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High-grade AV Block

Dawn's picture
Thu, 03/13/2014 - 14:42 -- Dawn

To continue on a topic started by Jason Roediger in his February ECG Challenge -

This series of two ECGs was taken from a 71-year-old man who complained of dizziness and near-syncope the day before these ECGs were done.  He was seen in an Emergency Dept., and advised to follow up with a neurologist. On the day of these ECGs, still feeling dizzy and like he would pass out, he called EMS again.  He denied chest pain.  We do not know his past medical history.  The first ECG was taken at 10:22 am.  His BP was 177/76 and SpO2 99%.  It shows a regular sinus rhythm (p waves marked by small asterisks) at a rate of about 75 / min.  There is a high-grade AV block, meaning that some P waves are conducted (beats 2, 4, 7), but most are not.  In addition, he has an escape rhythm, probably ventricular, at a rate of just over 40 / min.  The overall effect of the escape rhythm is to keep the heart rate above 40 beats per minute.

Fifteen minutes later, at 10:37 am, another ECG is taken.  The patient's BP is 154/86.   This ECG shows the high-grade AV block quite well, but this time, most of the QRS complexes on the strip are conducted from P waves.  It is difficult to see all the P waves in every lead, but if you remember that all three channels are run simultaneously, you will find evidence of the P waves in at least one of the three leads represented at any given time.  (Example:  V1, V2, and V3 - V3 shows the P waves well).  The next-to-last QRS on the page is interesting, as it has a different PRI than the normally conducting beats.  Is this a fusion beat or an aberrantly-conducted one?   It probably does not matter to the outcome of the patient. 

The slowing of the rate in the second strip gives us a clue as to why the patient felt dizzy, but the blood pressures recorded did not catch hypotension.  Possibly if the patient had been standing instead of lying on a stretcher, we would have seen more hemodynamic changes.

Unfortunately, we do not know the outcome of this patient, but it seems he is a candidate for an implanted pacemaker.

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Comments

ekgpress@mac.com's picture

     Agree completely with Dawn’s interpretation of these 2 serial tracings. I’d add several points.

  • Tracings like this can be confusing! Fortunately the patient was STABLE despite the scary appearance of these 2 12-lead tracings … This means there IS time to print out the tracing. While I prefer to always keep a good unmarked original for documentation - I myself had to PRINT this tracing out in order to interpret it.
  • Mark out the beats! Otherwise - it becomes hard to keep track of what is what. Because there is no long lead II rhythm strip - Calipers really are needed to verify that at least for the TOP tracing P waves are regular.
  • Once you KNOW that P waves are regular - you can verify that beats #2,4,7 in the TOP strip are the ones that are conducting. This IS important because you want to establish the etiology for this high-grade AV block if possible - and you’ll need to find conducting beats in various leads in order to determine if acute infarction is the cause (since you can’t reliably assess ST-T waves most of the time if you have ventricular escape beats … ). I see no sign of anything acute in the ST-T waves of beats #2,4,7 (which covers leads I,II,III; aVR,aVL,aVF; and V4,V5,V6 - which is enough to pretty much exclude acute MI).
  • In the TOP strip - several P waves in a row are NOT conducting. This is why we call this “high-grade” AV block. Note a P wave just before beat #5 with a PR far too short to conduct. This proves we are dealing with a ventricular escape rhythm.
  • As per Dawn - Find 2 escape beats in a row (ie, beats #5,6) - and this gives your your escape rate. QRS morphology of the escape focus looks like a LBBB - ergo the escape site is in the RV.
  • This AV block is 2nd degree because: i) it is not simple 1st degree; and ii) it is not simple 3rd degree (in which you have no conduction of any P waves despite adequate opportunity for conduction to occur). Realize that on rare occasions Mobitz I can drop several P waves in a row - so failure of multiple consecutive P waves in itself does not rule out Mobitz I. What is needed is to see 2 P waves in a row that DO conduct in order to be sure if this is Mobitz I vs Mobitz II. We see this in the BOTTOM strip - beats #1,2 and #4,5 in the BOTTOM strip conduct. I think the PR interval is constant for beats #1,2 and #4,5 - but I admit that I’m not certain due to lead change in both instances …. That said - in this case given very high-degree of AV block it probably doesn’t matter if this is Mobitz I or Mobitz II (it is probably Mobitz II) - because in either instance a pacemaker will be needed once “simple causes” of marked AV block that might be reversible (ie, drugs, recent infarction) have been ruled out.

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

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