Tuesday, January 1, 2013

ECG Interpretation Review #58 (2nd Degree - 3rd Degree AV Block - AV Dissociation)

The ECGs in Figure 1 and Figure 2 were both interpreted as showing 3rd degree (complete) AV block.
  • Do you agree?
Figure 1: Lead II rhythm strip. Is this complete AV block? (Figure reproduced from Case M in ACLS: Practice Code Scenarios-2013).
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Figure 2: Lead MCL-1 rhythm strip. Is this complete AV block? (Figure reproduced from Case M in ACLS: Practice Code Scenarios-2013). NOTE - Enlarge by clicking on Figures - Right-Click to open in a separate window.
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INTERPRETATION: We assess both rhythm strips by the Ps,Qs & 3R Approach, looking for: i) Presence (or absence) of P waves (atrial activity?); ii) QRS width (>0.10 sec is wide?); iii) Regularity of the rhythm; iv) Rate; and v) If P waves are present - Are P waves Related to the QRS (Are P waves conducting?).

FIGURE 1: - The QRS complex is narrow. The first 3 beats show sinus bradycadia at ~55/minute. The PR interval then noticeably shortens (ie, the PR preceding beats #4,5, and 6 is clearly too short to conduct!). Thus, there is transient AV Dissociation (since P waves are at least temporarily unrelated to the QRS).
  • Beats #7 and 8 represent a junctional rhythm at ~58/minute.
  • It is because the sinus rate slows (to 55/minute) that a slightly faster junctional rhythm can take over (beginning with beat #4).
  • We see NO evidence of any AV block on Tracing A - because P waves never fail to conduct when given a chance to do so (P waves before #4,5,6 don't have a chance to conduct ...).
FIGURE 2: - shows complete (3rd degree) AV Block. Note that the QRS does not necessarily have to be overly wide for there to be 3rd degree AV block.
  • The atrial rate is regular (marched out in Figure 3 with RED arrows).
  • There is also a regular ventricular rhythm - but P waves at all points in the cardiac cycle fail to conduct despite having adequate opportunity to do so (P waves "march through" the QRS).
Figure 3: Arrows indicate regular atrial activity that "marches through the QRS" in this patient with 3rd degree AV block (See text).
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AV Dissociation vs Complete AV Block
AV Dissociation is not the same as 3rd degree AV block. The term, "AV dissociation" merely means that one or more P waves is not related to a neighboring QRS. AV dissociation may be transient or permanent. It may be due to pathologic conditions such as 2nd or 3rd degree AV block - or it may be a benign manifestation of the simple fact that the sinus node temporarily slows down and is replaced in its pacemaker function by an appropriate AV nodal escape rate between 40-60/minute.
  • Always try to determine the cause of AV dissociation - of which there are three: 1) AV block itself (could be 2nd or 3rd degree AV block); 2) Usurpation - in which P waves transiently do not conduct because an accelerated junctional rhythm takes over ("usurps" - as commonly occurs with Dig toxicity); and 3) Default - in which a junctional escape rhythm takes over by default" (because of SA node slowing).
  • Figure 1 is an example of AV dissociation by "default" (slowing of the rate from sinus bradycardia that allows emergence of an appropriate junctional escape rhythm).
  • PEARL: In order to confidently diagnose complete AV block - the ventricular rate needs to be slow enough (usually <45-50/minute) in order to guarantee that P waves will occur in all phases of the cardiac cycle, yet still fail to conduct despite having adequate "opportunity" to conduct. This clearly occurs in Figure 3.
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- For more information - GO TO:
2) Case M from ACLS: Practice Code Scenarios-2013 (5th edition) -
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8 comments:

  1. I read and really impressed form your topic about "ECG Interpretation Review #58 (2nd Degree - 3rd Degree AV Block - AV Dissociation)". It's very comprehensive and informative and helpful for us. I was looking information about Dr. Luis Fandos, I am very happy to read it.

    Thanks for sharing.....

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  2. Hi Jabed. Thank you for the kind words about this ECG Blog #58.

    I must admit I am puzzled why you are asking me about Dr. Luis Fandos. I don't believe I have ever met him - and do not know the relation between Dr. Fandos and this ECG Blog?

    Thanks again for your interest in my material! - : )

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  3. I took care of a patient in ICU last night that also had an AV dissociation by default. He maintained a sinus rhythm as long as his rate was above 70. However, when he fell asleep and his rate dropped into the 60s a junctional rhythm took over. Thanks for the great info. On QRS complex 5 & 6, could the shortening of the PR interval represent some kind of atrial escape activity?

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  4. Thanks for your Comment Mike! A common time to see AV dissociation by default is at night during sleep (as happened with your patient). The sinus rate slows - and if the AV node doesn't also slow to the same degree - you'll sometimes see the AV node take over. This is precisely what is happening in Figure 1. The shortening of the PR interval tells us that the P waves in front of beats #4,5,6 are no longer conducting (the PR interval is too short for conduction to occur). The SA node continues to fire (in this case at the slow rate of ~ 55/minute) - but the slightly faster AV node (at 58/minute) takes over and fires before the SA nodal impulse can get down to the ventricles. If this was atrial escape - you would see another P wave morphology for these P waves.

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  5. Thanks Ken for the response! I forgot to mention also that the patient was on a nightly dose of metoprolol.

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  6. @Mike - the nightly dose of metoprolol sounds like the "culprit". The dose may need to be titrated down or stopped. Always a challenge ...

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  7. Thanks for all of the information! I work for a medical equipment company and wanted to learn more about the interpretation of the ekg. Although overwhelming at times, your information is great. Thanks again
    Andrew Schmidt
    Discount Cardiology
    www.cardiologyforless.com

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    Replies
    1. THANK YOU for the kind words Andrew. There are lots of links on this site (esp. top righthand column) if you are interested in additional ECG-related educational material. Feel free to e-mail me directly - Ken Grauer, MD (ekgpress@mac.com). THANKS again for your interest!

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