AV Block Case #2

Morning report ECG
Elias B Hanna, MD
LSU New Orleans, Cardiology
Analyze the following ECG with a focus on
the rhythm
1-What is the baseline rhythm?
• P is (-) in I, (+) in aVR
-this could mean the rhythm is not a sinus
rhythm, i.e., ectopic atrial rhythm
OR -it could be arm electrode reversal
• In case of arm lead reversal: P, QRS, and T will be
inverted in I and aVL, not just P wave. The P/QRS/T in
lead I will be flipped over, while the complexes in
leads aVL and aVR are switched
 this is not the case here, thus it is an ectopic atrial
2-How do you explain the pause?
• A pause has 3 causes:
1-Sinus pause: in this case, no blocked P is seen within the pause
(the sinus node is “sleeping”, thus no P or QRS are seen)
2-AV block (2nd degree): a blocked P is seen within the pause. The
blocked P marches out with the regularly occuring baseline P
3-Blocked PAC: : a blocked P is seen within the pause, sometimes
over the preceding ST/T. The blocked P does not march out with
the regularly occuring baseline P and actually comes
Which ones of these 3 causes of
pause are benign?
• Blocked PAC is the most benign of the 3
• Sinus pause, if short < 3sec or esp. if
asymptomatic, is benign
• AV block is benign if it is 2nd degree Mobitz 1.
It is ominous if it is 2nd degree Mobitz 2, even
if it is just an asymptomatic brief pause
• In our case, there is a blcked P within the
pause that marches out with the preceding P
wavesThis is AV block (see next Figure)
Blocked P marches
out with preceding
P waves
3-Is it a Mobitz 1 or Mobitz 2 seconddegree AV block?
• In Mobitz 1, PR progressively prolongs before
the pause, while in Mobitz 2, there is a sudden
block of P conduction without any preceding
PR prolongation
• However, in case of a very slowly progressive
PR prolongation, it may seem that PR is not
prolonging, and Mobitz 1 may be
misdiagnosed as Mobitz 2.
That is why it is best to look at the PR just
preceding the block and compare it to the PR
that follows the block
Ectopic P
Ectopic P
See how PR that precedes the block is
longer than PR that comes after the block Mobitz 1
Final ECG dx: Ectopic atrial rhythm rate of 95 bpm with Mobitz 1 (Wenckebach) AV
P drops without progressive PR prolongation=Mobitz 2. Rate~60 and the pt is
asymptomatic, it would seem innocuous but this is actually ominous because it is
Mobitz 2. Mobitz 1 is benign.
That is why it so important to carefully analyze every small pause. A benign Mobitz
1 or a blocked PAC may very well simulate the malignant Mobitz 2
Location of AV block
Mobitz 1 is usually a nodal
blockthe QRS is often narrow.
Mobitz 1 rarely progresses to
complete AV block and is often
Mobitz 2 is an infranodal blockthe QRS is
often wide. Mobitz 2 is ominous even if
asymptomatic, it can progress to a bad
complete AV block with slow ventricular
Am example of 2: 1 AV block
In 2:1 AV block, there is only one conducted QRS before the dropped QRS, thus you
cannot tell if the dropped QRS is preceded by progressive PR prolongation or not, i.e.
Mobitz 1 or 2. In order to say Mobitz 1 or 2 in case of 2:1 AV block, rely on the width of
QRS. If QRS is wide, it is an infranodal block, i.e. Mobitz 2, ominous; if QRS is narrow, it
is Mobitz 1. The 2:1 AV block on the current ECG is therefore Mobitz 2 AV block.

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