Thursday, July 8, 2021

ECG Blog #240 (55) — What Kind of Regular SVT?

Today's blog is all about the Regular SVT (SupraVentricular Tachycardia). The patient whose ECG is shown in Figure-1 presented with "palpitations". He was hemodynamically stable.

  • How would you interpret the ECG in Figure-1?
  • And — Can you tell the mechanism of this rhythm?
  • P.S. — How many of the 8 PEARLS that I list below were you aware of?

Figure-1: How would you interpret this ECG? (See text).



NOTE: Some readers may prefer at this point to listen to the 4:20-minute ECG Audio PEARL before reading My Thoughts regarding the ECG in Figure-1. Feel free at any time to refer to My Thoughts on this tracing (that appear below ECG MP-55).


Today's 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. 



My Initial THOUGHTS on ECG #1:

As usual — I approach rhythm interpretation in systematic fashion by addresing the KEY parameters contained in the Ps, Qs & 3Rs Approach (See ECG Blog # 185).

  • NOTE: It is not essential to perform this assessment in order (ie, I'll often look at QRS width or regularity of the rhythm before looking for P waves — depending on which of the 5 parameters is easiest to assess for the tracing at hand)


My sequential thought process for the rhythm in Figure-1 was the following:

  • The QRS complex is narrow — so the rhythm is supraventricular.
  • Sinus P waves are absent (ie, there is no upright P wave in lead II — although, as will be discussed momentarily — it appears that there may be other signs of atrial activity).
  • The ventricular rhythm is Regular.
  • The R-R interval is just over 2 large boxes in duration — so the Rate of the rhythm is just under 150/minute.

IMPRESSION: The rhythm in Figure-1 is a regular SVT rhythm at ~140/minute — but without clear sign of sinus P waves.

  • Looking at the rest of the ECG — there is marked left axis deviation (consistent with LAHB) — no chamber enlargement — early transition (ie, R wave height already equals S wave depth as early as between lead V1-to-V2) — and — some nonspecific ST-T wave abnormalities (that may be rate-related — and which are probably not acute).


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 followling 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.

  • NOTE: Most SVT rhythms manifest a narrow QRS complex. That said — an SVT rhythm may have a wide QRS if there is either preexisting bundle branch block or aberrant conduction due to the rapid rate.

PEARL #2: How Heart Rate helps in SVT Diagnosis:

  • Sinus Tachycardia usually does not exceed 160-170/minute in a "horizontal" adult (ie, in a patient you are examining, who has not just been running). This is not to say that sinus tachycardia will never go faster than 170/minute — but rather to suggest that when the rate of the regular SVT rhythm you are assessing is well over this rate range — then the rhythm will probably not be sinus tachycardia. NOTE: All bets are off in children — in whom sinus tachycardia over 200/minute is not that uncommon.
  • With AFlutter — the most common ventricular response in the patient who is not being treated with an antiarrhythmic medication is ~150/minute (usual range ~140-160/minute). This is because the atrial rate in untreated AFlutter is most often ~300/minute (usual range ~250-350/minute) — and since untreated AFlutter most often presents with 2:1 AV conduction — 300/2 ~150/minute. As a result — IF the ventricular rate of the regular SVT rhythm you are assessing is over ~170-180/minute — then AFlutter is less likely, because this rate would be faster-than-expected for 2:1 AV conduction, and too slow for 1:1 AV conduction. NOTE: This ~140-160/minute rate range is for untreated AFlutter. Patients who are already on antiarrhythmic medication may present with a slower atrial rate (and therefore slower ventricular response) for flutter.
  • It is well to remember that ATach is less common as a cause for a strictly regular SVT, especially in an otherwise healthy young-to-middle-aged adult. ATach is more likely to be seen in patients referred for EP (ElectroPhysiologic testing) — and in older adults with SSS (Sick Sinus Syndrome). I include ATach in the above differential diagnosis LIST for completeness — but take into account that it won't be seen as often as AFlutter and the reentry SVTs.
  • Therefore — IF the rate of a regular SVT without clear sign of sinus P waves is substantially faster than 160-170/minute — then a reentry SVT rhythm (ie, AVNRT or AVRT) becomes the most likely diagnosis. However, IF the rate of the regular SVT is close to 150/minute (ie, 140-160/minute) — then any of the 4 diagnostic entities in the above LIST could be present ( = sinus tach — AVNRT-AVRT — ATach — AFlutter).

