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Atrioventricular Blocks, Emergency Medicine

Basics

Description

• Impaired conduction between the atrium and the ventricle through the AV node or His-Purkinje system
• 1st-degree AV block:
  • Prolonged conduction through the AV node
  • Ventricular impulses are not lost.
  • Generally benign, and occurs in 1.6% healthy adults.
• 2nd-degree AV block:
  • Marked by a failure of some atrial impulses to reach ventricles
  • Mobitz Type I (Wenckebach):
    • Usually secondary to conduction deficit in AV node.
    • Progressive prolongation of the pulse-rate (PR) interval until a nonconducted P-wave and a dropped QRS complex occur
    • Generally benign, but may be a complication of an inferior wall MI
  • Mobitz Type II:
    • Conduction deficit is usually below the level of the AV node.
    • PR intervals are constant until single or multiple beats are abruptly dropped.
    • High likelihood of progression to complete heart block
    • Worse prognosis if associated with an acute MI
    • Less common than Type I
• 3rd-degree AV block:
  • Also known as complete heart block
  • All atrial impulses are unable to reach the ventricular conducting system; a ventricular escape pacemaker then takes over, resulting in AV dissociation.
  • Constant PP and RR intervals with variable PR intervals because PP and RR intervals are independent of each other.
  • More severe symptoms occur when the block is lower in the conducting system.
  • If secondary to toxicologic agents, often resolves upon omission of offending toxin
  • Never a benign condition

Etiology

• Essentially due to:
  • A structural lesion
  • Increase in inherent refractory period
  • Marked shortening of the supraventricular cycle
• MI:
  • 1st-degree block and Type I 2nd-degree AV block may be associated with an inferior wall MI:
    • These blocks are transient.
    • AV conduction usually returns to normal with no increased morbidity or mortality.
  • Type II 2nd-degree AV block may be associated with an anterior wall MI:
    • 5% anterior wall MIs are associated with AV blocks.
    • Increased mortality secondary to ventricular arrhythmias and left-heart failure
• Coronary artery disease:
  • Chronic ischemic injury can lead to fibrosis around the AV node
• Toxicologic:
  • Digoxin
  • β-blockers
  • Calcium-channel blockers
  • Amiodarone
  • Procainamide
  • Class 1C agents: Propafenone, encainide, flecainide
  • Clonidine
• Congenital
• Valvular heart disease
• Surgical trauma:
  • S/P coronary artery bypass graft or valvular replacement
• Increased vagal tone
• Infectious:
  • Syphilis
  • Diphtheria
  • Chagas disease
  • TB
  • Toxoplasmosis
  • Lyme disease
  • Myocarditis
  • Endocarditis
  • Rheumatic fever
  • Abscess formation in interventricular septum
• Collagen vascular diseases
• Infiltrative diseases:
  • Sarcoidosis
  • Amyloidosis
  • Hemochromatosis
• Cardiomyopathy
• Electrolyte disturbances:
  • Hyperkalemia
• Myxedema
• Hypothermia

• Occurs in children, but is often asymptomatic
• Associated mortality is highest in the neonatal period.
• Associated with:
  • Congenitally acquired maternal antibodies
  • Congenital heart disease
  • Infectious etiologies, such as rheumatic fever or myocarditis
• Be sure to consider potential toxic ingestions in pediatric patients with new AV block

Diagnosis

Signs and Symptoms

History

• 1st-degree AV block:
  • Asymptomatic
• Type I 2nd-degree AV block:
  • Pulse irregularities
• Type II 2nd-degree AV block and 3rd-degree block:
  • Exercise intolerance
  • Palpitations
  • Chest pain
  • Presyncope/syncope
  • Altered mental status
  • Dyspnea, orthopnea

Physical Exam

• 1st-degree AV block:
  • No discrete physical exam findings
• Type I 2nd-degree AV block:
  • Regularly irregular pulse
• Type II 2nd-degree AV block and 3rd-degree block:
  • Irregular pulse
  • Hypotension
  • Mental status changes
  • Signs of heart failure:
    • Rales
    • Cyanosis
    • Jugular venous distention

Essential Workup

• A 12-lead EKG to determine the type of block and identify evidence of infarction
• 1st-degree AV block:
  • PR interval >0.20 sec
• 2nd-degree AV block:
  • Type I: Progressive prolongation of PR interval until there is a nonconducted P-wave and a dropped QRS complex; occurs in repeated cycles; QRS is usually narrow.
  • Type II: PR interval remains constant; atrial impulses are not conducted intermittently, giving the appearance of an occasionally dropped ventricular beat; QRS may be prolonged depending on the level of the lesion.
• 3rd-degree AV block:
  • P-waves occur at consistent intervals.
  • QRS complexes occur independently from P-waves but also at consistent intervals.
  • QRS complexes are usually narrow unless there is an infranodal conduction disturbance or a ventricular escape rhythm.

