Monday, January 16, 2023

ECG Blog #357 — AV Block, Default or Usurpation?

The ECG in Figure-1 — was obtained from a previously healthy young adult woman who presented with palpitations.
  • How would YOU interpret the rhythm in Figure-1?
  • Is there evidence of AV block? 

Figure-1: 12-lead ECG and long lead II rhythm strip — obtained from a previously healthy young woman with palpitations. (To improve visualization — I've digitized the original ECG using PMcardio).

MY Thoughts on the Rhythm in Figure-1:
As always — I favor beginning with the rhythm by focusing on the long lead II rhythm strip — and assessing the Ps, Qs & 3Rs (See ECG Blog #185):
  • Technical NOTE: It is important to appreciate that the long lead II rhythm strip in today's tracing is not simultaneously-recorded with each of the sets of 3 leads. This is easiest to see — if you look directly above beats #10-13 in the long lead II rhythm strip. It should be apparent that the last 4 beats in this tracing (ie, beats #10-13) are faster and not synchronized with the recording of the 3 beats in leads V4,V5,V6 that are simultaneously recorded.
  • There are many different types of recording systems used in the too-numerous-to-count number of different ECG machine systems. The system for recording employed in today's tracing repeats beats #1,2,3 in each of the sets of 3 leads — and — in addition, provides an independent long lead II rhythm strip. 
  • Editorial Comment: This is not my favorite system for ECG recording — because I find it confusing. The advantage of this system, is that we do get to see what the first 3 beats in the long lead rhythm strip look like in all 12 leads. The decided disadvantage (in my opinion) — is that we have no idea what beats #4-thru-13 look like in leads other than in the long lead II.

Regarding Today's ECG: I will analyze in detail the long lead II rhythm strip beginning with Figure-2 below. For the moment — suffice it to say that today's rhythm is supraventricular (ie, all QRS complexes are narrow) — and that today's rhythm is somewhat irregular.
  • Keeping in mind that today's patient is a previously healthy young adult woman — I see no significant abnormalities in this patient's 12-lead ECG. Upright sinus P waves are seen for the 3 beats in lead II of Figure-1 — all intervals and the frontal plane axis are normal — there is no chamber enlargement — and — there are no acute ST-T wave changes in this ECG.

Taking a Closer LOOK at Today's Rhythm:
I've reproduced in Figure-2 — the long lead II rhythm strip in today's tracing. The rest of the 12-lead ECG that was seen in Figure-1 is of little help in rhythm determination — because after the first 3 beats, we do not see any of the remaining 10 beats in other leads.
  • As previously noted — the QRS complex is narrow for all 13 beats in the long lead II rhythm strip shown in Figure-2
  • The rhythm is not regular
  • P waves are present — although the relationship between P waves and neighboring QRS complexes is not immediately apparent on initial inspection of the long lead II rhythm strip.

Figure-2: For clarity — I've reproduced the long lead II rhythm strip in today's tracing. Is there a relationship between P waves and neighboring QRS complexes?

PEARL #1: The simple step of labeling P waves — is the easiest way I know to facilitate determining what (if any) is the relationship between P waves and neighboring QRS complexes.
  • RED arrows in Figure-3 — highlight P waves that I am certain are present.
  • I was unsure if additional P waves might be hiding within the QRS complex of beats #6-thru-9.
  • I suspect that the first 3 beats (ie, beats #1,2,3) and the last 2 beats (beats #12,13) in Figure-1 are all sinus conducted — because the PR interval is constant for each of these beats. At 0.12 second ( = 3 little boxes) in durationthis PR interval is long enough for sinus conduction to occur, especially given that the patient is a young adult (ie, the lower limit of the PR interval for normal sinus conduction is often shorter in younger individuals). 
  • In contrast — the PR interval before beats #4, 5, 10 and 11 appears to be too short to conduct! (ie, at least 0.12 second is usually needed for atrial activity to spread through the specialized intra-atrial pathways, and through the AV node).

