ECG Interpretation: ECG Blog #338 — Who is Related to Whom?

The 2-lead rhythm strip shown in Figure-1 — was obtained from a middle-aged woman, who presented to the ED (Emergency Department) with palpitations. Clinical information was limited — but an Echo did not reveal any underlying structural heart disease.

QUESTIONS:

How would YOU interpret the rhythm in Figure-1?
Is there AV block? If so — What kind of AV block?

Figure-1: 2-lead rhythm strip obtained from a middle-aged woman who presented to the ED with palpitations. What is the rhythm?

MY Thoughts on the ECG in Figure-1:

As always — I favor assessing the rhythm by the Ps, Qs 3R Approach, which helps to recall the 5 KEY Parameters (See ECG Blog 185).

PEARL #1: It does not matter in what sequence you choose to assess the 5 KEY Parameters — so I favor starting with whichever of these parameters are easiest to assess.

The QRS complex looks to be narrow (at least in the 2 leads we are given) — so I presumed that the rhythm was supraventricular.
P waves are present. Some of these P waves appear before QRS complexes with a PR interval having a reasonable chance of conducting. But other P waves (ie, the P wave that appears just before beat #3 — and the P wave that appears shortly after beat #6) — do not seem to have an adequate opportunity to conduct. So, some form of AV dissociation may be present.
Most of the QRS complexes look at least fairly Regular — at a Rate of ~80-85/minute.
Beat #8 occurs earlier than expected!

PEARL #2: At this point in my interpretation — I have found the simple step of labeling P waves to be amazingly helpful for: i) Determining IF there is an underlying regular (or almost regular) atrial rhythm; and, ii) Facilitating assessment as to whether some (or all) of the P waves identified are (or are not) Related to neighboring QRS complexes.

In Figure-1 — I began my “Search for P Waves” — by labeling those P waves that I was certain are present. I have done this with RED arrows in Figure-2.
As is often the case when I label atrial activity — the underlying atrial rhythm turns out to be surprisingly regular (ie, RED arrows in Figure-2).
Accounting for slight variation in the P-P interval that is so often seen — it seems logical to anticipate that a final P wave is likely to be hiding within the QRS of beat #4 (WHITE arrow in Figure-2). In support of this presumption — is the slight-but-real "extra widening" of the terminal portion of the QRS complex of beat #4 (that is not seen in any of the 9 other QRS complexes in lead II).
BOTTOM LINE: There appears to be a fairly regular sinus rhythm in today's tracing! (colored ARROWS in Figure-2). Recognition of this finding is critical for our next step in interpretation!
P.S.: As I have often emphasized — the above relationships are all-but-impossible to realize without the use of calipers! Using calipers — it took me no more than seconds to arrive at this point in my interpretation!

Figure-2: I've labeled the rhythm in Figure-1 with RED arrows highlighting definite P waves (and with a WHITE arrow highlighting where the remaining sinus P wave was likely to be hiding).

QUESTION:

Doesn't the labeling of P waves with arrows in Figure-2 — facilitate determining which of the P waves in today's tracing are likely to have a chance to conduct?

PEARL #3: When a complex arrhythmia is present, in which there is an underlying sinus rhythm with some form of AV dissociation (as is the case for today's rhythm) — my favorite CLUE that a beat is likely to be conducted — is IF you see a beat that occurs earlier-than-expected.

In addition to its earlier-than-expected occurrence — support that beat #8 in Figure-2 is probably conducted is forthcoming from: i) The fact that the QRS complex of beat #8 is narrow, and with a similar (albeit not identical) QRS morphology as is seen in other beats on this tracing; and, ii) Beat #8 is preceded by an on-time sinus P wave that manifests a PR interval with a reasonable chance to conduct.
KEY Advanced Points: Although the P wave preceding beat #8 manifests a longer-than-usual PR interval — the fact that this P wave occurs so close to the preceding beat (ie, it occurs during the ST-T wave of beat #7) — could easily account for some delay in conduction through the ventricles. The early occurrence of beat #8 may also account for the slight difference in QRS morphology (ie, most probably the result of some aberrant conduction).

