I. Anatomic classification (requires His bundle electrocardiography)
- AV nodal ("supra-Hisian")==>nl QRS width
- Intra-Hisian==>nl QRS width
- Infra-Hisian==>wide QRS
II. Clinical classification (any clinical type can occur with block at any anatomic level)
- First degree
- Results from slowing of AV nodal conduction, due to
- Age-related degenerative changes (usually)
- Dig
- Increased vagal tone
- AV nodal ischemia (can occur with IMI)
- Myocarditis
- Severe aortic regurgitation
- PR >0.2 sec at HR 70 or >0.17sec at HR 115
- Usually clinically silent and benign
- Second degree
- Mobitz type I (Wenkebach)
- See progressive prolongation of PR interval until a beat is not conducted at all; then sequence repeats
- Same causes as 1st degree AVB
- Delay is usually in the AV node rather than below, so PR interval decreases with atropine, unlike Mobitz II
- Often responds to atropine
- Mobitz type II
- Consists of intermittent interruption of AV conduction, identified as 2:1, 3:1, etc. based on how often beats are conducted
- Results from an infranodal block
- When beats are conducted, PR interval is nl
- Frequently progresses to complete heart block, unlike Mobitz type I
- Can get wide QRS is location of block is infra-Hisian
- Causes include anterior MI, degenerative changes, calcification of annuli of mitral or aortic valves
- Third degree ("complete heart block")
- All atrial beats are blocked and ventricles are driven by an escape rhythm
- 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
- Usually caused by degenerative aging changes; ischemia can contribute
- Complications include syncope and worsening of CHF
- Get signs of AV dissociation (see above), unless in Afib
- Tx: epinephrine, pacing
III. Avoid antiarrhythmics in pts with AV block and syncope, since they may suppress lower escape foci