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eMedicine Specialties > Emergency Medicine > Cardiovascular

Heart Block, First Degree

Michael D Levine, MD, Physician, Department of Medical Toxicology, Banner Good Samaritan Medical Center
David FM Brown, MD, Assistant Professor, Division of Emergency Medicine, Harvard Medical School; Vice Chair, Department of Emergency Medicine, Massachusetts General Hospital

Updated: Nov 16, 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.


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. Asymptomatic first-degree heart block is part of the spectrum of presentation of Lyme carditis in children. Lyme carditis is most likely in children with Lyme disease who are older than 10 years of age, those with arthralgias, and those with cardiopulmonary symptoms.5
  • Borderline first-degree heart block in patients with long-standing systemic lupus erythematosus (SLE) may be a clue to more significant cardiac disease, resulting from the progression of SLE; these patients require careful screening for underlying myocardial disease.6 Conduction disturbances may also be secondary to drugs used to treat SLE.
  • Patients who have undergone mitral valve replacement or mitral valve annuloplasty may have heart block postoperatively.7

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

Differential Diagnoses

Heart Block, Second Degree
Heart Block, Third Degree

Other Problems to Be Considered

Second-degree AV block
Third-degree AV block (complete heart block)
Junctional escape rhythms

Workup

Laboratory Studies

  • Routine laboratory studies are usually not indicated in the evaluation of first-degree atrioventricular (AV) block.
  • Electrolyte and drug screen can be obtained if a metabolic derangement or drug toxicity is suspected.

Imaging Studies

  • Routine imaging studies are not indicated for first-degree heart block.

Other Tests

  • Follow-up ECGs may be indicated in patients who are treated with AV nodal agents while in the ED as well as for patients with a concomitant MI.

Procedures

  • In general, no treatment is indicated for first-degree heart block. For those patients with a PR >300 msec, however, several uncontrolled trials have demonstrated an improvement in symptoms with the placement of a dual chamber pacemaker. However, there is little evidence suggesting an improvement in survival.2

Treatment

Emergency Department Care

No specific therapy is indicated for isolated first-degree atrioventricular (AV) block. Any associated condition (eg, myocardial infarction, digitalis intoxication) should be treated appropriately.

Consultations

No emergent consultation is necessary. Outpatient cardiology follow-up can be arranged, if desired.

Follow-up

Further Inpatient Care

  • In general, hospitalization specifically for first-degree atrioventricular (AV) block is not indicated. However, admission may be indicated for associated conditions (eg, myocardial infarction). Patients with a marked first-degree AV block (PR >300 msec) can present with symptoms similar to the pacemaker syndrome.2 In these individuals, admission may be indicated.
  • Guidelines from the American College of Cardiology, the American Heart Association, and the Heart Rhythm Society (ACC/AHA/HRS) recommend that permanent pacemaker implantation is reasonable for first-degree heart block with symptoms similar to those of pacemaker syndrome or hemodynamic compromise (class IIa recommendation; level of evidence, B).8 Additional ACC/AHA/HRS recommendations include the following:
    • Patients with first-degree AV block, with or without symptoms, may be considered for permanent pacemaker implantation if the block occurs in the setting of neuromuscular diseases such as myotonic muscular dystrophy, Erb dystrophy (limb-girdle muscular dystrophy), or peroneal muscular atrophy, because these patients may experience unpredictable progression of AV conduction disease ( class IIb recommendation, level of evidence, B).
    • Permanent pacemaker implantation is not indicated for asymptomatic first-degree AV block (class III recommendation; level of evidence, B).
  • Significant electrolyte abnormalities should be corrected.

Further Outpatient Care

  • In the absence of a disease process that requires admission, patients with first-degree AV block may be safely discharged and receive follow-up on an outpatient basis.
  • Patients should get serial follow-up ECGs to evaluate for progression to a higher-grade AV block.
  • Patients with first-degree AV block and coexistent bundle-branch block should be closely observed.

