Monday, March 11, 2024

ECG Blog #420 — A "Fast" Complete Heart Block?

I was asked to interpret the 2-lead rhythm strip shown in Figure-1 — without the benefit of any history. What are YOUR thoughts?
  • Is there AV block? If so — Is it complete AV block?

Figure-1: You are asked to interpret this 2-lead rhythm strip without the benefit of any history.


NOTE: Today's rhythm is challenging — especially if you have not seen this type of rhythm before. That said, attention to the sequential PEARLS I present in my discussion below can greatly facilitate recognizing this rhythm within seconds the next time you encounter it!


MY Thoughts on Today’s Rhythm: 

Assuming that today’s patient is not unstable hemodynamically (and does not require immediate synchronized cardioversion) — We can completely turn our attention to assessment of the rhythm. By the Ps, Qs & 3R Approach (which I review on ECG Blog #185):

  • The Rate of the rhythm in Figure-1 is somewhat fast — averaging ~100/minute (ie, with an R-R interval close to 3 large boxes in duration for most of the tracing).
  • The rhythm is clearly not Regular. The R-R interval is not the same throughout the tracing.
  • It appears that there are at least some P waves, albeit P waves are not seen in all parts of this tracing.
  • Although the QRS complex looks wider than it normally is — the QRS does not measure more than half a large box (ie, not more than 0.10 second) in either of the 2 monitoring leads shown in Figure-1

PEARL #1: Remember the saying, “12 leads are better than one”. The importance of determining whether the QRS complex of a tachycardia is wide or narrow — is that IF the QRS is narrow in all 12 leads, then the rhythm is supraventricular (and we have therefore ruled out the possibility of VT = Ventricular Tachycardia). The more monitoring leads we have available — the more accurate will be our determination about whether the QRS is wide or narrow.

PEARL #2: We only see 2 of the 12 leads of an ECG in Figure-1. This is important to appreciate — since sometimes a part of the QRS complex may lie on the baseline in one or more leads. When this happens — the QRS may “look” to be narrow in the lead(s) you are monitoring, whereas in reality the QRS is actually wide.
  • It is for this reason that I always favor doing a 12-lead ECG as soon as this is feasible for any arrhythmia of uncertain etiology. Clinical management of the patient clearly differs if you can rule out the possibility of a ventricular rhythm. Unfortunately in today’s case — No 12-lead ECG was available.

Assessing the 5th Parameter:
Thus far — We’ve determined the following: 
  • That the Rate of QRS complexes in today’s rhythm is fairly fast — but not quite Regular.
  • That the rhythm is probably supraventricular (because the QRS is not more than half a large box in either of the 2 monitoring leads we are given)
  • That at least some P waves are present.

  • The remaining parameter is the 3rd R — which  is determining IF any of the P waves in today’s rhythm are Related to neighboring QRS complexes. It turns out that this last parameter is KEY to solving today’s arrhythmia.

PEARL #3: The easiest way to assess for this 5th parameter — is to see if there are any PR intervals that repeat. And the BEST way to find any PR intervals that may be repeating — is to seek out any relative pauses in the rhythm — and to check out the PR interval before each beat that ends each pause.
  • NOTE: The “pause” in the rhythm may be brief — and not much longer than the other R-R intervals. Figure-2 illustrates this principle — in which the R-R intervals between beats #4-5; 8-9; and 12-13 are slightly longer than other R-R intervals in today’s tracing.
  • RED arrows highlight the P waves that precede each of the beats that end each of these short pauses. Isn’t the PR interval before beats #5, 9 and 13 equal?

PEARL #4: An important concept in “arrhythmia problem solving” — is based on the saying, “Birds of a Feather flock Together”. By this I mean that IF you see a certain ECG finding a number of times in a given tracing — then if you see a similar phenomenon that is not quite as precise — the chances are that you are seeing the same ECG finding!
  • Take beat #1 in Figure-2 — which occurs at the very beginning of this tracing (which means that we do not know what came before beat #1). As a result, we can not know if beat #1 was preceded by a slight pause in the same way that beats #5, 9 and 13 are all preceded by a slight pause of the same duration. 
  • BUT — Given the repetitive pattern of there being a slight pause after every 4th beat (ie, “Birds of a feather …” ) — and given that the PR interval preceding beat #1 ( = first RED arrow in Figure-2) is equal to the PR interval preceding beats #5,9,13 ( = the other RED arrows in Figure-2) — this is not by chance, and whatever relationship might be occurring between atrial activity and neighboring QRS complexes for beats #5-thru-16 — is almost certain to be the same phenomenon occurring for beats #1-thru-5! 

