COPY of Blog #507 — Teenager with Palpitations- EXTRA COPY

The ECG in Figure-1 — was obtained from an otherwise healthy male teenager with palpitations.

• How would you interpret this tracing?

Figure-1: The initial ECG in today's case — obtained from a teenager with palpitations. (To improve visualization — I've digitized the original ECG using PMcardio).

MY Thoughts on the ECG in Figure-1:

This is a complex tracing. I thought it best to break it down into parts.

• Beginning right after beat #6 — is a continuous run of a regular tachycardia which is wide until beat #21, when the QRS complex suddenly narrows!

NOTE: Although there is no single long lead rhythm strip — limb leads and chest leads are continuous, such that there are 30 consecutive beats on this tracing. In all — there is just under 10 seconds of monitoring.

• By the Every-other-Beat Method — the rate of the tachycardia is ~200/minute (See the ADDENDUM below for a brief ECG Video that reviews application of this concept).
• In Figure-2 — I have chosen the R wave of beat #8 in lead I as my "starting point" — because this begins precisely on a heavy grid line.
• We can see that the time it takes to record 2 beats (PINK numbers 1 and 2) — is 3 large boxes on ECG grid paper (BLUE numbers 1,2,3). This tells us that HALF of the rate = 300 ÷ 3 large boxes = 100/minute.
• Therefore, the actual rate = 100 X 2 = 200/minute.

Figure-2: Illustration of the Every-other-Beat Method for rapid estimation of fast rates. The rate of the regular tachycardia that begins after beat #6 is ~200/minute.

Today's Fast Rhythm is Supraventricular:

Although the QRS complex is wide from beat #7 through to beat #20 — the QRS then suddenly becomes narrow (ie, beginning with beat #21). This is best seen by focusing on the top row of beats in Figure-2 (ie, I suggest focusing on the 30 consecutive beats in leads I and V1 for my description below):

• PEARL #1: The facts that the last 10 beats in today's tracing clearly represent a regular SVT rhythm ( = narrow-complex tachycardia) — and that the transition from the wide tachycardia ( = beats #8-thru-20) — to the regular SVT that begins with beat #21 occurs without any pause or acceleration — suggests that this entire tachycardia is all supraventricular! (ie, If the wide beats represented VT — then it would be exceedingly unlikely for precise regularity of the rhythm to continue as the QRS narrows)!
• PEARL #2: QRS morphology during the wide tachycardia is consistent with RBBB aberrancy! (ie, The all positive QRS in lead V1 for beats #14-thru-20 — with slender initial R wave and wide terminal S wave in lead V6 for these beats — is completely consistent with RBBB conduction — as is the slender initial R wave with wide terminal S wave in lead I for beats #8-thru-13).
• PEARL #3: Putting together what we've established from PEARLS #1 and 2 — the tachycardia that begins after beat #6 is a regular SVT (albeit with a changing QRS morphology) at a rate of ~200/minute, but without sinus P waves. As I review in ECG Blog #240 — the rate of ~200/minute would be faster-than-expected for sinus tachycardia in a teenager — and unusual for AFlutter (since 2:1 AV conduction for untreated AFlutter typically results in a ventricular response close to 150/minute). Given that Atrial Tachycardia is an uncommon arrhythmia in an otherwise healthy teenager — this leaves a reentry SVT rhythm (either AVNRT or AVRT) as the most likely diagnosis for today's tachycardia, especially given the abrupt onset of this SVT rhythm.

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How Does Today's SVT Begin?

Now that we've established our "working diagnosis" as most probably being a reentry SVT rhythm — We can focus our attention on HOW this arrhythmia begins (See Figure-3):

• Beats #1,2,3 in Figure-3 — appear to be the last 3 beats in a previous SVT run at ~200/minute (that is probably of the same SVT mechanism with normal [narrow] QRS conduction as is seen in the chest leads for beats #21-thru-30).
• Beat #4 is sinus-conducted! (the 1st RED arrow in Figure-3 highlighting the sinus P wave).
• Beat #5 occurs early, and is preceded by a PAC (the 1st BLUE arrow that peaks the T wave of beat #4).
• Beat #6 is another sinus-conducted beat.
• Beat #7 is another PAC (2nd BLUE arrow in Figure-3). Note that this 2nd PAC produces a QRS complex that is wider than the normally-conducted QRS of beat #5.

