2nd Degree AV Block Type II (Mobitz 2)

2nd degree AV block type II ( Mobitz II), is a sinus rhythm having extra P waves with no
ventricular conduction (QRS complex) following them, or otherwise known as "dropped beats"
(see illustration above). The P waves march out evenly, the ventricular rate (R to Rs) is
irregular because of the dropped beats. 2nd degree block Mobitz II doesn't have a widening
PR pattern as with 2nd degree type 1 (Wenkebach). With 2nd degree Mobitz II, all PR intervals
are the same. In the example above, the dropped beat has two P waves between R to Rs. The
first P wave is blocked and the 2nd P wave conducts, having a QRS complex following.

Please notice that all PR intervals are equal, the dropped beats and the non-dropped beats
all have equal PR intervals. Winkebach and Third degree block both have unequal PR intervals.
If you see an extra P wave (dropped beat), first look at the PR interval. If it has equal PRs,
it's a Mobitz II. If not, then it's either Wenkebach or a Third degree block. First rule out a
Wenkebach because it is far more common to see than a third degree block.

With 2nd degree AV block, it is very important to know what type it is. Type 1 (Winkebach)
has a widening P-R interval before the dropped beat and is typically asymptomatic, rarely
requiring treatment. 2nd degree Mobitz II has a very high risk of going into third degree
block (total AV block), and has more severe symptoms, likely requiring hospitalization and
immediate treatment and usually requiring a Pacemaker. The Mobitz II block is located below
the AV node, at the Bundle of His or lower at the Bundle branches (see graph above). This
location is why Mobitz II is more dangerous because the electrical signal drops suddenly.

Wenkebach is located higher at the AV node and has a gradual lengthening drop, almost always
resetting after just one dropped beat. Whereas Mobitz II suddenly cuts out, and stands a
greater chance of continually dropping consecutive beats. Its location and that it suddenly
cuts out, puts it at a high level of risk to go into 3rd degree block, Stokes-Adams attack,
cardiac arrest or sudden cardiac death.

2nd degree Mobitz II can have more than two consecutive P waves within the same R to R segment.
The more consecutive dropped beats per QRS, the more high risk (unstable) the block becomes,
which would be called a High Grade 2nd degree block . The EKG strips below show a few different
examples of 2nd degree Mobitz II.

This strip shows variable ratios having 3:1 and 2:1 per QRS

This strip shows a ratio of three P-waves per QRS

This strip shows variable ratio with four P-waves per QRS

Causes of Mobitz II
  • Anterior MI (due to septal infarction with necrosis of the bundle branches).
  • Cardiac surgery (especially surgery occurring close to the septum, e.g. mitral valve repair)
  • Inflammatory conditions (rheumatic fever, myocarditis, Lyme disease).
  • Autoimmune (SLE, systemic sclerosis).
  • Infiltrative myocardial disease (amyloidosis, hemochromatosis, sarcoidosis).
  • Hyperkalaemia.
  • Drugs: beta-blockers, calcium channel blockers, digoxin, amiodarone.
  • Idiopathic fibrosis of the conducting system.

  • Clinical Significance
  • Mobitz II is associated with hemodynamic compromise and severe bradycardia
  • progression to 3rd degree heart block.
  • hemodynamic instability may be sudden, causing syncope/ Stokes-Adams attacks/ sudden cardiac death.
  • The risk of asystole is around 35% per year.
  • Requires admission for monitoring, temporary pacing and likely a permanent pacemaker.

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    Refrences for graphics and info used from: lifeinthefastlane.com