Friday, January 6, 2023

ECG Blog #355 — Why the Pauses with SVT?

The 2 lead II rhythm strips shown in Figure-1 — were obtained from a 50-year old woman who presented with palpitations. She was hemodynamically stable at the time both rhythm strips were recorded. 

  • How would YOU interpret the rhythm?
  • Why are there pauses in rhythm strip "B"?

Figure-1: These 2 rhythm strips were both obtained from a 50-year old woman with palpitations. Not much time passed between the recording of rhythm strip "A" and rhythm strip "B".

MY Thoughts on the Rhythm in Figure-1:
We are told that both of the rhythm strips in Figure-1 were obtained from the same patient — with a limited amount of time having passed between these 2 recordings.
  • That said — We are not told if this patient received any treatment. This is relevant — because there is an obvious difference between the 2 tracings.

Rhythm Strip A:

As always — I began my approach to rhythm assessment with review of the PsQs and 3Rs (as discussed in detail in ECG Blog #185).

  • To emphasize — When applying the Ps, Qs, 3R System — You do not have to go in sequence. Instead — I favor review of whichever parameters are easiest to assess.
  • The rhythm in "A" is fast and regular. The rate is ~135/minute. Although we only see a single monitoring lead — the QRS appears to be narrow. Sinus P waves do not appear to be present (at least not with a normal PR interval) — there is no obvious "relationship" between atrial activity and the QRS complex.

  • IMPRESSION: The rhythm in "A" is a Regular SVT (SupraVentricular Tachycardia) at a rate of ~135/minute, without clear sign of sinus P waves.

PEARL #1: Recognition that the rhythm in Figure-1 is a Regular SVT without clear sign of sinus P waves (ie, without a definite upright P wave in lead II) — should prompt consideration of the following differential diagnosis LIST:

  • i) Sinus Tachycardia (IF there is a possibility that sinus P waves might be hiding within the preceding ST-T wave)
  • ii) A Reentry SVT (either AVNRT if the reentry circuit is contained within the AV node — or AVRT if an AP [Accessory Pathway] located outside the AV node is involved)
  • iii) Atrial Tachycardia (ATach);
  • iv) Atrial Flutter (AFlutter) with 2:1 AV conduction.


KEY Point: Although other entities may also produce a regular SVT (ie, sinoatrial node reentry tachycardia, junctional tachycardia) — they are far less common in practice. Therefore, remembering to think of the 4 entities in the above LIST whenever you encounter a regular SVT rhythm without clear sign of sinus P waves — will greatly facilitate determining the correct diagnosis.

  • In Rhythm "A"any of the above 4 entities could be present. The clinical reality — is that treatment of this rhythm will need to begin before we know for certain which of these entities is the actual diagnosis. That said — initial management of this patient will be similar regardless of which of the above 4 entities the rhythm happens to be (ie, consideration of a vagal maneuver — and/or use of an AV nodal blocking agent such as Adenosine, Verapamil-Diltiazem, or a ß-Blocker).

PEARL #2: Assessment of Rhythm "B" provides the answer to Rhythm "A"! This is the principle that assessment of additional monitoring on the same patient may yield insightful information that facilitates diagnosis.

Rhythm Strip B:
The most obvious difference between Rhythm "A" and Rhythm "B" in Figure-1 — is that the rhythm in "B" is no longer regular. Instead — We note the following:
  • There are pauses in Rhythm B.
  • There is group beating (ie, groups of 2 beats — and of 3 beats — each separated by pauses that for the most part are of similar duration).
  • P waves are now clearly present in Rhythm B.

  • Beyond the Core: An important advanced observation that can be made with the use of calipers — is that the rate of the P waves that is seen in B (taking the P-P distance between any 2 P waves that are seen in a row) — is slightly slower than the ventricular rate in Rhythm A.
  • This finding is relevant — because we can suspect that perhaps this patient was started on some type of AV nodal blocking agent in A — which resulted in slight slowing of the atrial rate, and development of some form of AV block ... But let's work up to this in step-by-step fashion ...

Is There Conduction in Rhythm B?
What follows below entails assessment of Rhythm B. As noted above — there are P waves — and there is group beating in Rhythm B.
  • PEARL #3: The reason we know there is conduction of at least some of the P waves that we see in Rhythm B — is that the identical PR interval is seen before most of the beats that end a pause! (RED arrows in Figure-2). This is not by chance! Instead — it tells us that the first beat in each group is being conducted, albeit with a long PR interval (ie, ~0.44 second).

Figure-2: The first beat in each group is being conducted, albeit with a long PR interval (RED arrows).

The Atrial Rhythm in B Is Regular!
Using calipers facilitates (and greatly expedites!) establishing that the atrial rhythm in B is regular (RED arrows in Figure-3).
  • NOTE: PINK arrows highlight the 2 P waves in this rhythm strip that we don't see — because these P waves are hidden within the QRS of beats #2 and 7. But given how regular all other RED arrow P waves are — we know that on-time P waves are almost certain to be hiding under those PINK arrows!
  • This suggests that the underlying rhythm in Figure-3 — is ATach (Atrial Tachycardia) at ~130/minute (RED and PINK arrows showing regular P waves throughout the rhythm strip).

Figure-3: The atrial rhythm in B is regular (RED and PINK arrows).

PEARL #4: When you see ATach with group beating Think Wenckebach! As discussed in ECG Blog #261"Common things are common" — and Atrial Tachycardia is very commonly associated with Wenckebach conduction.
  • Recognition of group beating in Figure-2 — in which there is an underlying regular atrial rhythm + identical PR intervals that repeat + similar R-R interval duration for most of the pauses — strongly suggests that there is AV Wenckebach conduction.

