I. Anatomic classification (requires His bundle electrocardiography)

  1. AV nodal ("supra-Hisian")==>nl QRS width
  2. Intra-Hisian==>nl QRS width
  3. Infra-Hisian==>wide QRS

II. Clinical classification (any clinical type can occur with block at any anatomic level)

  1. First degree
  1. Results from slowing of AV nodal conduction, due to
  1. Age-related degenerative changes (usually)
  2. Increased vagal tone
  3. AV nodal ischemia (can occur with IMI)
  4. Myocarditis
  5. Severe aortic regurgitation
  6. Connective tissue disease
  7. Medications (beta-blockers, calcium channel-blockers, digoxin, adenosine)
  1. PR >0.2 sec at HR 70 or >0.17sec at HR 115, and doesn't change from beat to beat
  2. Usually clinically silent and benign though:
    1. Can result in delayed and ineffective mitral valve closure
    2. Associated with elevated risk for atrial fibrillation, pacemaker placement, and all-cause mortality (JAMA 301:2571, 2009)
  1. Second degree
  1. Mobitz type I (aka "Wenkebach")
  1. See progressive prolongation of PR interval until a beat is not conducted at all; then sequence repeats
  2. Same causes as 1st degree AVB
  3. Delay is usually in the AV node rather than below, so PR interval decreases with atropine, unlike Mobitz II
  4. Often responds to atropine
  1. Mobitz type II
  1. Consists of intermittent interruption of AV conduction, identified as 2:1, 3:1, etc. based on how often beats are conducted
  2. Results from an infranodal block
  3. When beats are conducted, PR interval is nl
  4. Frequently progresses to complete heart block, unlike Mobitz type I
  5. Can get wide QRS is location of block is infra-Hisian
  6. Causes include anterior MI, degenerative changes, calcification of annuli of mitral or aortic valves
  1. Third degree ("complete heart block")
  1. All atrial beats are blocked and ventricles are driven by an escape rhythm
  2. Escape rhythm is either junctional or idioventricular, depending on the location of the block; the latter with a wide QRS and ventr. rate 20-40
  3. Usually caused by degenerative aging changes; ischemia can contribute
  4. Complications include syncope and worsening of CHF
  5. Get signs of AV dissociation (see above), unless in Afib
  6. Tx: epinephrine, pacing

III. Avoid antiarrhythmics in pts with AV block and syncope, since they may suppress lower escape foci