Third Degree Atrioventricular Block (Third Degree Heart Block, Complete Heart Block)

Etiology

Congenital

  • Usually at the level of the AV node
  • Usually asymptomatic at rest, may be symptomatic with exertion (due to inability to increase heart rate)
  • In the absence of structural abnormalities, congenital complete heart block may be associated with maternal SS-A (Ro) and SS-B (La) antibodies

Acquired

Degenerative Disease

  • Lenègre Disease: sclerodegenerative process of the conduction system
  • Lev Disease: calcification of the conduction system and valves
  • Noncompaction Cardiomyopathy
  • Nail-Patella Syndrome
  • Mitochondrial Myopathy

Drugs

  • Class Ia Antiarrhythmics
    • Disopyramide (see xxxx, [[]])
    • Procainamide (see xxxx, [[]])
    • Quinidine (see xxxx, [[]])
  • Class Ic Antiarrhythmics
    • Encainide (see xxxx, [[]])
    • Flecainide (see xxxx, [[]])
    • Propafenone (see xxxx, [[]])
  • Class II Antiarrhythmics
    • Beta Blockerrs (see xxxx, [[]])
  • Class III Antiarrhythmics
    • Amiodarone (see xxxx, [[]])
    • Sotalol (see xxxx, [[]])
    • Dofetilide (see xxxx, [[]])
    • Ibutilide (see xxxx, [[]])
  • Class IV Antiarrhythmics
    • Calcium Channel Blockers (see xxxx, [[]])
  • Digoxin or Other Cardiac Glycosides (see see Digoxin, [[Digoxin]])

Iatrogenic

  • AV Nodal Slow/Fast Pathway Ablation
  • Cardiac Surgery: complete heart block occurs in 1-5.7% of cases
    • Risk Factors
      • Aortic Valve Annular Calcification
      • Aortic Valve Surgery
      • Bicuspid Aortic Valve
      • Female Gender
      • Pre-Existing Conduction System Disease (RBBB or LBBB)
  • Left Anterior Descending Coronary Artery Stenting
  • Septal Alcohol Ablation
  • Swan-Ganz Catheter Interference with Right Bundle Branch Conduction in Setting of Pre-Existing Left Bundle Branch Block (LBBB)

Infection

  • Endocarditis with Valve Ring Abscess (see Endocarditis, [[Endocarditis]])
  • Chagas Disease (see Chagas Disease, [[Chagas Disease]])
  • Lyme Disease: in endemic regions
  • Myocarditis (see Myocarditis, [[Myocarditis]])
  • Rheumatic Fever
  • Trypanosomiasis (see Trypanosomiasis, [[Trypanosomiasis]]): Trypanosoma cruzi
  • Varicella-Zoster Virus (see Varicella-Zoster Virus, [[Varicella-Zoster Virus]])

Infiltration

Ischemia/Infarction

  • General Comments
    • Early revascularization has decreased the incidence of AV block from 5.3% to 3.7% of cases
    • Occlusion of the right coronary artery is the most common source of AV block
      • Proximal RCA occlusion has a high incidence of AV block (24% of cases) because there is involvement of not only the AV nodal artery, but also of the right superior descending artery (which originates from the proximal portion of the right coronary artery)
  • Anterior Wall Myocardial Infarction with His-Purkinje (Infranodal) Block
  • Inferior Wall Myocardial Infarction with AV Nodal Block: complete heart block occurs in <10% of cases

Metabolic Disorders

  • Hypokalemia (see xxxx, [[]])
  • Hypothyroidism (see xxxx, [[]])
  • Hypoxia (see xxxx, [[]])

Neuromuscular Disease

  • Becker Muscular Dystrophy
  • Myotonic Muscular Dystrophy

Phase IV Block (Bradycardia-Related Block)

  • xxxxx

Rheumatic Disease

Toxic

  • Grayanotoxin (“Mad” Honey)
  • Nerium Oleander (see Nerium Oleander, [[Nerium Oleander]]): contains oleandrin and other less well-studied cardiac glycosides

Physiology

  • Complete Failure of Transmission of Sino-Atrial (SA) Node Firing to the Ventricle: absence of conduction, resulting in complete dissociation of atrial and ventricular electrical activity
    • Site of Block: 61% of cases have block below the His bundle
      • AV Node (20% of cases)
      • Bundle of His (<20% of cases)
      • Bundle Branch Purkinje System
    • Origin of Escape Rhythm: the ventricular escape rhythm can originate anywhere from the AV node to the bundle branch Purkinje system
  • AV Dissociation: while all cases of complete heart block have AV dissociation, not all cases of AV dissociation are due to complete heart block
    • Example: AV dissociation can occur in ventricular tachycardia (VT), where ventricular rate is faster than the sinus rate
    • Example: AV dissociation can occur in accelerated junctional tachycardia, where ventricular rate is faster than the sinus rate

Clinical Manifestations

Cardiac Manifestations

Bradycardia (see Bradycardia, [[Bradycardia]])

  • Complete AV Dissociation: no relationship between P waves and QRS complexes
    • Isorhythmic AV Dissociation: atrial and ventricular rates are so close to each other that the P waves appear to be normally conducting
      • Diagnosis of this requires close inspection of a long rhythm strip (to detect P-R interval variation) or pharmacologic acceleration of the atrial/sinus rate
  • Absence of Fusion Complexes
  • Variable QRS Duration: depends on the site of the block and the site of the escape rhythm pacemaker (pacemaker above His bundle produces a narrow-complex escape rhythm, while pacemaker at or below His bundle produces a wide-complex escape rhythm)
    • Block at Level of AV Node: escape is typically junctional at around 45-60 beats per min
      • Patient is usually hemodynamically stable
      • Heart rate increases in response to exercise and atropine
    • Block Below AV Node: escape arises from His bundle or bundle branch Purkinje system and is usually <45 beats per min
      • Patient is usually hemodynamically unstable
      • Heart rate does not increase in response to exercise and atropine

Hypotension (see Hypotension, [[Hypotension]])

  • Variable, depending on site of block and escape rhythm

Treatment

General Management

  • External/Transvenous Pacemaker: for emergent/short-term treatment
  • Permanent Pacemaker: for long-term treatment

Treatment of Complete Heart Block Associated with Swan-Ganz Catheter Interference with Right Bundle Branch Conduction in Setting of Pre-Existing Left Bundle Branch Block

  • Usually resolves with removal of catheter

Treatment of Complete Heart Block After Cardiac Surgery

  • Time course for recovery is variable
    • Many patients recover within 48 hrs of surgery
    • If no recovery occurs by post-op day 4-5, a permanent pacemaker should be implanted

Treatment of Complete Heart Block Associated with Ischemia/Infarction

  • Anterior Wall Myocardial Infarction with His-Purkinje (infranodal) Block: occlusion of the left anterior descending coronary artery (particularly proximal to the first septal perforator) usually requires permanent pacemaker implantation
  • Inferior Wall Myocardial Infarction with AV Nodal Block: often resolves within hrs-days (particularly with early coronary revascularization)

References

  • xxx