Sunday, June 20, 2021

ECG Blog #235 (51) — Is There Complete AV Block?

The ECG shown in Figure-1 was obtained from a patient with dizziness.

  • How would you interpret this ECG?
  • And — Is there Complete AV Block?


Figure-1: ECG obtained from a patient with dizziness (See text).



NOTE: Some readers may prefer at this point to listen to the 8:25 minute ECG Audio PEARL before reading My Thoughts regarding the ECG in Figure-1. Feel free at any time to refer to My Thoughts on this tracing (that appear below ECG MP-51).


Today’s ECG Media PEARL #51a (7:40 minutes Audio) — Reviews my thoughts on "Some Simple Steps to Help Interpret Complex Rhythms".



My THOUGHTS on the ECG in Figure-1:

This is a challenging tracing to interpret. A long lead rhythm strip is shown at the bottom of the 12-lead ECG. Note that neither the rhythm strip, nor the 12 leads in the ECG were recorded simultaneously — which complicates interpretation. That said — My sequential thought process for interpreting the rhythm was as follows:

  • Most of the beats in Figure-1 are wide. Because recordings for each of the 12 leads in the ECG are of such short duration — We are totally dependent on the long lead V4 rhythm strip for determining the rhythm.
  • The QRS for the first 5 beats in the rhythm strip are wide. The R-R interval is regular, and just over 6 large boxes in duration (which corresponds to a rate just under 50/minute).
  • Beats #6, 7 and 8 have a narrow QRS complex. There appears to be at least some conduction — because these last 3 beats are each preceded by a P wave with a constant (and normal) PR interval.
  • A number of other P waves are seen on this tracing — many of which do not appear related to neighboring QRS complexes. This suggests some degree of AV block.


PEARL #1: Note that beat #6 occurs earlier-than-expected, considering the longer R-R interval between beats #1-thru-5. This strongly suggests that beat #6 is conducted, especially since it is preceded by a P wave with a normal PR (that is shorter than any of the PR intervals seen for the first 5 beats on the tracing).

  • Since the PR interval preceding beats #7 and 8 is identical to the PR interval preceding beat #6 — we can presume that beats #7 and 8 are also conducted.



At this point in my interpretation — I needed to find the rest of the P waves.


QUESTION: Is the atrial rhythm in the long lead V4 rhythm strip of Figure-1 regular?

  • HINT: Use calipers!



ANSWER: I've labeled the P waves that I saw in Figure-1 with colored arrows (Figure-2).

  • RED arrows highlight P waves that I am certain about.
  • Setting my calipers to the P-P interval suggested by 2 consecutive P waves that I was certain about (ie, the 2 P waves just before and just after either beat #6 or beat #7) — I "walked out" where we would look for additional P waves IF the atrial rhythm was regular.
  • PINK arrows show tiny deflections that most likely represent hidden P waves.
  • The WHITE arrow shows where the remaining P wave would fall if the atrial rhythm was regular.

Figure-2: I've labeled P waves that I saw in Figure-1 with colored arrows (See text).


PEARL #2: The KEY to diagnosis of the various forms of AV block is to establish IF the atrial rhythm is regular, and if so — what relation there is (if any) between P waves and neighboring QRS complexes.

  • RED arrows in Figure-2 indicate that regular P waves are definitely seen throughout most of the long V4 rhythm strip.
  • While it is possible for there to be a "sudden development" of SA node exit block — it would seem far more likely that the tiny "extra deflections" suggested under the PINK arrows are also P waves.
  • There is no way to verify that a P wave occurs under the WHITE arrow — because the expected place where an on-time P wave would occur could be totally hidden within the QRS complex of beat #2. That said — Common things are common — and by far, the most likely scenario is that P waves in the long lead V4 are regular.


PEARL #3: Advanced rhythm interpretation entails working out theories based on logical assumptions — and then checking out these assumptions as more information is gathered. At this point in my interpretation — I strongly suspected (but could not yet prove) that the underlying atrial rhythm was regular.

  • 2 ways to "check out" one's theories are: i) To see if your theory allows a logical explanation of the rhythm; andii) To seek out additional monitoring on the patient which then does allow you to prove your theory.


