Thursday, May 23, 2019

ECG Blog #164 — PACs vs Wenckebach ... ?

The long lead II rhythm strip that appears in Figure-1 was sent to me without the benefit of clinical information. Unfortunately, no 12-lead ECG is available on this patient …
  • How do you interpret the rhythm in Figure-1?
  • Along the way to arriving at an ECG rhythm diagnosis — there are a bunch of observations that should be made. How many of these can you come up with?
  • What little device that experts in rhythm interpretation routinely carry around — will be invaluable for evaluating this rhythm?

Figure-1: Long lead II rhythm strip. No history available. How do you interpret the rhythm? 

  • NOTE #1: This long lead II rhythm strip was recorded at the standard recording speed of 25 mm/second. Although there is slight distortion due to some angulation of the ECG grid — the quality of this tracing is more than adequate for interpretation.

MSystem for Rhythm Interpretation:
Once you have looked at your patient to assess for hemodynamic stability — there are 5 KEY Parameters to assess in the interpretation of any rhythm. These 5 parameters are easily remembered by the saying, "Watch Your Ps, Qs & the 3Rs". It does not matter in what sequence you assess these 5 parameters — and, we often alter the sequence in which we look for them, depending on the tracing at hand. The important thing, is to be certain you address each of these 5 parameters for every rhythm strip you encounter. LOOK FOR:
  • P Waves (or better yet, for the presence or absence of atrial activity).
  • QRS Duration (We consider anything more than half a large box in duration on ECG grid paper as “wide” [ = >0.10 second] ).
  • Rate of the rhythm.
  • Is the rhythm Regular?
  • And, IF P waves are present — are P waves Related to neighboring QRS complexes? (ie, are P waves conducting?).
  • CLICK HERE — For more detailed explanation of my “Ps, Qs & 3R” Approach to Rhythm Interpretation. (If you click on SHOW MORE below this video on the YouTube page — You’ll see a detailed linked Contents of all that is contained in this video!).

MApproach to the Rhythm in Figure-1:
The rhythm in Figure-1 is a complex tracing! I fully acknowledge to not being 100% certain of my final diagnosis. That said — I believe that by using a systematic approach, we can deduce the most likely answer. My step-wise approach for interpreting this tracing is as follows:
  • Regarding assessment of Ps, Qs & the 3Rparameters — the QRS complex is narrow — the ventricular rhythm is not regular — butsome P waves are present! It appears that at least some of these P waves are related to neighboring QRS complexes (More details on the P wave rhythm below). Since the QRS complex is narrow — the rhythm is supraventricular.
  • Observation #1: There is group beating. This is most easily observed by stepping back a little bit from the rhythm in Figure-1. Thus, there are 3 groups of 2-beats each (beats #1-2; #6-7; and #8-9) — and 2 groups of 3-beats each (beats #3,4,5; and beats #10,11,12). Recognition of group beating is helpful — because it immediately tells us that some type of Wenckebach conduction may be present. 
  • NOTE: There are other potential reasons for group beating — including atrial or ventricular bigeminy or trigeminy. That said — prompt recognition of group beating puts us on alert, and this facilitates and expedites recognition of a Wenckebach conduction disturbance when one is present.
  • PEARL: BEFORE going any further — find a pair of CALIPERSAs is the case with other highly challenging arrhythmias — I predict that you will not be able to appreciate (or interpret) the fine points of this complex rhythm unless (until) you use calipers.

Since we have established that the rhythm in Figure-1 is supraventricular and manifests group beating — the next KEY to rhythm assessment is a search to identify all signs of atrial activity.
  • Observation #2: Start by looking for definite P waves — and, as we do so, we note that definite P waves are present! Definite P waves precede beats #1, 3, 6, 8, 10 and 13 (RED arrows in Figure-2). The PR interval preceding each of these beats is prolonged (ie, clearly more than 0.21 second) — and, this PR interval is the same in front of each of these beats. The fact that the PR interval preceding beats #1, 3, 6, 8, 10 and 13 is identical tells us that all of these beats are conducted, here with 1st-degree AV block.

Figure-2: We have added RED arrows to Figure-1 to highlight that each of these P waves is conducted with the same prolonged PR interval (See text).

We continue to look for other signs of atrial activity in the rest of the rhythm strip:
  • Observation #3: There appears to be notching in the ST segment of beats #2, 4, 7, 9 and 11 (YELLOW arrows in Figure-3). The fact that this notching occurs at approximately the same point in the ST segment of beats #2, 4, 7, 9 and 11 — but never appears in the ST segments of the 1st beat in each grouping (ie, there is no notching in the ST segment of beats #1, 3, 6, 8 and 10) — strongly suggests that this notching is not due to chance (and not due to artifact) — but instead, most likely represents additional atrial activity!
Figure-3: YELLOW arrows highlight consistent notching at the same point in the ST segment for the 2nd beat in each grouping. This notching indicates additional atrial activity (See text).