Figure-3: Focusing on how the regular SVT begins.

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QUESTION:
Can you explain WHY in Figure-3 — the QRS complex of beat #7 is wider and looks different than the QRS of beats #1-thru-6?

• HINT: The measurements (in milliseconds) that I’ve added under beats #3-thru-7 in lead II of Figure-4 provide an important clue to the Answer!

ANSWER:

As I explain in the ADDENDUM below — the concept of "cycle-sequence" comparison (as it relates to aberrant conduction) — and — the Ashman Phenomenon — explain why beat #7 is wider and looks different than beats #1-thru-6 in Figure-4 (See ECG Blog #70 — for detailed illustration of the Ashman Phenomenon).

• PEARL #4: My user-friendly way to synthesize the Ashman Phenomenon is simply to recall, “The funniest-looking beat follows the longest pause”. 
• The physiologic reason for this phenomenon — is that the longer the R-R interval preceding a given beat is — the longer the RP (Refractory Period) after that beat will be (with the RP consisting of both an absolute and relative refractory period — as in ECG Blog #70).
• In Figure-4 — the R-R interval preceding beat #6 is 740 msec. — which is longer than the 720 msec. R-R interval that precedes beat #4. Therefore, by the Ashman Phenomenon — the PAC that follows beat #6 will be more likely to conduct with aberration.

PEARL #5: The other component of cycle-sequence comparison — relates to the coupling interval, which is the distance from the onset of a QRS complex until the onset of the PAC that follows it. 

• It makes sense that the shorter the coupling interval — the greater the chance that a PAC will fall within the RRP (Relative Refractory Period), and therefore be conducted with aberration.
• In Figure-4 — it is the coupling interval of the 2nd PAC that is shorter (ie, 180 msec. vs 220 msec.) — therefore explaining why beat #7 is aberrantly conducted, but beat #5 is not.
• KEY Point: There is an "art" to applying the dual concepts of "cycle-sequence" comparison — in that both a longer preceding R-R interval and a shorter coupling interval may not be present, as they are for beat #7 in Figure-4.
• PEARL #6: Given the need for preciseness when applying cycle-sequence comparison for determining the likelihood of aberrant conduction — it should be obvious that use of calipers is a must to apply this concept!

PEARL #7: The ECG Video and content in Figures-7,-8,-9 in the ADDENDUM below — review the basics of aberrant conduction. As emphasized in this review — aberrantly conducted beats most often manifest some known form of conduction block (ie, RBBB, LBBB, and/or left anterior or posterior hemiblock).

• The predominantly positive R wave in lead I — with predominant negativity of the QRS in each of the inferior leads — suggest that beat #7 in Figure-4 is conducted with LAHB (Left Anterior HemiBlock) aberration.

Figure-4: How do the measurements that I’ve added under beats #3-thru-7 in lead II explain why the QRS of beat #7 is wider and looks different than the QRS of beats #1-thru-6?

PEARL #8: It is important to appreciate that the run of reentry SVT that begins with beat #7 in Figure-4 — is initiated by a PAC! 

• In contrast to ATach (ie, an ectopic Atrial Tachycardia) that usually begins with gradual acceleration of the ectopic atrial focus ("warm-up" phenomenon) — SVT reentry rhythms often start abruptly after a PAC — because this early beat arrives at the AV Node when the faster AV Nodal pathway is still refractory.
• This may serve to set up a reentry circuit — IF as a result of the PAC, the impulse starts down the other pathway, and is then able to complete the formation of a reentry circuit (ie, IF the faster AV Nodal pathway that was blocked has recovered in time to allow return of the impulse via retrograde conduction — as shown in Figure-5). 

Figure-5: The mechanism of a reentry SVT rhythm can be seen from this schematic figure — in that each time the impulse completes its path over the reentry circuit — retrograde conduction (back to the atria) as well as forward conduction to the ventricles (through the His-Purkinje system) occurs. As discussed below (as well as in ECG Blog #240) — this retrograde conduction back to the atria can sometimes be seen on the ECG during the tachycardia in the form of retrograde P waves.
= = = = =
KEY Point: With a reentry SVT — the impulse continues to circulate over the reentry circuit until this circuit is either interrupted (ie, by AV nodal blocking drugs or a vagal maneuver or another PAC or a PVC) — or — until the reentry SVT stops spontaneously.
= = = = =
NOTE: The reentry circuit shown here in Figure-5 depicts the mechanism for AVNRT (in which the reentry circuit is completely contained within the AV Node). This differs from the situation with AVRT — in which the reentry circuit extends outside of the AV Node via participation of an AP = Accessory Pathway (See PEARL #9).

