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Second-degree A-V Block

Thu, 03/27/2014 - 20:17 -- Dawn

Paramedic Erik Testerman has generously donated several excellent teaching ECGs to the Guru, and we will be featuring all of them soon.  This week, we show you the ECGs from a 59-year-old man with a blood glucose of 30 mg/dl.  He had no complaints .  After a bolus of Dextrose 10%, his blood glucose was 105 mg/dl.  He gave a past medical history of diabetes mellitus, hypertension, and left bundle branch block. Vitals were reported as normal and stable, except for the slow heart rate.

The first ECG, taken in the field, show a second-degree AV Block.  The conduction ratio is 2:1.  That is, there are two P waves for every QRS complex.  With this ratio, it is sometimes difficult to determine whether the patient has Type I (usually AV nodal) or Type II (Infranodal) AVB.  In order to diagnose Type I AVB (Wenckebach), we need to see TWO P waves in a row conducted, to see the prolongation of the PR interval.  It is not correct, however, to call ALL 2:1 AV blocks "Type II".  Often, simply taking a longer rhythm strip will expose a period of 3:2 conduction, showing progressive prolongation of the PRI.

This ECG, however, gives us some clues that it is probably TYPE II.  The patient has a left bundle branch block.  Type II AVBs are infranodal - that is, they affect the structures below the AV node:  the His Bundle and the Bundle Branches.  Type II AVBs represent INTERMITTENT TRI-FASCICULAR BLOCK, and that is common in the presence of RBBB  and LBBB (a bi-fascicular block).  This ECG probably represents an existing LBBB with an intermittent RBBB - When the right bundle is blocked, the patient has a tri-fascicular block, and no conduction to the ventricles.

Another clue that this is Type II is that the NON-CONDUCTED P waves fall CLEAR of the preceding T waves, meaning that they had ample opportunity to conduct, not being in the absolute refractory period.

Fortunately, the rhythm strip, taken one minute later, uncovers the diagnosis!  The sixth and seventh QRS complexes are conducted with a 3:2 ratio, showing PR intervals that stay the same, proving the rhythm is Type II. 

For this patient, the heart block and resulting bradycardia don't seem to be causing symptoms.  But infranodal blocks can easily progress to complete heart block and should be treated with implanted pacemakers.  The EMS crew in this case had transcutaneous pacer pads on the patient as a precaution, but he remained well-perfused and with a good BP the whole time. 

 

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ekgpress@mac.com's picture
     Excellent case presented by Dawn (donated by Erik Testerman). Rather than division of 2nd Degree AV Blocks into 2 types (and interpreting this particular ECG as “Mobitz I vs Mobitz II” AV Block) - it may be more practical classify the 2nd Degree AV Blocks as consisting of 3 Types: i) Mobitz I; ii) Mobitz II; and iii) 2nd Degree with 2:1 AV Block.
 
The importance of distinguishing between Mobitz I vs Mobitz II - is that a pacemaker will probably be needed with Mobitz II. In contrast - most of the time a pacer will not be needed with Mobitz I. Fortunately - you can usually get a pretty good idea of Mobitz I vs Mobitz II based on the following:
  • i) Statistics: Mobitz I is MUCH more common than Mobitz II. Although I’ve not seen frequency statistics published - in my experience, well over 90% of all 2nd Degree AV Blocks are Mobitz I. This is fortunate given the usually better prognosis of Mobitz I.
  • ii) QRS Width: A wide QRS strongly supports the likelihood of Mobitz II. That said - exceptions exist. On rare occasions - the QRS may be narrow with Mobitz II - and if preexisting BBB (Bundle Branch Block) exists - then you may have a wide QRS with Mobitz I.
  • iii) History: If the patient is having acute inferior MI - then Mobitz I becomes MUCH more likely. In contrast - Mobitz II is more likely if there is acute anterior MI.
  • iv) Duration of the PR Interval for Conducting Beats: If conducting beats manifest a long PR interval - then Mobitz I is more likely. If the PR is normal (or tending toward short) - then Mobitz II becomes more likely.
  • v) Is there Evidence of Mobitz I Elsewhere? If time permits - Search telemetry strips. Although not impossible - it is very unlikely for the rhythm to go back-and-forth from Mobitz I-to-Mobitz II. Therefore - finding definite Wenckebach on preceding telemetry strips strongly favors the 2:1 Block you are currently analyzing to being the same Mobitz I.
In the TOP tracing for this case - there is 2:1 AV Block with LBBB morphology for conducting beats. The PR interval of conducting beats is not long. Therefore - Mobitz II should be strongly suspected.  Note that the rate is in the mid 40s - so regardless of the “type” of 2nd degree AV block the patient has - there is likely to be trouble. But given slow rate and likely Mobitz II - one should optimize “pacing readiness”.
 
The BOTTOM tracing illustrates the point that is most often overlooked. You cannot diagnose Mobitz II with certainty from the surface ECG unless you see consecutively conducted beats. We finally see 2 beats conducted in a row (at the end of the BOTTOM strip) - which proves Mobitz II given no increase in the PR interval.

For reinforcement of these concepts regarding the various Types of 2nd Degree AV Block - and distinction between Mobitz I vs Mobitz II - Please check out these 3 ECG Blogs:

 

 

Ken Grauer, MD  www.kg-ekgpress.com   [email protected] 

Submitted by TestyLP on

Dawn and Dr. Grauer, I thank you so very much in expanding on this case I donated. I love everything about being a Paramedic, but I mostly love Cardiology!!! Further info on this pt, when I arrived at the hospital, his pulse was resting around 30 bpm, still in the 2:1 AV Block. The charge nurse unfortunately did not take me very seriously, and I was given a weird look as to why I had pacer pads on him.

Any words of advice either of you two may have for me to better myself would be greatly appreciated!!

Erik Testerman, AAS, LP, NREMT-P

Submitted by Dawn on

You don't really need my advice!  You are obviously a Paramedic who cares about doing a good job for your patients, and that is what matters.  In order to do this, you have to keep studying (you are doing that) and you have to follow up on your calls to learn as much as possible from them (you are doing that).  Thanks so much for sharing your ECGs with the ECG Guru so that others can learn, too.

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