Putting It All Together:
The underlying rhythm in Figure-3 appears to be sinus — with 5/13 beats in the long lead II rhythm strip being sinus-conducted.
  • We know there is at least transient AV dissociation — because there simply is not enough time for the P waves before beats #4,5,10 and 11 to conduct. Since the QRS complex for each of these beats is narrow and looks the same as the QRS of the 5 sinus-conducted beats — beats #4,5,10,11 must be junctional beats.
  • I suspect that sinus P waves continue throughout the entire long lead II rhythm strip in Figure-3 — most likely being hidden within the QRS complex of beats #6-thru-9. But regardless if P waves are (or are not) hidden within the QRS of these beats — we know that beats #6-thru-9 are also junctional beats, because the QRS of these beats looks like the QRS of sinus-conducted beats, and no P waves come before these beats.

  • The normal AV nodal (junctional) "escape" rate in adults is between 40-to-60/minute. Since the R-R interval that separates the 8 junctional beats in today's tracing is generally a bit less than 4 large boxes in duration (corresponding to a ventricular escape rate of ~75-80/minute) — the 8 junctional beats in today's tracing are slightly accelerated!

  • Sinus rhythm resumes at the end of today's tracing (ie, with beats #12 and 13). Note that the P-P interval that separates the last 3 RED arrows in Figure-3 has shortened to ~3 large boxes, which suggests that the rate of sinus P waves has increased to ~100/minute.

Figure-3: I've labeled those sinus P waves that I can definitely see. I was not certain if sinus P waves might be hiding within the QRS complex of beats #6-thru-9.

What Then is the Rhythm in Today's Tracing?
Although we have established the presence of AV dissociation for a part of the rhythm in Figure-3 — AV Dissociation is never a "diagnosis". Instead — this term is merely a description of the fact that at least temporarily, regular P waves are not related to neighboring QRS complexes. 

PEARL #2: As discussed; in detail in ECG Blog #192 — the recognition of ADissociation should prompt consideration of the Causes of this entity:
  • Cause #1: 2nd- or 3rd-degree AV Block (in which one or more P waves that should conduct do not conduct)
  • Cause #2: AV dissociation by Usurpation” (in which an accelerated junctional rhythm takes over the pacemaking function)
  • Cause #3: AV dissociation by Default” (in which slowing = “default” of the SA nodal pacemaker allows a junctional escape pacemaker to emerge).

PEARL #3: There is no evidence of any form of AV block in today’s rhythm. This is because none of the P waves that fail to conduct — had a chance to conduct, since the PR interval was either too short — or P waves were hidden within the QRS complex.
  • This is not to say that there might not be some degree of AV block — but only to say that the rhythm in Figure-3 is too short to answer this question (because we never see P waves occurring at points in the cardiac cycle when they should be expected to conduct, yet fail to do so).

PEARL #4: Although Pearl #2 above lists the 3 basic causes of AV dissociation — sometimes a combination of these causes may be present. This is especially true in younger adult patients — in whom it is more common to see a slightly accelerated junctional escape focus.
  • In today's tracing, there is a component of AV dissociation by "default" — because the 4th and 5th P waves (4th and 5th RED arrows in Figure-3) appear to be slowing down (ie, moving closer to the QRS of beats #4 and 5). However, there is also a component of "usurpation" — because as noted above, the rate of the junctional escape focus in today's tracing is slightly accelerated (to ~75-80/minute).

  • There also appears to be a component of "isorhythmic" AV dissociation that is operative — because both the junctional and sinus rates appear to increase independent of each other toward the end of today's rhythm strip! (ie, P waves are seen to move in-and-out of the QRS over the duration of the rhythm in Figure-3)


What is Isorhythmic AV Dissociation?