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NOTE: What follows below goes Beyond-the-Core. As an emergency provider — it is more than enough to recognize that the teenager in today's case is highly symptomatic with recurrent, rapid runs of a reentry SVT rhythm that merits referral to an EP cardiologist for EP study and probable ablation treatment.

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Beyond-the-Core:  Is there more Atrial Activity?

Take Another LOOK at Figure-4 that I've reproduced below ...

• Is there any evidence of atrial activity after beat #7?
• If so — What does this atrial activity suggest?

Figure-4: Take Another LOOK at Figure-4. Is there more atrial activity?

ANSWER:

There are a number of signs suggesting additional atrial activity that are seen after beat #7. I highlight some of these below in Figure-5:

• To Emphasize — I am not certain about all potential signs of additional atrial activity. That said — I thought this to be of less clinical importance, since this symptomatic teenager with recurrent runs of reentry SVT will need EP study regardless — to clarify the nature of his arrhythmia (and most likely for curative ablation).

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PEARL #9: Sinus P waves are upright in lead II (as well as in the other inferior leads). In contrast — retrograde P waves will be negative in the inferior leads. Other leads that commonly show retrograde atrial activity are leads aVR and V1 — in which retrograde P waves tend to be positive in these right-sided leads.

• As discussed in ECG Blog #240 — the 2 major types of reentry SVT rhythms are AVNRT (if the reentry circuit is completely contained within the AV Node) — and — AVRT (if an AP located outside of the AV Node is present and participating as the retrograde limb of the reentry circuit).
• The very sharp, negative deflections that are seen at the very end of the widened QRS complex for beats #8-thru-13 (highlighted by YELLOW arrows in Figure-6) look like retrograde P waves.
• I added yellow question marks highlighting possible retrograde P waves in the ST segment of beats #1 and 2, which are the last few beats of the narrow SVT rhythm that ends after beat #3.
• Perhaps the BLUE arrow that I added over the ST segment of beat #3 — represents another PAC that occurs with a very short coupling interval (180 msec.) but without a long preceding R-R interval, such that this PAC is blocked (therefore ending the SVT at the beginning of this tracing?).
• Perhaps the sharp, negative deflections seen in other leads during the widened SVT (ie, in lead aVF for beats #8-thru-13 — and at the very end of the QRS in leads V5,V6 for beats #14-thru-20) also represent retrograde P waves?

PEARL #10: I thought the above suggestions of retrograde atrial activity during the runs of reentry SVT in Figure-6 were occurring relatively late in the cycle — which, as illustrated in ECG Blog #240 — suggests participation of an AP (Accessory Pathway) in the reentry circuit (therefore defining the rhythm as orthodromic AVRT). 

• NOTE: The presence and participation of an AP in the reentry circuit will result in a longer reentry circuit than what occurs with AVNRT, in which the reentry circuit is completely contained within the AV Node. This is why the RP' interval tends to be longer with orthodromic AVRT compared to AVNRT (in which the reentry circuit is shorter, given that it is completely contained within the AV Node).

Figure-6: I've highlighted some indications of retrograde atrial activity.

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More about AVRT (AtrioVentricular Reciprocating Tachycardia):

A nice review of AVRT by Jabbour et al appears in StatPearls, 2024.

• A significant percentage of patients with a reentry SVT rhythm will have a "silent" AP (Accessory Pathway) that only allows retrograde conduction — which is why delta waves are never seen on a 12-lead ECG in these patients.
• Sinus P waves in these patients are preferentially conducted in the forward direction over the normal AV Nodal pathway — which is why the QRS complex is narrow.
• If a PAC occurs in such patients — it will be conducted over the normal AV Nodal pathway and, if it occurs "at just the right moment" — it may find that the AP has recovered its ability to conduct retrograde, thereby completing the formation of a reentry circuit. If the "right timing" persists (ie, with recovery of AP ability to conduct retrograde each time an impulse conducted over the normal AV Nodal pathway arrives at the His-Purkinje junction) — this may perpetuate a run of orthodromic AVRT (as appears to be happening in Figure-6).

