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).
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.
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).
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.

- For more information - GO TO:
2) Case M from ACLS: Practice Code Scenarios-2013 (5th edition) -



  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.....

  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! - : )

  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?

  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.

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

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

  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

    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!