My Proposed Laddergram for the Rhythm in Figure-2:

The most logical explanation for the rhythm in Figure-2 — is high-grade 2nd-degree AV block. For clarity (working on the assumption of a regular atrial rhythm) — I've labeled all P waves with letters in Figure-3:

  • The first 5 beats in the long lead V4 rhythm strip of Figure-3 look like complete AV block — because the QRS is wide, the ventricular rhythm is regular — and there appears to be complete AV dissociation (ie, the PR interval before these first 5 beats is constantly changing as P waves "march through" these first 5 QRS complexes).
  • The reason for QRS widening is a slightly accelerated ventricular escape focus (at a rate just under 50/minute — whereas 20-40/minute is the usual rate of an idioventricuar escape rhythm).
  • That said — the rhythm is not complete AV block — because the last 3 beats in this tracing are conducted, as indicated by: i) Earlier-than-expected occurrence of beat #6; andii) The constant and normal PR interval preceding beats #6, 7 and 8, that now manifest a normal (narrow) QRS complex.


Figure-3: My proposed laddergram for the rhythm in the long lead V4 (See text).

Bottom Line: I interpreted the rhythm in Figure-3 as showing "high-grade" 2nd-Degree AV Block. None of the earlier P waves in this tracing conduct — despite adequate opportunity for P waves a, c, e and g to conduct.

  • Therefore, there is complete AV dissociation early on in this tracing — but when P waves occur at "just the right moment" (as they do for P waves ik and m) — there is conduction! These last 3 beats conduct with 2:1 AV block.
  • The fact that when P waves do conduct, the PR interval is normal — suggests a lower level (and therefore more serious conduction disturbance) for this AV block (ie, below the AV node, within the bundle of His). This patient will almost certainly need a permanent pacemaker.


The Case Continues:

Additional rhythm strips were recorded on this patient. These additional non-continuous lead V4 rhythm strips are shown in Figure-4.


Figure-4: Rhythm A was previously shown in the long lead V4 of Figure-3. Rhythms BC and D are additional non-continuous rhythm strips recorded on this patient.

QUESTION: Do these additional rhythm strips confirm my previous assumptions? (ie, that the underlying atrial rhythm is regular — and that there is high-grade but not complete AV block?).

  • HINT: Use calipers!




ANSWER: I interpreted the additional rhythm strips shown in rhythms BC and D as supporting my prior assumptions — which I explain in Figure-5:

  • RED arrows highlight selected P waves in these tracings. Note that beat #8 in C — and beat #1 in D both manifest that same normal PR interval as did conducted beats #6, 7 and 8 in A.
  • Beats #2-thru-7 in B — beats #1-thru-7 in C — and beats #3-thru-7 in D all show additional episodes transient complete AV dissociation with a slightly accelerated ventricular escape rhythm.
  • Final proof that the narrow QRS complexes in these tracings are being conducted — is forthcoming from the Fusion beats that we see (ie, beat #1 in B — and beat #2 in D).
  • The laddergram in Figure-5 is drawn from Rhythm D. Note depiction of the Fusion beat (ie, beat #2 in D) — that manifests a QRS complex and T wave intermediate in morphology between conducted beat #1 and ventricular beats #2-thru-7.


Bottom Line: The rhythm in today's case is "high-grade" 2nd-Degree AV Block — which almost certainly arises from an infrahiscian level. The patient will need a permanent pacemaker.



Figure-5: I've labeled Figure-4. Rhythm A was previously shown in the long lead V4 of Figure-3. Rhythms BC and D are additional non-continuous rhythm strips recorded on this patient. The laddergram is drawn from Rhythm D.




Acknowledgment: My appreciation to Dr. Prabhakar Deshpande (from Nagpur, India) for allowing me to use this tracing — and to Dr. Mehul for making me aware of this case.



Related ECG Blog Posts to Today’s Case: 

  • ECG Blog #185 — Reviews my Systematic Approach to Rhythm Interpretation using the “Ps, Qs & 3R Approach”.
  • ECG Blog #188 — Reviews the essentials for reading (and/or drawingLaddergrams, with LINKS to numerous Laddergrams I’ve drawn and discussed in detail in other blog posts.

  • ECG Blog #191 — Reviews distinction between AV Dissociation vs Complete AV Block.
  • ECG Blog #192 — Reviews the 3 Causes of AV Dissociation.
  • ECG Blog #186 — Reviews some basics on AV Blocks, and WHEN to suspect Mobitz I ( = AV Wenckebach) 2nd-degree AV Block. 
  • ECG Blog 189 — More on determining What TYPE of AV Block is present?

