eMedicine Specialties > Emergency Medicine > Cardiovascular

Heart Block, First Degree

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

Updated: Mar 9, 2009

Introduction

Background

On an electrocardiogram (ECG), the PR interval is defined as the time interval between the initial deflection of the P wave to the start of the QRS complex. Normally, this interval should be between 120 and 200 msec. First-degree heart block, or first-degree atrioventricular (AV) block, is defined as prolongation of the PR interval on the ECG to more than 200 msec.1 First-degree heart block is considered "marked" when the PR exceeds 300 msec.2  While the conduction is slowed, there are no missed beats.

ECG in a patient with first-degree heart block.

ECG in a patient with first-degree heart block.

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ECG in a patient with first-degree heart block.

ECG in a patient with first-degree heart block.


Pathophysiology

With first-degree atrioventricular (AV) block, every atrial impulse is transmitted to the ventricles, resulting in a regular ventricular rate. This type of AV block can arise from delays in the conduction system in the AV node itself, the His-Purkinje system, or a combination of both. Overall, dysfunction at the AV node is much more common than dysfunction at the His-Purkinje system. If the QRS complex is of normal width and morphology on the ECG, then the conduction delay is almost always at the level of the AV node. If, however, the QRS demonstrates a bundle-branch morphology, then the level of the conduction delay is often localized to the His-Purkinje system.

Occasionally, the conduction delay can be the result of an intra-atrial conduction defect. Some causes of atrial disease resulting in a prolonged PR interval include endocardial cushion defects and Ebstein anomaly.3

Frequency

United States

In the United States, the prevalence of first-degree atrioventricular (AV) block among young adults ranges from 0.65-1.6%. Higher prevalence is reported in studies of trained athletes. First-degree AV block is more common among African Americans compared with Caucasian populations. The prevalence of first-degree AV block increases with advancing age.4

Mortality/Morbidity

In and of itself, first-degree AV block is a benign condition, with no associated increase in morbidity or mortality.

Clinical

History

  • Patients may have a history of past heart disease, including myocarditis or myocardial infarction (MI).
  • Patients may be highly conditioned athletes with a high degree of vagal tone, or they may be on medications that slow conduction through the AV node.
  • A history of an infectious disease, such as Lyme disease, may be present.

Physical

No findings on the physical examination are associated with first-degree AV block; it is generally an incidental finding noted on an ECG.

Causes

  • The following are the most common causes of first-degree atrioventricular (AV) block:
    • Intrinsic AV nodal disease
    • Enhanced vagal tone
    • Acute MI, particularly acute inferior wall myocardial infarction (MI)
    • Myocarditis
    • Electrolyte disturbances (eg, hypokalemia, hypomagnesemia) 
    • Drugs (especially those drugs that increase the refractory time of the AV node, thereby slowing conduction)
  • Drugs that most commonly cause first-degree AV block include the following:
    • Class Ia antiarrhythmics (eg, quinidine, procainamide, disopyramide)
    • Class Ic antiarrhythmics (eg, flecainide, encainide, propafenone)
    • Class II antiarrhythmics (beta-blockers)
    • Class III antiarrhythmics (eg, amiodarone, sotalol, dofetilide, ibutilide)
    • Class IV antiarrhythmics (calcium channel blockers)
    • Digoxin or other cardiac glycosides
    • Magnesium

More on Heart Block, First Degree

Overview: Heart Block, First Degree
Differential Diagnoses & Workup: Heart Block, First Degree
Treatment & Medication: Heart Block, First Degree
Follow-up: Heart Block, First Degree
Multimedia: Heart Block, First Degree
References
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References

  1. John AD, Fleisher LA. Electrocardiography: the ECG. Anesthesiol Clin. Dec 2006;24(4):697-715, v-vi. [Medline].

  2. Barold SS, Ilercil A, Leonelli F, Herweg B. First-degree atrioventricular block. Clinical manifestations, indications for pacing, pacemaker management & consequences during cardiac resynchronization. J Interv Card Electrophysiol. Nov 2006;17(2):139-52. [Medline].

  3. Sherron P, Torres-Arraut E, Tamer D, Garcia OL, Wolff GS. Site of conduction delay and electrophysiologic significance of first-degree atrioventricular block in children with heart disease. Am J Cardiol. May 1 1985;55(11):1323-7. [Medline].

  4. Upshaw CB Jr. Comparison of the prevalence of first-degree atrioventricular block in African-American and in Caucasian patients: an electrocardiographic study III. J Natl Med Assoc. Jun 2004;96(6):756-60. [Medline].

  5. Mymin D, Mathewson FA, Tate RB, Manfreda J. The natural history of primary first-degree atrioventricular heart block. N Engl J Med. Nov 6 1986;315(19):1183-7. [Medline].

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Further Reading

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Keywords

heart block, first-degree heart block, first degree heart block, arrhythmia, cardiac arrhythmia, abnormal heart rhythm, atrioventricular block, first-degree atrioventricular block, AV block, first-degree AV block, prolongation of the PR interval, P wave, PR interval

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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 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, Department of Medicine, Division of Emergency Medicine, Harvard Medical School; Associate-Chief, Attending Physician, 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: Schering  Honoraria Speaking and teaching

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: American College of Emergency Physicians, Council of Emergency Medicine Residency Directors, New York Academy of Medicine, 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: Nothing to disclose.

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, Department of Medicine, Division of Emergency Medicine, Harvard Medical School; Associate-Chief, Attending Physician, 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: Schering  Honoraria Speaking and teaching

 
 
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