Tuesday, February 9, 2021

ECG Blog #192 (ECG MP-9) — AV Dissociation by Usurpation – Default – AV Block?

I will label today's case as Part-2 of a mini-series on AV dissociation (Part-1 was ECG Blog #191). The rhythm that I want to discuss is shown in Figure-1.

QUESTION: Which of the following choices gives the BEST interpretation of the rhythm shown in Figure-1?

  • HINT: More than 1 choice is partially correct — but only 1 choice provides the BEST answer to this question.


Answers to Choose From:

Choice A:  The rhythm is AV dissociation. There are sinus and junctional rhythms.

Choice B:   The rhythm is 2nd-degree AV block. There is junctional escape — but no sinus-conducted beats.

Choice C:  There is sinus bradycardia with sinus arrhythmia that temporarily results in a junctional escape rhythm. This leads to AV dissociation by “default”.

Choice D:  There is high-grade 2nd-degree AV block — because at least several P waves are not conducted.

Choice E:  There is 3rd-degree (complete) AV block.


Figure-1: Which of the above 5 choices provides the BEST interpretation of this rhythm?




NOTE #1: Some readers may prefer at this point to watch to the 6-minute ECG Video before reading My Thoughts regarding the rhythm in Figure-1. Feel free at any time to review to My Thoughts (that appear below ECG MP-9).



Today’s ECG Media PEARL #9 (5:40 minutes) — reviews the 3 Causes of AV Dissociation. This ECG video provides needed insight for optimal interpretation of the rhythm in Figure-1.



NOTE #2: Lots more on AV Dissociation was featured in my ECG Blog #191 — including ECG Media Pearl #8 and a 7-page PDF on distinction between AV dissociation vs complete AV block.





MY Thoughts on the Rhythm in Figure-1:

As always — I find the most accurate and time-efficient process for arrhythmia interpretation is to use the Ps, Qs & 3R Approach (For review of this systematic approach to arrhythmia interpretation — Please see ECG Blog #185).

  • PEARL #1 — For clarity, I’ve labeled P waves in today’s tracing (Figure-2). BLUE arrows highlight P waves (or at least, portions of P waves) that I can be certain about. Using calipers tells us that almost certainly, another P wave is hiding within the QRS complex of beat #2 (WHITE arrow). I have found the simple act of labeling P waves in this way to be of invaluable assistance for clarifying the relationship between atrial activity and neighboring QRS complexes.
  • Continuing with Assessment of the Ps, Qs & 3Rs — The QRS complex looks narrow on this single monitoring lead. The R-R interval appears constant (at ~ 7 large boxes, which corresponds to a rate ~45/minute) for the first 5 beats on the tracing.
  • PEARL #2: IF you used calipers — you should have instantly recognized that beat #6 occurs slightly earlier-than-expected (ie, the R-R interval between beats #5-6 is less than the R-R interval for the preceding 5 beats!)Recognizing that a beat occurs earlier-than-expected is often an excellent CLUE that this beat is conducted! The fact that the following beat (ie, beat #7) occurs even earlier — and — the finding of a reasonable and equal PR interval preceding beats #6 and 7 tells us that both of these beats are sinus-conducted!
  • It makes sense that beats #6 and 7 in Figure-2 are sinus-conducted — and, that the prior 5 beats are not conducted. This is because the PR interval before the first 5 beats is simply too short to conduct (or in the case of beats #1, 2 and 3 — the P wave coincides with the QRS complex). Since the QRS complex for beats #1-thru-5 is narrow and not preceded by P waves that conduct — these first 5 beats in Figure-2 represent a junctional escape rhythm at ~45/minute.

Figure-2: I’ve added ARROWS to indicate where my calipers suggest the presence of P waves (See text).

PEARL #3: Note that although the QRS complex of all 7 beats on this tracing is narrow — that QRS morphology is slightly different for the 5 junctional escape beats (ie, the R wave is clearly taller for beats #1-thru-5, compared to the R wave for beats #6 and 7). Because the site within the AV node from which a junctional escape beat may arise can be slightly different than the site in the AV node that a normal, sinus-conducted beat passes through — sometimes (not always!) you may see slightly different QRS morphology for the junctional beats. When this finding occurs, it can be very helpful in telling you whether a certain beat is or is not conducted.

  • For example — the PR interval preceding beat #5 is ~0.16 second, or potentially long enough to be conducted. But the reasons we know that beat #5 is not sinus-conducted are: i) Because the next beat ( = beat #6) occurs slightly earlier-than-expected; ii) Because the PR intervals preceding sinus-conducted beats #6 and 7 are identical and longer (~0.21 second) than the PR interval preceding beat #5; andiii) Because QRS morphology of beat #5 is the same as QRS morphology for the other junctional beats (ie, beats #1-thru-4) — and different than QRS morphology for the 2 sinus-conducted beats at the end of the tracing (ie, beats #6 and 7).