As introduced in ECG Blog #195 — the phenomenon of nearly equal sinus and junctional escape rates that continue to vary slightly, yet somehow maintain a close (yet slightly variable) proximity of P wave to QRS over time is known as isorhythmic Adissociation

  • This unusual form of AV dissociation is defined by its name. Both pacemakers are "isorhythmic" (ie, nearly equal rates for sinus P waves and the junctional rhythm)and — there is AV "dissociation" (ie, the atria and AV node beat independently from each other). In its pure form — there may be no conduction of sinus impulses for as long as the rhythm lasts. Or, there may be intermittent "recapture" of the pacemaking function if the sinus nodes speeds back up.
  • NOTE: The colorful term, “accrochage” (from the French word for “hanging on” or “hooking” onto something— has often been used to refer to the uncanny pursuit of P waves to neighboring QRS complexes, despite slight variation in the rates of these P wave and QRS rhythms. The less colorful term, "synchronization" is then preferred when the phenomenon of isorhythmic AV dissociation is longer lasting.

CLINICALLY — The significance of AV dissociation (be it isorhythmic or otherwise) depends on the clinical setting in which it occurs. 

  • As emphasized earlier — there is no evidence of any AV block in today’s tracing. Assuming this lack of evidence for AV block continued with additional monitoring — the clinical course for the previously healthy young adult woman in today's case is most likely to be benign.
  • NOTE: AV dissociation by “default” is not uncommon among otherwise healthy, athletic individuals — because of the sinus bradycardia so commonly seen in the resting state with the resultant increase in vagal tone. Having such individuals exercise briefly is usually enough to sufficiently increase the sinus rate — which when the rhythm is benign, will result in sinus P waves regaining control of the rhythm (and overriding the resting junctional escape rate).

  • BOTTOM Line: I found today's rhythm interesting because it defies precise classification. Transient AV dissociation is seen that appears to be most likely the result of a combination of factors, including sinus arrhythmia, slight acceleration of the junctional escape rate — and components of intermittent isorhythmic AV dissociation. That said — What counts clinically, is that this rhythm is most likely to be benign in this otherwise healthy young adult woman.

  • P.S.: Would be good to rule out extracardiac factors that might be contributing to this arrhythmia (ie, verifying that TSH and serum electrolytes are normal — and that there is no excess stress, anxiety, dehydration, or use of stimulant agents such as alcohol, cocaine, or sympathomimetic drugs).

For clarity in Figure-4 — I've drawn my proposed laddergram to illustrate the most likely mechanism for today's rhythm: 
  • Although I suspect that fairly regular P waves continue throughout today's rhythm — I drew P waves f-thru-i that are seen within the Atrial Tier in PINK, because I can not prove that P waves are hiding within the QRS complexes of beats #6-thru-9.
  • I suspect beats #1,2,3 and #12 and 13 are all sinus-conducted — because the PR interval is long enough to conduct in a young adult (ie, ~0.12 second in duration) — and because the PR interval for each of these beats is the same! That said — I can not prove that P waves a,b,c and l,m are conducting (We would need a longer period of monitoring to do so).
  • We know that beats #4-thru-11 must be slightly accelerated junctional beats — since QRS morphology remains the same, and none of these beats are preceded by P waves with a "chance" to conduct.

Figure-4: My proposed laddergram for today's rhythm.


Acknowledgment: My appreciation to Arron Pearce (from Manchester, UK) for the case and this tracing.


ADDENDUM (1/16/2023):

ECG Media PEARL #12 (6:40 minutes Audio) — reviews the entity known as isorhythmic AV Dissociation.

  • 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.


Related ECG Blog Posts to Today’s Case:

  • ECG Blog #205 — Reviews my Systematic Approach to 12-lead ECG Interpretation.
  • ECG Blog #185 — Review of the Ps, Qs, 3R Approach for systematic rhythm interpretation.

  • ECG Blog #188 — Reviews how to read and draw Laddergrams (with LINKS to more than 50 laddergram cases — many with step-by-step sequential illustration).

  • ECG Blog #195 — Reviews the phenomenon known as Isorhythmic AV Dissociation.
  • ECG Blog #192 — Reviews the 3 Causes of AV Dissociation (and emphasizes why AV Dissociation is not the same thing as Complete AV Block).

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