Saturday, February 6, 2021

ECG Blog #191 (ECG MP-8) — Is AV Block Complete? (3rd Degree vs AV Dissociation)

The 2-lead rhythm strip shown in Figure-1 was obtained from an elderly woman who presented to the ED following a syncopal episode. On the basis of this rhythm strip — she was diagnosed as being in complete AV Block.

  • Question #1Is there AV Dissociation in Figure-1?
  • Question #2: Do YOU agree that the rhythm shown in this figure represents complete ( = 3rd-degree) AV Block?

  • Extra Credit: Why do I suspect that a majority of those reading this post will get the answer to Question #2 wrong?


Figure-1: This 2-lead rhythm strip was obtained from an elderly woman with syncope. Do YOU agree that this rhythm represents complete AV block?  (See text).


What were YOUR Answers?

  • Your Answer to Question #1 —

  • Your Answer to Question #2 —




NOTE: Some readers may prefer at this point to watch today's ECG Media Pearl #8 before moving on to my final interpretation. That said — Feel free at any time to review my My Thoughts on ECG #1 (that appear just below ECG MP-8).


Today’s ECG Media PEARL #8 (7:00 minutes Video) — is an ECG video that reviews the difference between AV Dissociation vs Complete AV Block — and illustrates this difference while explaining the interpretation of the rhythm in Figure-1.

  • NOTE: See also ECG Blog #192 — which emphasizes the 3 Causes of AV Dissociation.



MY Thoughts on the Rhythm in Figure-1:

Note that the above ECG Media PEARL #8 reviews interpretation of the rhythm in Figure-1 in detail. For clarity — I reproduce below selected KEY figures from this video.

  • We see a total of only 5 beats in Figure-1. The ventricular rhythm is regular at a rate just under 50/minute. It is difficult to assess QRS duration from just these 2 leads — but it looks like there may be slight QRS widening (at the least, we see a multiphasic QRS complex in lead V1).
  • The atrial rhythm is fairly regular at a rate of ~100-110/minute (RED arrows highlighting P waves that we are certain about). Use of calipers tells us to expect another P wave at the spot highlighted by the PINK arrow (Figure-2). So, given the overall regular appearance of the 10 other P waves in this tracing (RED arrows) — we can be almost certain that P wave regularity continues, even though 1 P wave is hidden by simultaneous occurrence of the QRS complex of beat #3.


Figure-2: I’ve added RED arrows to Figure-1 to indicate P waves were are certain about. Given the overall regularity of the P wave rhythm — we strongly suspect another P wave is hiding within the QRS complex of beat #3 (PINK arrow).



MY Thoughts on the Rhythm (Continued):

By the systematic Ps, Qs & 3R Approach to rhythm interpretation (which we discussed in detail in ECG Blog #185) — the remaining parameter not yet assessed in Figure-2 is the 3rd “R” = Are P waves Related (ie, “married” ) to neighboring QRS complexes?

  • The easiest way to assess if P waves are related to neighboring QRS complexes — is to look in front of each QRS complex — and see if the PR interval stays the same or constantly changes.
  • In Figure-2 — it should be readily apparent that the PR interval in front of each of the 5 QRS complexes is never the same. 
  • PEARL #1: To emphasize — that although the PR intervals that precede beat #1 and beat #2 look similar — use of calipers immediately shows that the PR interval preceding beat #2 is longer. Calipers also immediately show that the PR interval preceding beat #3 is longer than the PR interval preceding beat #2. Thus, the PR intervals in Figure-2 are constantly changing.

Putting This Together: In Figure-2 — there is a slightly widened QRS complex with a regular ventricular rhythm (at just under 50/minute) — a fairly regular atrial rhythm (at ~100-110/minute) — and — a PR interval before each of the 5 QRS complexes that constantly changes!

