Atrioventricular Blocks

Basics

Description

  • Normally, P waves are paired with QRS complexes with a fixed PR interval of 120–200 ms
  • AV blocks represent 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: Occurs in 0.7% healthy adults
  • 2nd-degree AV block:
    • Failure of some atrial impulses to reach ventricles
    • Mobitz type I (Wenckebach):
      • Usually secondary to conduction deficit within AV node
      • Progressive prolongation of the 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
    • 2nd-degree, High grade: 2 or more consecutive nonconducted P waves, but with continued AV association
  • 3rd-degree AV block:
    • Also known as “complete heart block” due to total AV dissociation
    • All atrial impulses are unable to reach the ventricular conducting system resulting in the activation of a junctional or ventricular escape rhythm
    • Constant PP and RR intervals with variable PR intervals, as atria and ventricles are acting independently
    • More severe symptoms occur when the block is lower in the conducting system
    • Never a benign condition

Etiology

  • Congenital:
    • Structural heart disease
    • Immune mediated
  • Acquired:
    • Age-related degeneration and fibrosis of the conduction system is the most common cause of acquired (>65 yr)
  • MI:
    • 1st-degree block and type I 2nd-degree AV block may be associated with an inferior wall MI. Usually transient and typically resolves with no increased morbidity or mortality
    • Type II 2nd-degree AV block may be associated with an anterior wall MI and is associated with 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 (not inclusive):
    • Digoxin
    • β-blockers: Prazosin
    • β-blockers (even ophthalmic)
    • Calcium channel blockers
    • α2-agonists: Clonidine, dexmedetomidine
    • Adenosine
    • Antiarrhythmics: Amiodarone, procainamide, flecainide, propafenone
    • Donepezil
    • Lithium
    • Methadone
    • Remdesivir
  • Electrolyte disturbances
  • BRASH syndrome (synergistic effect of therapeutic AV nodal blocker with mild hyperkalemia)
  • Valvular heart disease
  • Postsurgical (CABG, valve replacement)
  • Infectious:
    • Syphilis
    • Diphtheria
    • Chagas disease
    • TB
    • Toxoplasmosis
    • Lyme disease
    • Myocarditis
    • Endocarditis
    • Rheumatic fever
    • In one study, 5% of hospitalized COVID patients developed a new AV block
  • Collagen vascular diseases
  • Muscular dystrophy
  • Infiltrative diseases:
    • Sarcoidosis
    • Amyloidosis
    • Hemochromatosis
  • Cardiomyopathy
  • Myxedema
  • Hypothermia
  • Blunt cardiac trauma
  • Channelopathies
  • Increased vagal tone (as in sleep or athletes)

Pediatric Considerations

  • Occurs in children, but is often asymptomatic
  • Mortality is highest in the neonatal period
  • Usually congenital or infectious, but consider potential toxic ingestions

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