Observation #4: There appears to be an underlying regular atrial rhythm (ie, a regular P-P interval) throughout the rhythm in Figure-3! This is not immediately apparent. We deduce the presence of a regular underlying atrial rhythm in the following manner.
  • Common things are commonSince we know that each of the P waves highlighted by RED arrows is conducting in Figure-3 — and, since the distance between the YELLOW arrow and the RED arrow found in between beats #2-3; #7-8; and 9-10 looks to be equal — the most common (and logical) explanation for the colored arrows that we see in Figure-3, is that there must be an underlying regular P wave rhythm.
  • This is where your CALIPERS come in! Set your calipers to the distance that separates the YELLOW and RED arrows that are found between beats #7-8. Then, IF you move this distance to the LEFT with your calipers — you will fall upon some extra peaking in the T wave of beat #6 (BLUE arrow in Figure-4). This extra peaking is the result of this next on-time P wave occurring at the same time that the T wave of beat #6 peaks (BLUE arrow in Figure-4).
Figure-4: Moving your calipers to the LEFT by an amount equal to the distance that separates the YELLOW and RED arrows between beats #7-8 — places you precisely on the extra peaking of the T wave of beat #6 (BLUE arrow). This extra peaking is the result of superposition of the next on-time P wave, that occurs at the same time that the T wave of beat #6 peaks (See text).

  • Keeping your calipers set to this precise distance — you can now walk out extra peaking in the T waves of beats #1, 3, 6, 8 and 10 — and, an extra subtle notch in the upslope of the T wave of beats #5 and 12 (BLUE arrows in Figure-5). For practical purposes — the regular colored arrows in Figure 5 confirm the presence of an underlying regular atrial rhythm (BLUE arrows).
Figure-5: There appears to be a regular, underlying sinus rhythm — with a consistent P-P interval between each of the arrows (See text).

Observation #5: After establishing that the underlying atrial rhythm is regular — it becomes apparent that most of "the Footprints of Wenckebach" are present for the rhythm in Figure-5. This suggests that the rhythm is 2nd-Degree AV Block, Mobitz Type I ( = AV Wenckebach). These are the "Footprints" that we see:
  • Group beating.
  • Lengthening of the PR interval until a beat is dropped — after which the cycle resumes and the PR interval shortens.
  • A regular atrial rhythm (ie, a regular, or at least fairly regular P-P interval).
  • The pause that contains the dropped beat is less than twice the shortest R-R interval.
  • Progressive shortening of the R-R interval within groups of beats, until a beat is dropped.

PEARL ( = Beyond-the-Core): In addition to group beating — there are a number of other characteristics that suggest a periodicity consistent with some type of Wenckebach conduction. Marriott has colorfully labeled these characteristics as the Footprints of Wenckebach. We emphasize that Wenckebach conduction does not always manifest each of these findings — but recognition that several of these characteristics are present goes a long way toward suggesting the diagnosis!
  • The best known of these “footprints” (and the easiest to spot) — is group beating. As we emphasized in Observation #1 — there are 3 groups of 2 beats, and 2 groups of 3 beats in Figure-5.
  • Although Mobitz I, 2nd-degree AV block is defined by progressive lengthening of the PR interval until a beat is dropped — not every case of Wenckebach conduction “reads the textbook”. As we will see momentarily — this case is a little atypical — in that the PR interval increases between the 1st and 2nd beats in a group, but not between the 2nd and 3rd beats in a group.
  • By definition — the atrial rhythm (P-P interval) should be regular (or at least almost regular) when there is AV block. I love this “footprint” — because recognition of significant irregularity in the underlying atrial rhythm is the best “tip-off” that you are dealing with something other than AV Wenckebach (ie, PACs, sinus pause or arrest, etc.).
  • The last 2 “footprints” are not generally appreciated. The shortest R-R intervals in Figure-5 are the intervals between beats #4-5 and #11-12. It is easy to see with calipers that each of the pauses (ie, the interval between beats #2-3; #5-6; #7-8; #9-10; and #12-13) is less than twice the shortest R-R interval. This relationship is often not seen when the reason for group beating is sinus pauses or PACs.
  • With Wenckebach groups that consist of 3-or-more beats — the R-R interval within the group tends to progressively decrease. The reason for this — is that the greatest increment (ie, increase) in PR interval with typical Wenckebach tends to be between the 1st and 2nd beats in the group. Thus, in Figure-5 — note that the R-R interval between beats #3-4 is greater than the R-R interval between beats #4-5. Similarly, the R-R between beats #10-11 is greater than that between beats #11-12. Appreciation of this relationship is especially helpful in recognizing Wenckebach conduction when there are no P waves (ie, with AFib) and with SA block. (NOTE: The R-R interval will not always progressively decrease within groups with Wenckebach conduction — but it is a helpful feature when it is present.).