  • Therefore — there is complete AV Dissociation for the 5-beat (6.4-second duration) rhythm strip shown in Figure-2 — because none of the P waves are being conducted to the ventricles.
  • PEARL #2: Although there is complete AV dissociation for this 5-beat rhythm strip in Figure-2 — we have not proved that there is complete AV block. Often ignored by many clinicians is the all-important criterion for complete AV block that requires the rhythm strip to be long enough — and — the rhythm to be slow enough for P waves to occur in all phases of the cardiac cycle, yet still fail to conduct. That is, before you can prove the presence of complete AV block — you have to demonstrate that P waves have adequate opportunity to conduct (by occurring in all parts of the R-R interval) — yet still fail to do so. The 6.4-second rhythm strip in Figure-2 is unfortunately just too short to guarantee that there has been adequate opportunity for this to occur.
  • As to the interpretation of this rhythm — although we have not proved the presence of complete AV block — there at the least appears to be significant 2nd-Degree AV Block — because we would normally expect the P waves occurring near the middle of each R-R interval to conduct, yet there is no evidence of any conduction (ie, no QRS complex follows closely after any of these P waves).


The Case Continues: In Figure-3 — the next 4 beats that come after beat #5 are now shown.

  • Do YOU still think complete AV block is present?


Figure-3: The rhythm from Figure-2 is continued — and the next 4 beats are shown. Is there still complete AV dissociation?

What do YOU Think?

  • In Figure-3 — Is there still complete AV dissociation?


MY Thoughts on Figure-3:

A number of changes are evident in Figure-3, now that we have seen the next 4 beats in the tracing.

  • Note that the QRS complex narrows and changes in morphology for the last 4 beats in Figure-3.
  • Regular P waves continue throughout the entire tracing. Yet as opposed to what we saw with the shorter (5-beat) tracing in Figure-2 — the PR interval preceding beats #6, 7, 8 and 9 in Figure-3 now appears to be constant! This tells us that beats #6, 7, 8 and 9 are sinus conducted — which confirms that the rhythm in Figure-2 was not complete AV block, because when P waves finally have an opportunity to conduct, they do!
  • These last 4 beats in the rhythm strip also confirm the conduction defect as a form of 2nd-Degree AV Block, in which there is 2:1 AV conduction.  


PEARL #3: I emphasized in the above ECG Media Pearl #8 — the difference between complete AV block vs AV dissociation without complete AV block. The KEY for making this distinction, is to look for evidence of conduction.

  • The fact that the PR interval preceding beats #6, 7, 8 and 9 remains the same tells us that these beats are sinus conducted!
  • Sinus conduction is further supported by narrowing of the QRS complex (with a change in QRS morphology) for these last 4 beats in Figure-3.
  • NOTE: Most of the time with complete AV block — the ventricular rhythm will be regular. This is because escape rhythms are in general regular. Although extremely subtle in Figure-3 (You will need calipers to verify this!) — the R-R interval preceding beat #6 is ever-so-slightly shorter than the R-R interval preceding each of the prior 5 beats in this tracing. Recognition in a tracing with AV dissociation that a beat occurs earlier-than-expected is an excellent clue that the reason this beat occurs slightly earlier is that it is being conducted!

PEARL #4: To clarify the difference between AV dissociation vs complete AV block — I've added an ADDENDUM to this post, in which I reproduce a 7-page excerpt from my ACLS-2013 Arrhythmias (Expanded Version) book. Among the KEY "take home" points from this excerpt are the following:

  • The term, “AV dissociation” — simply means that for a certain period of time, sinus P waves are not related to neighboring QRS complexes. That is, P waves preceding the QRS are not being conducted to the ventricles.
  • AV dissociation may be as short-lived as occurring for 1 or 2 beats — or, it may be persistent as occurs with complete AV block. 
  • AV dissociation is never a “diagnosis”. Instead — it is a condition cause by “something else”. The task for the clinician is to figure out what the cause of AV dissociation is for any given rhythm. There are 3 possible causes: i) AV block itself (either from 2nd- or 3rd -degree AV block); ii) Usurpation — in which P waves transiently do not conduct because an accelerated junctional or ventricular rhythm takes over the pacemaking function (ie, “usurps” the rhythm); and/oriii) Default — in which a junctional or ventricular escape rhythm takes over by “default”, because the rate of the sinus pacemaker has slowed down for whatever reason. 


