AUTHOR INFORMATION

Section 1 of 10

Authored by Michael D Levine, MD, Staff Physician, Department of Emergency Medicine, Brigham and Women's Hospital, Massachusetts General Hospital

Coauthored by David FM Brown, MD, Assistant Professor, Department of Medicine, Division of Emergency Medicine, Harvard Medical School; Vice-Chair, Department of Emergency Medicine, Massachusetts General Hospital

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, and Society for Academic Emergency Medicine

Edited by Theodore Gaeta, DO, MPH, Residency Director, Clinical Associate Professor of Emergency Medicine in Medicine, Department of Emergency Medicine, New York Methodist Hospital; Francisco Talavera, PharmD, PhD, Senior Pharmacy Editor, eMedicine; 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; John Halamka, MD, Chief Information Officer, CareGroup Healthcare System, Assistant Professor of Medicine, Department of Emergency Medicine, Beth Israel Deaconess Medical Center; Assistant Professor of Medicine, Harvard Medical School; and Jonathan Adler, MD, Attending Physician, Department of Emergency Medicine, Massachusetts General Hospital; Division of Emergency Medicine, Harvard Medical School

Author's Email:	Michael D Levine, MD
Editor's Email:	Theodore Gaeta, DO, MPH

eMedicine Journal, September 5 2006, VOLUME 7, Number 9

INTRODUCTION

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Background: On an electrocardiogram (ECG), the PR interval, which is defined as the time from the initial deflection of the P wave to the start of the QRS complex, 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.

Pathophysiology: With first-degree 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.

Frequency:

• In the US: In the United States, the prevalence of first-degree AV block among young adults ranges from 0.65-1.6%. Higher prevalence is reported in studies of trained athletes (8.7%) and medical students (8%). The incidence is 1.13 per 1000 persons. It is more common among African Americans compared with Caucasian populations. The prevalence of first-degree AV block increases with advancing age.

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

CLINICAL

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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 AV block:

• Intrinsic AV nodal disease

• Enhanced vagal tone

• Acute MI, particularly acute inferior wall MI

• Myocarditis

• Electrolyte disturbances

• 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

DIFFERENTIALS

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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

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Lab Studies:

• Routine laboratory studies are usually not indicated in the evaluation of first-degree AV block.

• Electrolyte and drug screen can be obtained if the physician has a suspicion of a metabolic derangement or drug toxicity.

Imaging Studies:

• Routine imaging studies are not indicated.

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.

TREATMENT

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Emergency Department Care: No specific therapy is indicated for isolated first-degree 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

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Further Inpatient Care:

• Hospitalization specifically for first-degree AV block is not indicated. However, admission may be indicated for associated conditions (eg, myocardial infarction).

• 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.

• Physicians should closely observe patients with first-degree AV block and coexistent bundle-branch block.

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 AV block is very good.

TEST QUESTIONS

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CME Question 1: An asymptomatic 50-year-old man is noted to have first-degree heart block on his electrocardiogram. Appropriate management includes which of the following?

A: Oral beta-blockers
B: Intravenous beta-blockers
C: Intravenous atropine
D: Admission to a telemetry bed
E: None of the above

The correct answer is E: Patients with first-degree heart block generally are asymptomatic and require no specific therapy.

CME Question 2: Which of the following may cause first-degree heart block?

A: Intrinsic AV nodal disease
B: Enhanced vagal tone
C: Acute myocardial infarction
D: Myocarditis
E: All of the above

The correct answer is E: First-degree heart block may be caused by any of these, as well as by electrolyte disturbances and certain drugs.

Pearl Question 1 (T/F): The normal PR interval on electrocardiogram is 0.12-0.20 seconds.

The correct answer is True: The PR interval is defined as the time from the initial deflection of the P wave from the baseline to the beginning of the QRS complex. A normal PR interval is 0.12-0.20 seconds.

Pearl Question 2 (T/F): Calcium channel blockers can cause first-degree heart block.

The correct answer is True: Calcium channel blockers, beta-blockers, and cardiac glycosides all can cause first-degree heart block.

Pearl Question 3 (T/F): First-degree heart block commonly is seen in acute inferior myocardial infarction.

The correct answer is True: First-degree heart block is not uncommon in the presence of acute inferior myocardial infarction but does not worsen the prognosis.

Pearl Question 4 (T/F): The presence of first-degree heart block in a patient is not associated with morbidity or death.

The correct answer is True: Morbidity and mortality rates from heart disease appear to be unaffected by the presence of a first-degree block.

PICTURES

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Caption: Picture 1. ECG in a patient with first-degree heart block.
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Caption: Picture 2. ECG in a patient with first-degree heart block.
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BIBLIOGRAPHY

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• Bexton RS, Camm AJ: First degree atrioventricular block. Eur Heart J 1984 Mar; 5 Suppl A: 107-9[Medline].
• Hesler RA: Cardiovascular principals. In: Goldfrank's Toxicologic Emergencies. Goldfrank LR, Howland MA, Flomenbaum NE, et al, eds. 7th ed. New York: McGraw Hill; 2002: 315-34.
• Mymin D, Mathewson FA, Tate RB, Manfreda J: The natural history of primary first-degree atrioventricular heart block. N Engl J Med 1986 Nov 6; 315(19): 1183-7[Medline].
• Upshaw CB: Comparison of the prevalence of first-degree atrioventricular block in African-American and in Caucasian patients: an electrocardiographic study III. J Natl Med Assoc 2004 Jun; 96(6): 756-60[Medline].
• Yealy DM, Delbridge TR: Dysrhythmias. In: Rosen's Emergency Medicine. Concepts and Clinical Practice. Marx JA, Hockberger RS, Walls RM, eds. 5th ed. Mosby; 2002: 1053-98.
• Zipes DP: Specific arrhythmias: Diagnosis and treatment. In: Braunwald E, ed. Heart Disease: A Textbook of Cardiovascular Medicine. 3rd ed. WB Saunders Company; 1988: 658-716.

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Author Information | Introduction | Clinical | Differentials | Workup | Treatment | Follow-up | Test Questions | Pictures | Bibliography