Figure-2: Applying the concept of “Looking for a slight pause in the rhythm” I’ve labeled the 4 P waves that we clearly see with RED arrows. Note that the PR interval for these 4 P waves is the same!

PEARL #5: It’s time to determine IF there is an underlying regular atrial rhythm — which could be the case IF some P waves are hiding within certain ST-T waves. Figure-3 illustrates How to Look for “Hidden” P waves:
  • Search the tracing looking for 2 P waves in a row that you can clearly see. RED arrows in Figure-3 illustrate 2 such P waves that are clearly seen in today’s rhythm.
  • Using the P-P interval set by these 2 RED arrows locates the next spot to search for a “P wave in hiding”. Isn’t the T wave under the WHITE arrow in Figure-3 “fatter” than most other T waves in this tracing?
  • The reason the T wave under the WHITE arrow is “fatter” — is that this is where the next consecutive P wave occurs.
  • NOTE: Using calipers greatly facilitates the process. Using calipers instantly makes you “smarter” — and allows me to “solve” an arrhythmia such as today’s tracing within seconds that providers who do not use calipers are simply not able to solve.

Figure-3: How to find “hidden” P waves (See text).

Now that we have identified the P-P interval for 3 consecutive P waves (as shown by the 3 arrows in Figure-3) — it should take no more than seconds to “walk out” where the regularly-occurring P waves are located throughout the rest of today’s tracing (Figure-4):

Figure-4: RED arrows highlight where regular atrial activity is located throughout the rhythm in today’s case.

PEARL #6: Try to determine if the underlying atrial rhythm is a “sinus” rhythm — or an ectopic atrial rhythm. This determination is not always easy to make.
  • A sinus rhythm — is defined by the presence of an upright P wave in lead II. This is because when limb lead electrodes are correctly placed — the direction of atrial depolarization as impulses arising from the SA node travel toward the AV node will be oriented close to +60 degrees in the frontal plane, which corresponds to the 60 degree electrical perspective of standard lead II in Einthoven’s Triangle.
  • Therefore — IF P waves are not upright in lead II, then the rhythm is not sinus!
  • The problem is that when P waves are upright in standard lead II (as they are in Figure-4) — then it may be difficult to distinguish between a sinus rhythm vs an ectopic atrial rhythm!

PEARL #7: Determining the atrial rate may help. In Figure-4 — the atrial rate is ~125/minute (which can be quickly estimated by the Every-Other-Beat Method reviewed in ECG Blog #210)

  • By the Every-Other-Beat Method Given that the P-P interval of every-other-P-wave in Figure-4 is just under 5 large boxes in duration — this means that half the atrial rate is slightly faster than 300 ÷ 5 (ie, slight faster than 60/minute) — which means the actual atrial rate is ~125/minute.

  • The PEARL: The finding of a fast and regular atrial rate in association with an irregular supraventricular (narrow-QRS) rhythm — is much more likely to represent ATach (an ectopic Atrial Tachycardia) — than sinus tachycardia. The reason for this finding, is that Wenckebach conduction is commonly seen with ectopic ATach.

Proving My Theory . . .
Using calipers — I was able to prove within seconds that my suspicion of ATach with Wenckebach conduction was the answer to today's rhythm.
  • For clarity in Figure-5 — I've reproduced Figure-2, in which we see group beating (ie, beats #1-thru-4; #5-thru-8; #9-thru-12; #13-thru-16— with each group separated by a slight pause of similar duration (ie, between beats #4-5; #8-9; #12-13) — with each pause ending with a conducted P wave (ie, the RED arrow P waves, each with an identical PR interval before beats #1,5,9,13). The repetitive pattern of these similarities can not be by chance!
  • As soon as my calipers confirmed the underlying regular atrial rhythm (RED arrows in Figure-4) — I knew today's rhythm had to be ATach with Wenckebach conduction.

Figure-5: I've reproduced Figure-2 to highlight the repetitive pattern of "group beating" in today's rhythm (See text).

