Atrioventricular Blocks

Atrioventricular Blocks is a topic covered in the 5-Minute Emergency Consult.

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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

Pediatric Considerations
  • 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

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Citation

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TY - ELEC T1 - Atrioventricular Blocks ID - 307044 Y1 - 2016 PB - 5-Minute Emergency Consult UR - https://emergency.unboundmedicine.com/emergency/view/5-Minute_Emergency_Consult/307044/all/Atrioventricular_Blocks ER -