Complications

  • Patients with first-degree block can occasionally progress to higher-grade AV blocks. Usually, such a progression is only to a Mobitz type I, second-degree heart block, but occasionally, higher-grade block can occur. The later scenario is particularly seen in patients with an acute MI, myocarditis, or acute drug overdoses.
  • Drugs that slow conduction through the AV nodal system increase the risk of progression to higher-grade heart blocks. Administering such agents to a person with a coexisting first-degree AV block should be done with caution.

Prognosis

  • The prognosis for isolated first-degree atrioventricular (AV) block is very good. Progression from isolated first-degree heart block to high-degree block is very uncommon.9
  • Heart block in children with Lyme carditis tends to resolve spontaneously, with median recovery in 3 days (range: 1-7 days).5
  • Cheng et al found that first-degree heart block is associated with increased long-term risks of atrial fibrillation, pacemaker implantation, and all-cause mortality.10 Their community-based cohort included 7575 individuals from the Framingham Heart Study who had baseline routine 12-lead electrocardiography in 1968-1974 and underwent prospective follow-up through 2007.
    • Compared with individuals whose PR intervals were 200 msec or shorter, those with first-degree atrioventricular block had a 2-fold adjusted risk of atrial fibrillation (hazard ratio [HR], 2.06; 95% confidence interval [CI], 1.36-3.12; P<.001), a 3-fold adjusted risk of pacemaker implantation (HR, 2.89; 95% CI, 1.83-4.57; P<.001), and a 1.4-fold adjusted risk of all-cause mortality (HR, 1.44; 95% CI, 1.09-1.91; P=.01).
    • Each 20-msec increment in PR interval was associated with an adjusted HR of 1.11 (95% CI, 1.02-1.22; P = .02) for atrial fibrillation, 1.22 (95% CI, 1.14-1.30; P<.001) for pacemaker implantation, and 1.08 (95% CI, 1.02-1.13; P=.005) for all-cause mortality.

Multimedia

ECG in a patient with first-degree heart block.

Media file 1: ECG in a patient with first-degree heart block.

ECG in a patient with first-degree heart block.

Media file 2: ECG in a patient with first-degree heart block.

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. Costello JM, Alexander ME, Greco KM, Perez-Atayde AR, Laussen PC. Lyme carditis in children: presentation, predictive factors, and clinical course. Pediatrics. May 2009;123(5):e835-41. [Medline].

  6. Makaryus JN, Catanzaro JN, Goldberg S, Makaryus AN. Rapid progression of atrioventricular nodal blockade in a patient with systemic lupus erythematosus. Am J Emerg Med. Oct 2008;26(8):967.e5-7. [Medline].

  7. Berdajs D, Schurr UP, Wagner A, Seifert B, Turina MI, Genoni M. Incidence and pathophysiology of atrioventricular block following mitral valve replacement and ring annuloplasty. Eur J Cardiothorac Surg. Jul 2008;34(1):55-61. [Medline].

  8. [Guideline] Epstein AE, DiMarco JP, Ellenbogen KA, Estes NA 3rd, Freedman RA, Gettes LS, et al. ACC/AHA/HRS 2008 Guidelines for Device-Based Therapy of Cardiac Rhythm Abnormalities: a report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines (Writing Committee to Revise the ACC/AHA/NASPE 2002 Guideline Update for Implantation of Cardiac Pacemakers and Antiarrhythmia Devices) developed in collaboration with the American Association for Thoracic Surgery and Society of Thoracic Surgeons. J Am Coll Cardiol. May 27 2008;51(21):e1-62. [Medline].

  9. 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].

  10. Cheng S, Keyes MJ, Larson MG, McCabe EL, Newton-Cheh C, Levy D, et al. Long-term outcomes in individuals with prolonged PR interval or first-degree atrioventricular block. JAMA. Jun 24 2009;301(24):2571-7. [Medline].

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

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.

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