Laddergram Illustration:

To better illustrate the mechanism of the 9-beat rhythm we see in Figure-3 — I’ve drawn a laddergram (Figure-4):

  • A fairly regular atrial rhythm is seen throughout the tracing (RED arrows).
  • There is complete AV dissociation for the first 5 beats in the tracing — because none of the P waves during this initial part of the rhythm are conducted to the ventricles. 
  • It is difficult to be certain from where in the ventricles the escape rhythm arises during these first 5 beats (ie, could be from the ventricles, or from one of the hemifascicles or bundle branches) — but the escape rhythm almost certainly arises from below the AV node.
  • The P wave preceding beat #6 is conducted! Note that the PR interval preceding beats #7, 8 and 9 is the same as the PR interval preceding beat #6, because all 4 of these beats are conducted.
  • There is 2nd-Degree AV Block, with 2:1 AV conduction for these last 4 beats on the tracing. 
  • The reason for AV dissociation in Figure-4 during the initial part of the tracing is 2nd-Degree AV Block. It is impossible to be certain from this single rhythm strip, whether this represents Mobitz I or Mobitz II 2nd-degree AV block — because we never see 2 conducted P waves in a row (so we don’t know if the PR interval would progressively increase IF given a chance to do so).
  • Beyond-the-Core: We also can not tell whether or not this rhythm represents a form of “high-grade” 2nd-degree AV block — because we never see 2 P waves in a row that should conduct yet fail to do so during the early portion of this rhythm strip in which there is complete AV dissociation.
  • P.S.: For those wanting review on how to read (and/or draw) laddergrams — Please CHECK OUT my ECG Blog #188.

Figure-4: Laddergram illustration of the mechanism for the 9-beat rhythm strip shown in Figure-3.

  • What follows below is a 7-page excerpt from my ACLS-2013 Arrhythmias (Expanded Version) book, in which I review the distinction between AV dissociation vs complete AV block.


  1. Hello. Thank you for this excellent ecg tracing. I thought this ecg was AVD and not a CHB just by looking at figure 1. I believe that the P wave in beat # 5 is conducting and related to its complex because it looks normal and the PRI has a normal duration. That is why I did not think the heart block is complete. Of course, after you posted figure 2, it became clearer that the following pr intervals were conducted. Am I correct or not correct that in beat 5 the p wave is conducted? Thank you so much for your explanation and your expertise. Good day to you.

    1. Thank you so much for your excellent comment! When all we saw was 5 beats — you could not tell if beat #5 (the last beat in the short strip that I initially showed) — but you just could NOT tell if those 5 beats represented simple AV dissociation with less than complete AV block or AV block. Remember that “AV dissociation” is NOT a “diagnosis”, but rather a condition that could be due to: i) AV block or ii) AV dissociation from “default” (sometimes without any AV block) or iii) “usurpation”.

      Once I show you the complete 9 beats — we see that beats #6-thru-9 definitely DO conduct. Although the PR interval before beat #5 looks “potentially normal” — it is the SHAPE of the QRS that tells me that like the 4 beats before it — beat #5 is NOT conducted. And, IF you look closely — the P wave of beat #6 begins right ON the heavy grid line — and the QRS begins just a tiny bit before the next heavy line. In contrast — the P wave of beat #5 clearly begins BEFORE the heavy grid line — and the QRS begins EARLIER with respect to the next heavy grid line compared to beat #6 — so the PR interval IS shorter before beat #5 — which confirms that beat #5 is NOT being conducted!

      THANKS again for your interest! — :)