Diagnosis Tests & Interpretation

Additional studies aid in confirming the etiology of the identified AV block.  
Lab

• Electrolytes
• Calcium, magnesium
• Cardiac enzymes:
  • Especially for Type II 2nd-degree and 3rd-degree blocks
• Digoxin level, if patient has been exposed to this medication

Imaging

• CXR:
  • May identify cardiomyopathy or CHF
• ECG:
  • May identify regional wall motion abnormalities or valvular dysfunction

Differential Diagnosis

• Accelerated junctional rhythm
• Idioventricular rhythm
• Sinus bradycardia
• SA block

Treatment

Pre-Hospital

• Transcutaneous pacing for unstable Type II 2nd- or 3rd-degree block
• Atropine:
  • Avoid with Type II 2nd-degree block because it may precipitate complete heart block
  • Contraindicated in 3rd-degree heart block with a widened QRS complex
• Attempts should be made to prevent increases in vagal tone.

Initial Stabilization/Therapy

• Transcutaneous pacemaker:
  • Necessary for the unstable patient with signs of hypoperfusion:
    • Hypotension
    • Chest pain
    • Dyspnea
    • Mental status changes
• Atropine:
  • Can be administered in:
    • Complete heart block with a narrow QRS
    • Symptomatic sinus bradycardia

Ed Treatment/Procedures

• 1st-degree AV block:
  • No treatment required
  • Avoid AV nodal blocking agents
  • Evaluate for associated MI, electrolyte abnormalities, medication excess in the appropriate clinical scenarios
• Type I 2nd-degree AV block:
  • Usually no treatment needed
  • If symptomatic, atropine will enhance AV conduction
• Type II 2nd-degree AV block:
  • Temporary transcutaneous or transvenous pacemaker
  • Atropine is not effective and should be avoided
• 3rd-degree AV block:
  • 1st line of treatment: Emergent pacemaker
  • May transiently respond to atropine with narrow QRS complexes
  • If block is identified to be toxin-mediated, specific treatments include:
    • Digoxin-specific antibodies (digoxin overdose)
    • Glucagon and calcium (β-blocker or calcium-channel blocker overdose)

Medication

• Atropine: 0.5-1.0 mg (peds: 0.01-0.03 mg/kg) IV q5min as necessary
• Digoxin-specific antibodies: 10 vials (380 mg) is an appropriate loading dose if digoxin toxicity is strongly suspected:
  • Serum level — weight (kg) = number of vials to be administered
• Glucagon: 5-10 mg (peds: 50 μg/kg) IV over 5 min
• Calcium chloride: 250-500 mg (peds: 20 mg/kg) IV

Follow-Up

Disposition

Admission Criteria
Monitored bed:  

• Type II 2nd-degree block
• 3rd-degree block

Discharge Criteria
Asymptomatic 1st-degree and Type I 2nd-degree blocks: Ensure follow-up for further outpatient workup.  

Followup Recommendations

Asymptomatic 1st-degree and Type I 2nd-degree blocks can follow-up with a cardiologist on a routine outpatient basis.  

Pearls and Pitfalls

• Obtaining an EKG rapidly in symptomatic patients is paramount.
• Once a high-degree AV block has been diagnosed, initiate transcutaneous pacing immediately.
• Obtain a complete history from all available resources; it may help you identify an offending toxin rapidly.
• Common pitfalls:
  • Failure to interpret EKG properly
  • Failure to diagnose AV block appropriately
  • Failure to initiate transcutaneous pacing in a timely fashion
  • Failure to consult cardiology for permanent pacemaker in a timely fashion

Additional Reading

• Harrigan  RA, Chan  TC, Moonblatt  S, et al. Temporary transvenous pacemaker placement in the emergency department. J Emerg Med.  2007;32(1):105-111.
• Olgin  JE, Zipes  DP. Specific arrhythmias: Diagnosis and treatment. In: Libby  P, ed. Braunwalds Heart Disease: A Textbook of Cardiovascular Medicine. 8th ed. Philadelphia, PA: Saunders Elsevier; 2008:913-923.
• Ufberg  JW, Clark  JS. Bradydysrhythmias and atrioventricular conduction blocks. Emerg Med Clin North Am.  2006;24(1):1-9.
• Yealy  DM, Delbridge  TR. Dysrhythmias. In: Marx  JA, et al., eds. Rosen's Emergency Medicine: Concepts and Clinical Practice. 7th ed. St. Louis, MO: CV Mosby; 2010:93-100.

See Also (Topic, Algorithm, Electronic Media Element)

• Bradyarrhythmias
• Cardiac Pacemakers

Codes

ICD9

• 426.10 Atrioventricular block, unspecified
• 426.11 First degree atrioventricular block
• 426.13 Other second degree atrioventricular block
• 426.0 Atrioventricular block, complete
• 426.12 Mobitz (type) II atrioventricular block
• 426.1 Atrioventricular block, other and unspecified

ICD10

• I44.0 Atrioventricular block, first degree
• I44.1 Atrioventricular block, second degree
• I44.30 Unspecified atrioventricular block
• I44.2 Atrioventricular block, complete
• I44.39 Other atrioventricular block
• I44.3 Other and unspecified atrioventricular block

SNOMED

• 233917008 Atrioventricular block (disorder)
• 270492004 First degree atrioventricular block (disorder)
• 195042002 second degree atrioventricular block (disorder)
• 27885002 Complete atrioventricular block (disorder)
• 195039008 partial atrioventricular block (disorder)
• 28189009 Mobitz type II atrioventricular block (disorder)
• 54016002 Mobitz type I incomplete atrioventricular block (disorder)

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