Description

• Impaired conduction between the atrium and the ventricle through the AV node or His–Purkinje system
• First-degree AV block:
  • Prolonged conduction through the AV node
  • Ventricular impulses are not lost
  • Generally benign
  • Occurs in 1.6% healthy adults
• Second-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
• Third-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:
  • First-degree block and type I second-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 second-degree AV block may be associated with an anterior wall MI:
    • 5% anterior wall MIs and up to 28% of inferior 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, ecainide, 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
• Blunt cardiac trauma

• 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