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.
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".
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).