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eMedicine Journal
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Emergency Medicine
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Cardiovascular
Heart Block, Second Degree Synonyms, Key Words, and Related Terms: 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 |
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Author Information | Introduction | Clinical | Differentials | Workup | Treatment | Medication | Follow-up | Test Questions | Pictures | Bibliography
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 | AUTHOR INFORMATION | Section 1 of 11    |
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 | Section 2 of 11  |
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:
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 11 |
History:
Physical:
Causes:
| DIFFERENTIALS | Section 4 of 11 |
Heart Block, First Degree
Heart Block, Third Degree
Myocardial Infarction
Other Problems to be Considered:
Heart block, congenital
| WORKUP | Section 5 of 11 |
Lab Studies:
Imaging Studies:
Other Tests:
| TREATMENT | Section 6 of 11 |
Prehospital Care:
Emergency Department Care:
Consultations:
| MEDICATION | Section 7 of 11 |
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.
| Drug Name | Atropine (Atropair, Atropine-Care, Isopto) -- Enhances sinus node automaticity. In addition, blocks the effects of acetylcholine at the AV node, thereby decreasing the refractory time and speeding conduction through the AV node. Insufficient doses can cause paradoxical effects, further slowing the heart rate |
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| Adult Dose | 0.5 mg rapid IV push; for patients in PEA arrest, administer 1 mg; 0.04 mg/kg IV maximum; can also be administered via endotracheal tube, although absorption is less predictable compared with IV administration; if administered via endotracheal tube, dose should be increased 2-3 fold |
| Pediatric Dose | 0.02 mg/kg IV push, with minimum of 0.1 mg; any single dose should not exceed 0.5 mg/dose IV in children or 1 mg/dose in adolescents; maximal total IV dose is 0.04 mg/kg; can be administered via endotracheal tube, although absorption is less predictable compared with IV administration; if administered via endotracheal tube, dose should be increased 2-3 fold |
| Contraindications | Documented hypersensitivity to belladonna alkaloids or related products; concomitant acute myocardial ischemia/infarction, thyrotoxicosis, narrow-angle glaucoma, or tachycardia; Down syndrome or brain damage in children (may show hyperreactive response to topical atropine); coronary artery disease; congestive heart failure; hypertension |
| Interactions | Coadministration with other anticholinergics have additive effects; pharmacologic effects of atenolol and digoxin may increase with atropine; antipsychotic effects of phenothiazines may decrease with this medication; tricyclic antidepressants with anticholinergic activity may increase effects of atropine |
| Pregnancy | C - Safety for use during pregnancy has not been established. |
| Precautions | Caution in Down syndrome and/or children with brain damage to prevent hyperreactive response; caution also in coronary heart disease, tachycardia, congestive heart failure, cardiac arrhythmias, hypertension, peritonitis, ulcerative colitis, hepatic disease, and hiatal hernia with reflux esophagitis; in prostatic hypertrophy, prostatism can have dysuria and may require catheterization |
| FOLLOW-UP | Section 8 of 11 |
Further Inpatient Care:
Further Outpatient Care:
Prognosis:
| TEST QUESTIONS | Section 9 of 11 |
CME Question 1: A 60-year-old woman presents with light-headedness that began several hours ago. The electrocardiogram reveals sinus rhythm with a Mobitz II heart block and no acute ST changes. Which of the following is not considered appropriate management?
A: Application of transcutaneous pacing patches
B: Continuous cardiac monitoring while in the ED
C: Discharge from the ED once stabilized
D: Cardiac consultation
E: Admission to a telemetry bed
The correct answer is C: Patients with a Mobitz II second-degree heart block require admission and further evaluation by a cardiologist because of the high likelihood of progression to complete heart block.
CME Question 2: Which of the following statements regarding Wenckebach block is incorrect?
A: It most commonly is due to a conduction block at the atrioventricular (AV) node.
B: It is characterized by progressive prolongation of the PR interval, causing progressive R-R interval lengthening until a P wave fails to conduct to the ventricle.
C: It frequently is asymptomatic.
D: It may be seen in the setting of an acute myocardial infarction.
E: It may be seen in states of enhanced vagal tone, as in a high-performance athlete.
The correct answer is B: Wenckebach block is characterized by progressive prolongation of the PR interval causing progressive R-R interval shortening until a P wave fails to conduct to the ventricle.
Pearl Question 1 (T/F): Mobitz II block is more likely than Mobitz I block to be associated with an acute myocardial infarction (MI) in the ED.
The correct answer is True: Mobitz II block is more likely to be associated with acute MI, although Wenckebach block also may be seen in that setting.
Pearl Question 2 (T/F): Patients with a new onset of Mobitz II heart block must be admitted to the hospital.
The correct answer is True: Second-degree heart block may be the only evidence of an impending myocardial infarction.
Pearl Question 3 (T/F): The dose of atropine for a hemodynamically stable 50-kg adult who has a Mobitz II second-degree block is 0.5 mg/kg.
The correct answer is False: The dose of atropine for an adult with a pulse is 0.5 mg. The dose of atropine for a patient in cardiac arrest is 1 mg. Atropine can be given via an endotracheal tube, but the dose should be increased 2-3 fold. In the pediatric patient, the dose of atropine is 0.02 mg/kg, with a minimal dose of 0.1 mg. The maximum dose of atropine is 0.04 mg/kg.
Pearl Question 4 (T/F): Atropine can be given via endotracheal tube.
The correct answer is True: Atropine may be given via endotracheal tube, but its absorption may be unreliable when administered in this manner.
| PICTURES | Section 10 of 11 |
| Caption: Picture 1. An electrocardiogram of a patient with Mobitz I (Wenckebach) second-degree AV block. | |
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| Picture Type: ECG | |
| Caption: Picture 2. An electrocardiogram of a patient with Mobitz II second-degree AV block. | |
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| Picture Type: ECG | |
| BIBLIOGRAPHY | Section 11 of 11 |
| NOTE: |
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| Medicine is a constantly changing science and not all therapies are clearly established. New research changes drug and treatment therapies daily. The authors, editors, and publisher of this journal have used their best efforts to provide information that is up-to-date and accurate and is generally accepted within medical standards at the time of publication. However, as medical science is constantly changing and human error is always possible, the authors, editors, and publisher or any other party involved with the publication of this article do not warrant the information in this article is accurate or complete, nor are they responsible for omissions or errors in the article or for the results of using this information. The reader should confirm the information in this article from other sources prior to use. In particular, all drug doses, indications, and contraindications should be confirmed in the package insert. FULL DISCLAIMER |
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Author Information | Introduction | Clinical | Differentials | Workup | Treatment | Medication | Follow-up | Test Questions | Pictures | Bibliography
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