Putting It All Together:

The above deductions suggest the following:

The rhythm in today's case is supraventricular — since all QRS complexes are narrow.
There is an underlying regular atrial (presumably sinus) rhythm — as suggested by the regular colored arrows in Figure-2.
The ventricular rhythm is also fairly regular, with the exception of beat #8 — which clearly occurs earlier than expected.
Given the presence of an underlying sinus rhythm — the fact that early-occurring beat #8 is preceded by an on-time P wave strongly suggests that beat #8 is a sinus-conducted "capture" beat.
In contrast — some form of AV dissociation is present elsewhere on the tracing, because: i) The PR interval in front of other beats varies; and, ii) A number of P waves are clearly not conducting — because the PR interval is either too short to conduct (as for the P wave before beat #3) — OR — the P wave occurs too soon after the QRS to allow conduction (as for the P waves of beats #4, 5 and 6).
The P-P interval (ie, distance between colored arrows in Figure-2) — and the R-R interval for all QRS complexes except early-occcurring beat #8 — is almost the same!
CONCLUSION: There is no evidence of AV block in today's tracing. Instead — the clinical entity that explains all of the above findings is isorhythmic AV dissociation! (See ECG Blog #195 — for full discussion of this entity).

PEARL #4: As emphasized in ECG Blog #192 and ECG Blog #191 — AV dissociation is not a diagnosis. Instead — AV Dissociation is an ECG finding that is the result of one of 3 Causes. These are: i) 2nd- or 3rd-degree AV Block (in which one or more P waves that should conduct do not conduct because of some type of AV block); ii) AV dissociation by "Usurpation" (in which an accelerated junctional rhythm takes over the pcemaking function); and/or, iii) AV dissociation by "Default" (in which slowing = "default" of the SA nodal pacemaker allows a junctional escape pacemaker to emerge).

There is no clear evidence of 2nd or 3rd-degree AV block in today's tracing! — because I don't see any P waves in Figure-2 that have an opportunity to conduct, yet fail to do so. This is not to say that there might not be some form of AV block — but rather that there is no evidence of AV block in the single tracing we have been given.
The cause of AV dissociation in Figure-2 is also not the result of "default" — because the sinus rate does not drop below 60/minute.
By the process of elimination — the cause of AV dissociation in Figure-2 is "Usurpation" of the underlying sinus arrhythmia by an accelerated junctional rhythm, which at ~80-85/minute is clearly faster than the usual 40-60/minute AV nodal escape rate.

Editorial NOTE:

I fully acknowledge that I was not certain which P waves in Figure-2 (other than early-occurring beat #8) were conducting to the ventricles. That said — not knowing this did not in the least alter my clinical impression:

The fact that there is AV dissociation by usurpation — with 1 "capture" beat — and otherwise slightly variable but nearly identical sinus and AV nodal rates — establishes the diagnosis of isorhythmic AV dissociation (as described in detail in ECG Blog #195).
Clinical Management of this patient remains the same — regardless of whether beats #1, 2, 9 and 10 are (or are not) conducting to the ventricles. This is because accelerated junctional rhythms are not overly common in adults. As a result — the KEY is to try to find the cause of this rhythm, with the hope of correcting it (ie, ischemia/infarction — shock — acid-base or electrolyte abnormality — post-operative state — digitalis toxicity — "sick" patient).

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Final Confirmation of the Rhythm in Today's Case:

The BEST way to demonstrate the etiology of a complex cardiac rhythm — is by construction of a Laddergram — which I illustrate in Figure-4.

NOTE: For more on how to read (and/or draw) Laddergrams — Please check out our ECG Blog #188 (which includes teaching aids + LINKS to more than 50 laddergrams I have published).

Figure-3: It's easiest to begin construction of a laddergram by indicating atrial activity. Vertical RED lines in the Atrial Tier correspond to P waves (RED arrows) in the long lead II rhythm strip.

Figure-4: Next — I've filled in the Ventricular Tier with RED arrows that correspond to each of the QRS complexes in the long lead II. Note that it will often be easy to fill in the Atrial and Ventricular Tiers (as I've done in Figures-3 and -4). The "challenge" for solving the arrhythmia — usually begins with filling in the AV Nodal Tier (This comes next!).

Figure-5: It's time to start filling in the AV Nodal Tier. It's easiest to begin with whichever beat(s) that you know are conducting ( = beat #8 — which occurs earlier-than-expected, and which is preceded by an “on-time” P wave with a reasonable PR interval).