Bottom Line: In the case of the rhythm shown in Figure-1 — my familiarity with looking for “the Footprints of Wenckebach” allowed me to suspect the diagnosis of this arrhythmia in less than 10 seconds! This is because I instantly recognized group beating — I next recognized definite conduction of the 1st beat in each group — and I saw what looked to be regularly-spaced notching and/or extra peaking that strongly suggested a regular underlying atrial rhythm. Confirming the regularity of this underlying atrial rhythm took mere seconds with use of calpiers — and these relationships, together with "the overall look" of this arrhythmia was enough to strongly suggest Wenckebach conduction to me.
  • We devote our ECG Blog #236 to the 2nd-Degree AV Blocks. We highlight the "Footprints of Wenckebach", beginning at 1:20 minutes in the 15-minute Video Pearl on this blog post.
CHALLENGE: PROVE to yourself how useful awareness of the “Footprints of Wenckebach” can be.
  • How long does it now take you to suspect the correct rhythm diagnosis in our ECG Blog #55?

Our Final Observation: It is possible for certain complex arrhythmias to have more than a single plausible rhythm interpretation. The way to PROVE that your interpretation is one of the plausible explanations — is to draw Laddergram that makes sense, and which illustrates your theory.
  • CONFESSION: It took me many years to become comfortable drawing laddergrams. The “good news” — is that it takes no more than a few minutes (and a little bit of practice) to become comfortable reading laddergrams. Our laddergram for the rhythm in this case is shown in Figure-6. Our explanation of events follows in the Figure legend.
  • NOTE: For more on the basics of HOW to Read (and/or Draw) Laddergrams — Please see my ECG Blog #188

Figure-6: Laddergram to illustrate our interpretation of this rhythm. RED arrows highlight the underlying regular atrial rhythm (schematically shown by the 18 short vertical RED lines in the Atrial Tier).
  • The 1st beat in each group conducts with 1st-degree AV block and an identical PR interval (beats #1, 3, 6, 8, 10 and 13)
  • The 3 groups of 2 beats (ie, beats #1-2; #6-7; #8-9) manifest typical Wenckebach conduction — in that the PR interval increases until a beat is dropped. 
  • The 2 groups of 3 beats (ie, beats #3,4,5; and #10,11,12) are slightly atypical — in that the PR interval preceding beats #5 and 12 is not longer than the PR interval preceding beats #4 and 11. I believe the reason for this discrepancy is simply due to the disproportionately large difference in R-R intervals within the 3-beat groupings. 

  • BOTTOM LINE: Despite this slightly atypical feature in the two 3-beat groupings — I feel this laddergram confirms that the most likely diagnosis for this rhythm is sinus with 2nd-Degree ABlock, Mobitz Type = AV Wenckebach), in which there is 4:3 AV conduction ( = 4 P waves, of which only 3 are conducted for beats #3,4,5; and #10,11,12)and3:2 AV conduction ( = 3 P waves, of which only 2 are conducted for beats #1-2; #6-7; and #8-9).

  • Correction: My THANKS to Wai Shein — for his "eagle eye" in catching the error in my original description of Figure-6 — which I have now corrected = May 9, 2024).

ADDENDUM: For those wanting to see the stepwise approach used to arrive at the laddergram I showed in Figure-6 — I’ve added Figures #7-thru-13.
  • CLICK on Figure #7 — which will magnify this Figure for you. Then advance one-by-one the next 6 Figures in the magnified mode to see our stepwise approach for drawing this laddergram.
Figure-7: Laddergram STEP-1. We find it easiest to first complete the Atrial Tier, that shows atrial activity.
Figure-8: Laddergram STEP-2. We next fill in the Ventricular Tier, to show QRS complexes that we are sure about.
Figure-9: Laddergram STEP-3. We connected the P wave from the Atrial Tier for beat #1 — to the QRS complex that it conducts to.
Figure-10: Laddergram STEP-4. We've now filled in all of the sinus-conducted beats (ie, those beats that we are certain about = the first beat in each group).
Figure-11: Laddergram STEP-5. It should now be easy to see that conduction of the 2nd P wave in each grouping takes a little bit longer ( = the principal characteristic of Wenckebach conduction).
Figure-12: Laddergram STEP-6. We've now added conduction properties for the 3rd QRS complex in the 2 3-beat groups. Use of a laddergram facilitates seeing that the PR interval preceding beats #5 and 12 is atypical for AV Wenckebach, in that it is not longer than the PR interval preceding beats #4 and 11.
Figure-13: Finished Laddergram (STEP-7). We complete the laddergram by adding a terminal "butt" to those P waves that are not conducted. 
CONCLUSION: My proposed laddergram shows the rhythm to be sinus with 2nd-Degree AV Block, Mobitz Type I ( = AV Wenckebach) — in which there is 4:3 and 3:2 AV conduction.

Acknowledgment: My THANKS to Thant Sin Win (from Myanmar) for allowing me to use this tracing and clinical case.