Miscellaneous additional notes re AVRT rhythms:

• Rarely (in only ~5% of AVRT episodes) — conduction of a supraventricular impulse may arrive in the ventricles via forward conduction over the AP — with retrograde conduction to complete the reentry circuit occurring over the AV Nodal pathway (ie, antidromic AVRT). In such cases, since the forward limb of the reentry circuit passes directly to the ventricles over the AP — the QRS complex will be wide, and antidromic AVRT may look identical to VT.
• On occasion — a patient may have more than a single AP (with this consideration relevant to the conclusion of today's case — as I discuss below).
• The ability of an AP to conduct retrograde (and participate in the reentry circuit of an AVRT rhythm) is not necessarily lifelong. Especially in children — the ability of a "silent" AP to conduct retrograde often resolves as the child becomes older.
• The abrupt switch of a reentry SVT that begins with QRS widening (ie, with either RBBB or LBBB conduction) — but then suddenly normalizes QRS duration without appreciable change in the R-R interval between wide vs narrow beats — suggests that an AP may be participating in the reentry cycle (This is seen beginning with beat #21 in Figure-6). 
• 
  
• Coumel's law may then help to predict localization of the AP:
• IF the R-R interval is slightly longer during the SVT with RBBB conduction than when the QRS is narrow — then the AP is right-sided. 
• IF the R-R interval is slightly longer during the SVT with LBBB conduction than when the QRS is narrow — then the AP is left-sided.

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CASE Follow-Up:

EP study was done on today's patient — and verified the presence of a participating AP in the reentry circuit (thereby confirming orthodromic AVRT as the mechanism of the arrhythmia).

• Noted in the EP report was that atrial stimulation induced orthodromic AVRT mediated by a concealed left lateral AP.
• Intermittent tachycardia was seen with RBBB conduction while maintaining the same cycle length — and with LBBB conduction manifesting a longer cycle length. This confirmed left-sided localization of the AP ( = Coumel's sign).
• The pathway was ablated — after which it was no longer inducible on EP study.

Unfortunately — Holter monitoring following ablation showed some recurrence of the SVT rhythm.

• I do not have further follow-up at this time.
• As I noted earlier — it is known that on occasion more than a single AP may be present. I suspect this may be the case with today's patient — as alternating participation by more than a single AP would seem the most logical explanation for recurrence of this patient's reentry SVT rhythm after ablation of only one of the APs.
• I suspect a 2nd EP study may be needed to identify one or more additional APs that may need to be ablated.

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Acknowledgment: My appreciation to Amelia Aria (from Bucharest, Romania) for the case and this tracing.

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Related ECG Blog Posts to Today’s Case: 

• ECG Blog #185 — Reviews the Ps, Qs & 3Rs Approach to systematic rhythm interpretation.
• ECG Blog #240 — and ECG Blog #345 — Atrial activity in reentry SVT Rhythms.

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ADDENDUM #1 (11/XXXX/2025): Included below is the following:

• The Every-other-Beat Method for rapid estimation of fast rates.
• More on aberrant conduction.

ECG Media Pearl #27 (3:00 minutes Video) — ECG Blog #210 — Reviews the Rule of 300 for estimating heart rate — and — @ 1:25 minutes in the video, the Every-Other-Beat Method for Estimating Rate with fast rhythms (4/2/2021).

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ECG Media PEARL #28 (4:45 minutes Video) — Reviews WHY some early beats and some SVT rhythms are conducted with Aberration (and why the most common form of aberrant conduction manifests RBBB morphology).

• NOTE: I have excerpted a 6-page written summary regarding Aberrant Conduction from my ACLS-2013-ePub. This appears below in Figures-7, -8, and -9).
• CLICK HERE — to download a PDF of this 6-page file on Aberrant Conduction. 

Figure-7: Aberrant Conduction — Refractory periods/Coupling intervals (from my ACLS-2013-ePub).

 

Figure-8: Aberrant Conduction (Continued) — QRS morphology/Rabbit Ears.

 

Figure-9: Aberrant Conduction (Continued) — Example/Summary.