Figure-6: I next focused on beats #9 and 10. As I acknowledge in my earlier discussion — I was not at all certain IF beats #9 and 10 are conducting. This is difficult to determine — because although the R-R intervals preceding these beats seems to be a little bit shorter than the R-R interval before beat #7 — and, both beats #9 and 10 are preceded by on-time sinus P waves with a similar PR interval — this tracing is curved (and therefore measurements are somewhat distorted).

NOTE: If beats #9 and 10 are conducting — then I will have to explain on my laddergram why the PR interval preceding beat #8 is longer than the PR interval preceding beats #9 and 10 (which I will do shortly).

Figure-7: I propose that beats #3-thru-7 are all slightly accelerated junctional escape beats (BLUE circles that I've drawn within the AV Nodal Tier). The QRS complex is narrow for each of these beats — their R-R interval is the same — and none of the P waves close to these beats have a chance to conduct (because neighboring RED arrow P waves either manifest an overly short PR interval — or P waves occur too soon after the QRS to conduct).

On the other hand — I'm not certain IF beats #1 and 2 are (or are not) being conducted — because both beats are preceded by a potentially reasonable PR interval (therefore the BLUE question marks).

Figure-8: Assuming (as I did in Figure-7) that beats #3-thru-7 are slightly accelerated junctional escape beats — there will be some retrograde conduction (dotted BLUE butt ends within the AV Nodal Tier). This would explain why none of the P waves near beats #3-thru-7 are able to make it through to the ventricles. It could also explain why the PR interval preceding beat #8 is longer than the PR interval preceding beats #9 and 10 (ie, "concealed" retrograde conduction from junctional escape beat #7 delays conduction of the next sinus P wave — with resultant PR interval lengthening before beat #8).

Figure-9: I then completed my proposed laddergram by assuming beats #1 and 2 are also junctional escape beats (BLUE circles within the AV Nodal Tier) — because I think the PR intervals preceding beats #1 and 2 are different, therefore suggesting continuation of AV dissociation by usurpation.

CONCLUSION: I think the underlying rhythm in today's case is sinus. This sinus rhythm is interrupted by an accelerated junctional rhythm — which continues until an on-time sinus P wave is able to recapture the ventricles ( = beat #8). Thus, there is AV dissociation by "usurpation" (from an accelerated junctional pacemaker). Because the rate of the accelerated junctional pacemaker and the underlying sinus rate are so close to each other — there is isorhythmic AV dissociation (See ECG Blog #195 — for full discussion of this entity).

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Editorial NOTE:

I fully acknowledge that I am not certain that the laddergram I drew in Figure-9 is correct (or that this represents the only potential solution) to the mechanism of today's interesting arrhythmia.

Unfortunately — because of technical reasons (ie, today's tracing is slanted and angled — therefore distorting measurements slightly, but enough to make it impossible to be certain about true P-P and R-R intervals) — it is not possible to be certain IF the atrial rhythm successfully recaptured the ventricles for the last 2 beats on the tracing.
That said — regardless of whether beats #1,2; and #9,10 are sinus-capture beats or resumption of the accelerated junctional rhythm — I think it clear that the "underlying problem" here is isorhythmic AV dissociation (that arises from an accelerated junctional rhythm).
BOTTOM LINE: The "Take Home" Point and clinical management of today's case remain the same as I suggest above = Try to find out the reason for the accelerated junctional rhythm — and then — Try to "fix" this cause.

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Acknowledgment: My appreciation to Willis Kwandou (from Watampone, Indonesia) for the case and this tracing.

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Related ECG Blog Posts to Today’s Case:

ECG Blog #185 — Use of a Systematic Approach to Rhythm Interpretation.
ECG Blog #188 — Reviews how to read (and/or draw) Laddergrams (plus LINKS to more than 50 clinical examples of laddergrams I have drawn).
ECG Blog #186 — and ECG Blog #236 — for review on the basics of 2nd-degree AV Block.
ECG Blog #192 — Reviews the 3 Causes of AV Dissociation — and emphasizes why AV Dissociation is not the same thing as Complete AV Block.
ECG Blog #195 — Reviews the entity of Isorhythmic AV Dissociation.
ECG Blog #191 — Emphasizes the difference between AV Dissociation vs Complete AV Block.
ECG Blog #159 — Reviews another case to illustrate distinguishing between AV Dissociation vs Complete AV Block.
ECG Blog #202 — and ECG Blog #257 — Review cases regarding HOW to tell if there is (or is not) Complete AV Block.
ECG Blog #247 — Reviews a complex case with AV Dissociation.