To facilitate recognition of this Wenckebach conduction — I've color-coded the P waves in Figure-4:

  • RED arrows highlight the initial P wave in most of the groups. Note that the PR interval of these P waves (ie, the PR interval that precedes beats #4, 6, 9, 11, and 14)is identical (ie, prolonged to ~0.44 second in duration).
  • Dark BLUE arrows highlight the 2nd P wave in 6 of the groups. The PR interval of these P waves (that precede beats #2, 5, 7, 10 and 12) — is longer than the PR interval highlighted by the RED arrows (ie, ~0.50 vs 0.44 second).
  • Light BLUE arrows highlight the location of where a 3rd P wave is hidden in 2 of the groups. The PR interval of these P waves is longer still (ie, ~0.54 second).
  • YELLOW arrows highlight the last P wave in a given group. These YELLOW P waves are not conducted because of the Wenckebach block. Note that after each of these YELLOW arrows — a RED arrow follows, thereby beginning the next group.

Figure-4: To facilitate appreciation of Wenckebach conduction — I've color-coded the P waves in this rhythm (See text).

Laddergram Illustration:
It is much easier to appreciate the presence of Wenckebach conduction for the ATach rhythm in Figure-4 — by use of the Laddergram that I've drawn in Figure-5:
  • I've kept the color-coding shown in Figure-4.
  • It's easiest to follow events in the laddergram — by picking any group of beats, and starting by following the path of the RED arrow P wave in that group. For example — the RED arrow P wave before beat #6 manifests progressive PR interval lengthening for the dark BLUE and then light BLUE arrows that follow — until the YELLOW arrow P wave (that appears just before beat #8) is not conducted.
  • The next Wenckebach group then begins with the RED arrow P wave that precedes beat #9.
  • Wenckebach conduction in Figure-5 manifests 4:3 and 3:2 AV conduction.

  • NOTE: The PR interval of the PINK arrow P wave is slightly longer than the PR interval for all of the RED arrow P waves. I'm not quite sure why. I drew the laddergram to reflect that this PINK arrow P wave is being conducted to the ventricles (but I added a question mark to acknowledge that I am not certain this P wave is conducted). That said — this does not matter clinically, because the "theme" of this rhythm is unchanged (ie, ATach with Wenckebach conduction).

Figure-5: Laddergram depiction of the color-coded tracing in Figure-4.

Final Laddergram:
I typically do not color-code the lines used to construct my laddergrams. I've therefore made all lines in the laddergram uniform in color. With Figure-5 as background — it should now be easy to follow the final laddergram depiction in Figure-6.

Figure-6: Final laddergram for Rhythm B.

CASE Review:
Take another LOOK at the original 2 rhythms that were shown in Figure-1. Note in Figure-7 — that I've added selected RED arrows to highlight atrial activity.
  • Now that we've established that Rhythm B represents ATach with 4:3 and 3:2 AV Wenckebach conduction — it should be apparent that the Rhythm A was ATach with 1:1 AV conduction and a long and constant PR interval (of ~0.32 second). What was thought to be a "pointed" T wave in Rhythm A — turned out to be the P wave of an ectopic Atrial Tachycardia.

  • Putting It All Together: We were not told if today's patient received any treatment in between the time that Rhythm A and Rhythm B were recorded. I strongly suspect that some form of AV nodal blocking treatment was administered (ie, Adenosine, Verapamil-Diltiazem or a ß-Blocker) because: i) The ventricular (and therefore the atrial) rate of Rhythm A is slightly faster than the atrial rate in Rhythm B (ie, ~135/minute vs 130/minute); ii) The PR interval for the 1st conducted beat in each of the Wenckebach groupings in Rhythm B is longer than the PR interval during 1:1 AV conduction in Rhythm A (ie, 0.44 second — compared to 0.32 second); and, iii) Wenckebach conduction is seen in Rhythm B, whereas there was 1:1 AV conduction in Rhythm A.
  • The most logical conclusion is that some AV nodal blocking agent was administered to the patient when she presented in Rhythm A — and that this resulted in slight slowing of the atrial rate, an increase in the PR interval — and Wenckebach conduction in which every 3rd or 4th beat was blocked.

Figure-7: I've labeled selected P waves in today's original tracings .

Final POINT: Rather than calling Rhythm B some form of 2nd-degree AV "block" — I favor considering this rhythm as ATach with Wenckebach conduction. This type of Wenckebach conduction is often physiologic, as a result of the rapid atrial rate. It often does not represent a specific conduction defect — and Wenckebach conduction will often resolve if the ATach is controlled.


Acknowledgment: My appreciation to Hafiz Abdul Mannan Shahid (from Lahore, Pakistan) for the case and these tracings.



ADDENDUM (1/6/2023): 

  • The Audio Pearls below may be helpful in assessment of the rhythms in today's case.

ECG Media PEARL #51a (7:40 minutes Audio) — Reviews of "Some Simple Steps to Help Interpret Complex Rhythms" ).

ECG Media PEARL #55 (4:20 minutes Audio) — What does the term, "SVT" mean? This Audio Pearl reviews the semantics and clinical application involved in use of this term. 


Relevant ECG Posts to Today's Case:


  1. thank you for your explanation! I had a same EKG case but I couldn't interpret it like you!

    1. THANK YOU for the kind words! Glad my explanation was helpful! — :)

  2. Wow. That was brilliant. Classic KG Genius. I don't think anyone else could have solve this riddle, thanks Prof

    1. THANKS for the kind words. KEY = Use of calipers — :)