PEARL #4: By definition — there is transient AV Dissociation on today’s tracing because: i) There are a number of P waves that are not related (ie, not conducting) to neighboring QRS complexes (ie, We see this for beats #1-thru-5); andii) The last 2 beats on the tracing ( = beats #6 and 7are sinus-conducted, so that AV dissociation is not present by the end of the tracing.


Putting It All Together: As emphasized in the above ECG Media Pearl #9 (and also in the 7-page PDF excerpt I posted in ECG Blog #191) — there are Causes of ADissociation. These are: i) 2nd- or 3rd-degree AV Block (in which one or more P waves that should conduct do not conduct)ii) AV dissociation by Usurpation” (in which an accelerated junctional rhythm takes over the pacemaking function)and/or iiiAV dissociation by Default” (in which slowing = “default” of the SA nodal pacemaker allows a junctional escape pacemaker to emerge).

  • The correct ANSWER in today’s case is Choice C — because the primary rhythm disorder is sinus bradycardia with sinus arrhythmia. During the period of time in which the sinus node rate drops below the inherent AV nodal escape rate — there is a junctional escape rhythm (ie, this occurs for beats #1-thru-5). Toward the end of the tracing — the P-P interval shortens as the sinus P wave rate speeds up, allowing sinus P waves to “capture” the QRS for the last 2 beats in the tracing. Thus, there is AV Dissociation by “Default” due to slowing of the sinus rhythm that temporarily results in a junctional escape rhythm.
  • Choice A is not correct — because it is not correct to say that a rhythm “is” AV dissociation. AV dissociation is never a rhythm. Instead, AV dissociation is simply a description of the fact that for some period of time (ie, for the first 5 beats in this tracing) — P waves temporarily are not related to neighboring QRS complexes.
  • Choice B is not correct — because there is no evidence of any AV block on this tracing. This is because none of the P waves that fail to conduct during the initial part of this tracing have a “chance” to conduct (ie,they all occur too close to the neighboring QRS). Another reason Choice B is incorrect — that that the last 2 beats on this tracing are sinus-conducted.
  • Choices D and E are also not correct — because as per Choice B, there is no evidence of any AV block at all on this tracing (ie, none of the P waves that don’t conduct had a “chance” to conduct).


PEARL #5: We have no idea of the clinical significance (or lack thereof) of this tracing This rhythm could be benign — or — it could be pathologic. It all depends on the clinical situation!

  • Clinical Scenario #1: IF the patient in today’s case was an otherwise healthy, completely asymptomatic, athletically-inclined individual who had this ECG done because someone heard a very slow and slightly irregular heartbeat — then there most likely would be no cause for concern. Training effect with increased vagal tone could account for sinus bradycardia and arrhythmia with a rate that occasionally dips below 50/minute. AV dissociation by “default” is not uncommon in such individuals — in whom a junctional escape rhythm may be a normal (expected) response for periods during the day when the sinus rate significantly slows.
  • Clinical Scenario #2: IF instead, the patient was an older adult with a history of syncope — then the rhythm in Figure-2 could be consistent with SSS (Sick Sinus Syndrome). This is not to say that a permanent pacemaker would be immediately needed — but the natural history of SSS is often that of a long prodromal period (sometimes up to a decade or more) — in which sinus bradycardia with sinus arrhythmia are the most common initial rhythm manifestations. In this case — additional monitoring of the patient would clearly be indicated to see if additional arrhythmias consistent with SSS might be detected (ie, sinus pauses or sinus arrest; more profound bradycardia episodes; alternation between tachy- and bradyarrhythmias, etc.).


For clarity — I conclude today’s case with an illustrative laddergram (Figure-3).

  • PEARL #6: As I emphasized in ECG Blog #188 — regardless of whether or not you are comfortable drawing laddergrams — laddergrams serve as a wonderful teaching tool that can instantly clarify what the clinician suspects is going on. For example — Isn’t it EASY to appreciate the mechanism I propose for the rhythm in Figure-3?


Figure-3: Laddergram illustration of the mechanism in today’s rhythm. None of the first 5 P waves are conducted (BLUE and WHITE arrows) — because these P waves occur too close to neighboring QRS complexes. Instead — there is a regular junctional escape rhythm at ~45/minute. Retrograde conduction from the first 5 beats (dotted RED lines within the AV nodal Tier) prevents forward conduction of these first 5 P waves — until beat #6, when the sinus P wave rate increases just enough to capture the ventricles for beats #6 and 7 (the 2 RED P waves seen in these last 2 beats are able to conduct to the ventricles).


  1. Thank you for your scientific and detailed interpretation. You elegantly identified P waves with BLUE arrows. I always use your "Ps, Qs & 3R Approach" and calipers.