  • ECG Blog #133 — Illustrates use of Fusion beats and AV Dissociation in the diagnosis of VT.
  • ECG Blog #128 — Reviews the concept of Fusion Beats. 
  • ECG Blog #129 — Late-cycle (End-diastolic) AIVR and Fusion beats in diagnosis of ventricular beats.


ADDENDUM (6/21/2021): I received a comment today on this tracing from David Richley, who is well known to most ECG enthusiasts who frequent any of the many ECG internet forums. Dave always offers the most astute commentary on complex arrhythmia interpretation. NOTE: What follows below goes way beyond-the-core! But for those readers who love complex arrhythmia diagnosis — I think you'll find what follows is compulsive!

  • As often happens — Dave's comment "sent me back to the drawing board". I had noticed in Figure-3 that the PR interval had been progressively shortening during the ventricular escape rhythm until finally, with beat #6 — P wave i was able to conduct. But prior to Dave's comment — I had no idea as to why this was so.
  • I would amend Dave's comment (that I show below) to clarify that by the RP interval (which he calculates for us in his Figure-6) — he is actually referring to the interval between the preceding ventricular escape beat — and the onset of the 2nd P wave within that R-R interval.
  • BOTTOM LINE: I went "back to the drawing board" and postulated retrograde conduction out of the ventricular escape rhythm (dotted butt-end lines that I drew into the AV Nodal Tier in my Figure-7). Being meticulous in drawing these dotted lines all at precisely the same length and angle — it becomes apparent (as per Dave)that the "degree" of AV block does not change — but because of the changing relation of dissociated P waves during the period of AV dissociation — P wave i happens to fall at "just the right moment" to enable conduction with 2:1 AV block.
  • Final PEARL: The original reason this ECG was sent to me, was to inquire if I thought there was complete AV block. Dave's theory proves that the degree of AV block was never complete — because when P waves occur "at just the right moment" (ie, beginning with beat #6) — they can and do conduct to the ventricles (albeit with 2:1 AV block). NONE of this changes the fact that this patient has severe conduction system disease — and needs a permanent pacemaker. But it does make for a captivating discussion!
  • Final THOUGHT: Go back to Figure-4. Rhythm A is the rhythm that I show again in my Figure-7. But take another look at Rhythm C in Figure-4. You'll note beat #8 in C is conducted. And, exactly as per Dave's comment — note that during the prior 7 beats in which there is complete AV dissociation and a ventricular escape rhythm — that the PR interval preceding beats #2-thru-7 progressively decreases (because of progressive increase in the RP interval — which is why beat #8 is able to conduct!).


Figure-6: Submitted by David Richley, in which he calculated successive RP intervals.

Figure-7: My revised laddergram for Figure-3. The dotted lines within the AV Nodal Tier represent RP-dependent conduction (See Comment by David Richley below).


BELOW is the COMMENT I Received Today from David Richley:

This is a fascinating ECG and I agree completely with your analysis. I'd like to make some additional points. I think the ECG provides a good example of what Marriott termed "RP-dependent conduction".

  • It might be puzzling that there is a transition from complete AV dissociation to 2:1 block. Why should this happen? Is there a sudden "improvement" in the functioning of the conduction system?  The reason for the AV dissociation and ventricular escape rhythm for the first 5 beats — is that none of the atrial impulses occurs at such a time that they can conduct — the conduction system is refractory. 
  • However, during this period — because of the AV dissociation — the RP interval is increasing beat by beat. When the RP interval reaches 920 ms (just before beat 6)the conduction system has had sufficient time to recover — and beat 6 is a conducted (or capture) beat
  • Subsequent RP intervals of about 1140 ms obviously mean that alternate P waves will then conduct, so there is 2:1 block. 
  • KEY: The important point is that there is no change in the status of the conduction system during this rhythm strip! Instead, it is the timing of the P waves in relation to the preceding QRS (and the failure of the escape rhythm to reassert itself) — that explain the apparent change in conductivity. This tracing illustrates beautifully why you can not diagnose complete AV block with confidence from a short section of ECG.


  1. Great explanation sir. Thank you!

    1. @ Mehul K — THANK YOU for bringing this case to my attention! — :)