Knowing (from Figure-4) where each of the underlying regular P waves are located — allowed me to focus on the 2nd P wave in each group — which I've highlighted with PINK arrows in Figure-6. These PINK arrow P waves are each conducting the 2nd QRS complex in each group of beats (ie, PINK arrow P waves are conducting beats #2,6,10 and 14).
  • Note that the PR interval of each PINK arrow P wave is longer than the PR interval before the 1st beat in each group (highlighted by each RED arrow P wave).

Figure-6: I've highlighted the 2nd P wave in each group with a PINK arrow.

Next, in Figure-7 — I focused on the 3rd and 4th P waves in each group (highlighted by light and dark BLUE arrows, respectively).
  • Note progressive lengthening of the PR interval for each of the colored arrows in each of the 4 groups!

Figure-7: I've highlighted the 3rd and 4th P waves in each group with light and dark BLUE arrows, respectively

Finally, in Figure-8 — I highlight with YELLOW arrows, the location of the next on-time P wave.
  • It is these "on-time" YELLOW arrow P waves that represent the dropped (non-conducted) P wave in each of the Wenckebach cycles in today's rhythm.
  • This is as it should be with Wenckebach conduction — in that these non-conducted YELLOW P waves occur at the beginning of the slight pause between each group — after which the following on-time P wave (ie, the next RED arrow P wave) begins a new cycle with a shorter PR interval.

Figure-8: YELLOW arrows highlight the non-conducted P wave in each group — after which the next Wenckebach cycle begins.

The Laddergram . . .
The above derivation for today's rhythm is rendered easier to see in Figure-9 by use of laddergram illustration.
  • For clarity in Figure-9 — I've kept the coloration of arrows from Figure-8, to facilitate appreciation of progressive lengthening of each PR interval within each of the groups — until the 5th P wave in each group ( = the YELLOW arrow P waves) are non-conducted — followed at the end of each short pause, by resumption of conduction with PR interval shortening for the 1st beat in the next group (highlighted by each RED arrow P wave).

Figure-9: Laddergram illustration of today's rhythm.

Figure-10 is what today's laddergram looks like without coloration.

Figure-10: Final laddergram.

FINAL Thoughts on Today's Rhythm:
As per my Note at the beginning of today's case — today's rhythm is a challenging one if you have not seen this entity before. But once you are aware of this arrhythmia presentation — it becomes EASY to recognize within seconds of seeing the tracing.
  • Repetitive patterns are unlikely to be due to chance! Cement this concept in your mind by going back to the original tracing (which is why I once again show Figure-1 below, without any P waves labeled).

  • As you now take another LOOK at this initial tracing — Isn't it now easier to appreciate group beating with a repetitive pattern of similar intervals — in which each of the short pauses are terminated by easily-identifiable P waves with the same PR interval before beats #1,5,9 and 13?
  • IF you used calipers to facilitate recognizing these relationships — Wasn't it EASY to establish within seconds these similarities in intervals?

  • BOTTOM Line: Once you recognize that today's rhythm is supraventricular — with group beating — and with fast, regularly-occurring P waves — that show clear conduction of the 1st P wave at the end of each pause — you have essentially established that the rhythm is ATach with Wenckebach conduction.
  • Today's rhythm is not complete AV block — because complete ( = 3rd-degree) AV block is most often associated with a regular escape rhythm — and the ventricular rhythm in Figure-1 is clearly not regular!
  • The "good news" — is that most of the time when ATach is associated with the group beating pattern of 2nd-degree AV Wenckebach — the conduction defect usually resolves once the ATach is controlled.

Figure-1: Take another LOOK at today's rhythm!


Acknowledgment: My appreciation for the case and this tracing that was anonymously sent to me for my opinion.

Related ECG Blog Posts to Today’s Case:
  • ECG Blog #185 — reviews the Ps, Qs & 3R Approach (Listen to Audio Pearl #3 in this post).
  • ECG Blog #188 — Reviews how to read and draw Laddergrams (with LINKS to more than 90 laddergram cases — many with step-by-step sequential illustration).
  • ECG Blog #267 — Reviews with step-by-step laddergrams, the derivation of a case of Mobitz I with more than a single possible explanation.

  • ECG Blog #210 — for review of the Every-Other-Beat Method for rapid estimation of heart rate (See Video Pearl #27 in this post).

  • ECG Blog #229 — reviews distinction between AFlutter vs ATach.
  • The November 12, 2019 post in Dr. Smith's ECG Blog — in which I review my approach to a Regular SVT rhythm.

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