Atrioventricular Block

Aka: Atrioventricular Block, Atrioventricular Node Block, AV Block, AV Node Block, First Degree Atrioventricular Block, Second Degree Atrioventricular Block, Third Degree Atrioventricular Block, Complete Atrioventricular Block, Sinoatrial Exit Block, Wenckebach AV Block, Mobitz I AV Block, Mobitz II AV Block, Heart Block

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II. Pathophysiology

  1. Impaired conduction in Atria, AV Node or His-Purkinje

III. Types: First Degree Atrioventricular Node Block

  1. Causes
    1. Normal finding in >1% of healthy adults
    2. Inferior wall ischemia (right Coronary Artery) if onset in acute presentations
    3. Increased vagal tone (responds to Atropine)
    4. Hypothyroidism
    5. Medications
      1. Digitalis affect
      2. Non-Dihydropyridine Calcium Channel Blocker
      3. Beta Blocker
  2. Findings
    1. Regular narrow-complex rhythm at 40-60 beats/minute
    2. Prolonged PR Interval >0.20 seconds
  3. Treatment
    1. None needed

IV. Types: Second Degree Atrioventricular Node Block

  1. Also known as Sinoatrial Exit Block
  2. Type I Second Degree Atrioventricular Node Block
    1. Known as Mobitz I AV Block or Wenckeback Conduction
    2. Acute condition affecting AV Node
      1. Most often due to excess vagal tone
    3. EKG Findings
      1. Progressive increase PR Interval until beat/QRS Complex dropped (non-conducted P Wave)
      2. Cycle repeats after beat dropped, with PR Interval resetting aftet the dropped beat
    4. Causes: Functional, reversible defects
      1. Inferior Myocardial Ischemia
      2. Rheumatic Fever
      3. Digitalis Toxicity
      4. Increased vagal tone
      5. Beta Blocker
    5. Response to Maneuvers
      1. Bradycardia improves in response to Atropine and Exercise
      2. Worsens with carotid massage
  3. Type II Second Degree Atrioventricular Node Block
    1. Known as Mobitz II Block
    2. Chronic condition affects infranodal conduction path
      1. Intermittent conduction defect with a consistent frequency of non-conducted P Wave
      2. Described as a ratio (e.g. 3:2) of P Waves to QRS Complexes
    3. EKG Findings
      1. Regularly dropped QRS waves at consistent interval is typical
        1. Ratios of P Waves to conducted QRS (ratio 2:1, 3:2, 4:3)
        2. Intermittent non-conducted P Waves at a constant frequency of missed beats
        3. However, non-conducted P Wave may also occur without pattern
      2. Other findings
        1. No change in PR Interval (remains constant unlike Mobitz 1)
        2. RR interval is constant, except for the missed beat, which is then double the PP interval
        3. QRS Duration does not differentiate Mobitz 1 from 2 (either may have a normal or prolonged QRS)
        4. Although typically infranodal, the defect may lie within the AV Node in some cases
    4. Causes: Structural, non-reversible defects
      1. Anteroseptal Myocardial Infarction
      2. Lenegre Disease
      3. Lev Disease
      4. Cardiomyopathy
    5. Response to Maneuvers
      1. Mobitz II AV Block is an infranodal disorder, that does not respond to Atropine
        1. Atropine may worsen degree of Mobitz 2 block (e.g. from 2:1 to 4:1)
      2. Mobitz II AV Block also worsens in response to Exercise
    6. Prognosis
      1. Worse than Mobitz I Block
      2. High risk of progression to complete Heart Block (third degree block)
    7. Management
      1. Urgent cardiology Consultation
      2. Pacemaker indicated

V. Types: Third Degree Atrioventricular Node Block (Complete AV Nodal Block)

  1. Findings
    1. Complete electrical and mechanical AV dissociation
    2. P and QRS waves are present but unrelated
  2. Images
    1. cv_ekg_avBlock3.jpg
  3. Subtypes
    1. Congenital Third Degree AV Node Block (narrow complex)
      1. Narrow complex escape rhythm at 45 to 60 bpm (vagal or above the AV Node)
      2. Associated with limited Exercise tolerance
    2. Acquired Third Degree AV Node Block (wide complex)
      1. Wide complex escape beat at 30 to 45 bpm (infranodal)
      2. Hemodynamic instability requires stabilization
        1. Transcutaneous Pacing

VI. References

  1. Berberian, Brady, Mattu (2024) Crit Dec Emerg Med 38(7): 12-3

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