2nd degree AV block



Mobitz 2



2nd degree Mobitz 2 heart block is a defect or disease of the HIS bundle (His-Purkinje system)
located below the AV node. The electrical signal from the SA node contracts the atria as usual
(a P-wave on the ekg strip). However, after the AV node sends the signal down to contract the
ventricles, it suddenly gets cut off (blocked) at the HIS bundle/bundle branches and the
ventricles do not contract. Looking on an ekg strip (see illustration above), it shows no QRS
following the P-wave, AKA "dropped beat". Mobitz 2 heart block has no P-R widening, all
P-R intervals are the same. Mobitz 2 is more like an electrical shortage. Unlike Mobitz 1
(wenckebach), whereas the signal is delayed by a repolarizing exhaustion of the AV node
which builds with consecutive beats. Mobitz 2 causes a sudden electrical failure and is more
dangerous/threatening due to its location and instability. Mobitz 2 carries an elevated risk
of having no escape rhythm emerging in the event of going into complete blockage/failure.



High Grade 2nd degree block:
When there are multiple dropped beats between each QRS (see above),
it is a higher risk. If there are three or more dropped beats between QRS's,
it is considered a "high grade" block

Both 2nd degree blocks; Mobitz 1 and Mobitz 2, prevent the ventricles from contracting. However,
the mechanism in which they work are quite different from each other due to their location.

Mobitz 1 (wencke):
is located at the AV node and is often called, "AV nodal block".

Mobitz 2:
resides in the HIS bundle or lower in the bundle branches. It is referred to as
"infranodal block" (below the AV node), or a "HISIAN Block" (HIS bundle).

Causes:
    Mobitz 2 is usually caused by structural damage to the conduction system
  • Myocardial Infarction
  • Cardiac Surgery
  • Heart muscle Infections
  • Idiopathic Fibrosis
  • Infections
  • Hyperkalemia
  • Autoimmune diseases
  • Clogged arteries
  • Autoimmune diseases (SLE, systemic sclerosis).
  • Cardiac medications / Beta blockers, calcium channel blockers, digoxin, amiodarone.
  • Rheumatic fever and sarcoidosis

Symtoms:
  • Irregular heart rhythm
  • Dizziness/syncope
  • Chest pain
  • Fatigue
  • Shortness of breath

Treatment:
  • Stop medications if they are the cause
  • If symptomatic, immediate admission/cardiac monitoring
  • Medical evaluation as to causes
  • Pacemaker insertion if irreversible


Return to Home Page