 2005;112:IV-67–IV-77. [Full Text].

[ CLOSE WINDOW ]

Further Reading

Clinical guidelines

Adult basic life support: 2005 International Consensus Conference on Cardiopulmonary Resuscitation and Emergency Cardiovascular Care Science with Treatment Recommendations.Adult basic life support. In: 2005 International Consensus Conference on Cardiopulmonary Resuscitation and Emergency Cardiovascular Care Science with Treatment Recommendations. Circulation 2005 Nov 29;112(22 Suppl):III5-16.

Advanced life support: 2005 International Consensus Conference on Cardiopulmonary Resuscitation and Emergency Cardiovascular Care Science with Treatment Recommendations. Advanced life support. In: 2005 International Consensus Conference on Cardiopulmonary Resuscitation and Emergency Cardiovascular Care Science with Treatment Recommendations. Circulation 2005 Nov 29;112(22 Suppl):III25-54.

[ CLOSE WINDOW ]

Keywords

atrioventricular block, second-degree atrioventricular block, AV block, A-V block, second-degree AV block, second-degree A-V block, Mobitz I, Mobitz I heart block, Mobitz I atrioventricular block, Mobitz I AV block, Mobitz I A-V block, Mobitz II, Mobitz II heart block, Mobitz II atrioventricular block, Mobitz II AV block, Mobitz II A-V block, second-degree heart block, atrial impulses, cardiac conduction system, nonconducted atrial impulse

[ CLOSE WINDOW ]

Contributor Information and Disclosures

Author

Michael D Levine, MD, Physician, Department of Medical Toxicology, Banner Good Samaritan Medical Center
Michael D Levine, MD is a member of the following medical societies: Alpha Omega Alpha, American College of Emergency Physicians, American College of Medical Toxicology, American Medical Association, Emergency Medicine Residents Association, Phi Beta Kappa, and Society for Academic Emergency Medicine
Disclosure: Nothing to disclose.

Coauthor(s)

David FM Brown, MD, Assistant Professor, Division of Emergency Medicine, Harvard Medical School; Vice Chair, Department of Emergency Medicine, Massachusetts General Hospital
David FM Brown, MD is a member of the following medical societies: American College of Emergency Physicians and Society for Academic Emergency Medicine
Disclosure: Nothing to disclose.

Medical Editor

Theodore J Gaeta, DO, MPH, FACEP, Clinical Associate Professor, Department of Emergency Medicine, Joan and Sanford Weill Medical College at Cornell University; Vice Chairman and Program Director of Emergency Medicine Residency Program, Department of Emergency Medicine, New York Methodist Hospital; Academic Chair, Adjunct Professor, Department of Emergency Medicine, St George's University School of Medicine
Theodore J Gaeta, DO, MPH, FACEP is a member of the following medical societies: Alliance for Clinical Education, American College of Emergency Physicians, Clerkship Directors in Emergency Medicine, Council of Emergency Medicine Residency Directors, New York Academy of Medicine, and Society for Academic Emergency Medicine
Disclosure: Nothing to disclose.

Pharmacy Editor

Francisco Talavera, PharmD, PhD, Senior Pharmacy Editor, eMedicine
Disclosure: eMedicine Salary Employment

Managing Editor

Eddy Lang, MDCM, CCFP (EM), CSPQ, Assistant Professor, Department of Family Medicine, McGill University; Consulting Staff, Department of Emergency Medicine, The Sir Mortimer B Davis-Jewish General Hospital
Eddy Lang, MDCM, CCFP (EM), CSPQ is a member of the following medical societies: American College of Emergency Physicians
Disclosure: Nothing to disclose.

CME Editor

John D Halamka, MD, MS, Associate Professor of Medicine, Harvard Medical School, Beth Israel Deaconess Medical Center; Chief Information Officer, CareGroup Healthcare System and Harvard Medical School; Attending Physician, Division of Emergency Medicine, Beth Israel Deaconess Medical Center
John D Halamka, MD, MS is a member of the following medical societies: American College of Emergency Physicians, American Medical Informatics Association, Phi Beta Kappa, and Society for Academic Emergency Medicine
Disclosure: Nothing to disclose.

Chief Editor

David FM Brown, MD, Assistant Professor, Division of Emergency Medicine, Harvard Medical School; Vice Chair, Department of Emergency Medicine, Massachusetts General Hospital
David FM Brown, MD is a member of the following medical societies: American College of Emergency Physicians and Society for Academic Emergency Medicine
Disclosure: Nothing to disclose.

 
 
HONcode

We subscribe to the
HONcode principles of the
Health On the Net Foundation

All material on this website is protected by copyright, Copyright© 1994- by Medscape.
This website also contains material copyrighted by 3rd parties.

DISCLAIMER: The content of this Website is not influenced by sponsors. The site is designed primarily for use by qualified physicians and other medical professionals. The information contained herein should NOT be used as a substitute for the advice of an appropriately qualified and licensed physician or other health care provider. The information provided here is for educational and informational purposes only. In no way should it be considered as offering medical advice. Please check with a physician if you suspect you are ill.