PEARL #3: Other Helpful Diagnostic Features:

In addition to the ventricular rate of a regular SVT rhythm — there are a number of other ECG findings that may help with differential diagnosis. These include:

  • Detection of Atrial Activity! — I have reviewed on many occasions how using calipers can facilitate detection of 2:1 AV conduction diagnostic of AFlutter (ie, See my discussion in the November 12, 2019 post of Dr. Smith's ECG Blog). As I'll discuss momentarily — detection of retrograde atrial activity can be diagnostic of a reentry SVT rhythm.
  • Look for a “Break” (or subtle Change) in the Rhythm! — Even a slight pause in the SVT rhythm, may be all that is needed to reveal underlying atrial activity that had been hidden by the regular tachycardia. 
  • Look for the “Onset” (and/or Termination) of the Rhythm! — The KEY clue to the etiology of an SVT often lies with capturing either the beginning and/or the end of the SVT. ATach often begins gradually, with progressive acceleration of the ectopic focus (ie, “warm-up” phenomenon). Then, there may be gradual slowing (ie, “cool-down) of the rhythm as it ends. In contrast — SVT reentry rhythms often start with 1 or more PACs that block conduction down one of the AV nodal pathways, which serves to promptly set up the reentry circuit as the impulse starts down the other pathway. The onset of a reentry SVT is often abrupt (ie, accounting for the previous term used to designate this rhythm = PSVT = Paroxysmal SVT)The termination of reentry SVT rhythms is also usually quick (though this may occur over several beats).


PEARL #4: While our goal is to arrive at as precise of a rhythm diagnosis as possible — the "good news" is that for practical purposes — initial diagnostic and/or treatment measures in the field or in the ED for an undifferentiated regular SVT rhythm are similar (ie, consideration of a vagal maneuver — and/or use of an AV nodal blocking agent such as Verapamil/Diltiazem; a ß-Blocker, and possibly Adenosine).



The CASE Continues:

Take another look at the rhythm in Figure-1.

  • QUESTION: Do you see any sign of atrial activity?






We have already emphasized how sinus P waves are not seen in Figure-1 — as there is no upright P wave that precedes the QRS complex in lead II.

  • That said — RED arrows in Figure-2 indicate that there are retrograde P waves! 


PEARL #5: Retrograde atrial activity is most easily recognized by the finding of negative P waves in one or more of the inferior leads. This negative P wave deflection may be seen as a notch in the terminal portion of the QRS complex — as a notch in the ST-T wave — or sometimes beyond the end of the T wave. 

  • In Figure-2 — retrograde atrial activity is best seen in leads III and aVF (as a negative notch) — and in lead V1 (as a small positive notch that appears just after the r' of the QRS complex). I suspect the negative deflection of the QRS in lead II reflects a combination of S wave + retrograde P wave, though it is more difficult to distinguish a distinct retrograde P wave in this lead.
  • KEY Point: The distance from the onset of the R wave until the beginning of the retrograde P wave is known as the RP' interval.
  • IF retrograde P waves are seen in a regular SVT rhythm — and the RP' interval is short (ie, ≤70 msec.) — then AVNRT ( = AtrioVentricular Nodal Reentrant Tachycardia) is likely! (González-Torrecilla et al: Ann Noninvasive Electrocardiol 16(1):85-95, 2011).
  • Because precise measurement of the RP' interval is often difficult — I consider the RP' interval to be short (ie, predictive of AVNRT) if retrograde P waves either notch the end of the QRS, or occur very soon after the QRS (as they do in Figure-2, as shown by the RED arrows).


Figure-2: I've added RED arrows to Figure-1 to indicate retrograde atrial activity. The RP' interval is short (ie, retrograde P waves immediately follow the end of the QRS complex) — therefore suggesting AVNRT as the mechanism for this SVT rhythm (See text).

Looking Closer at the Retrograde P Waves:

Consider the 3 tracings shown in Figure-3. They each show a Regular SVT rhythm, in which sinus P waves (ie, upright P waves in lead IIare not clearly seen.

  • ECG #2  shows a regular SVT at just under 150/minute. The upright deflection near the midpoint in the R-R interval in lead II is almost certainly just a T wave. Against there being a sinus P wave hidden within this T wave — is the fact that this would make the PR interval long (ie, ~0.26 second) — and if anything, the PR interval shortens when there is tachycardia. Although a rate of ~150/minute should always make you think about the possibility of AFlutter — other than possibly lead V6, none of the other 11 leads even remotely suggest 2:1 AV conduction. Instead — the RED arrows in ECG #2 suggest retrograde atrial conduction with a very short RP' interval that notches the terminal portion of the QRS complex (with a positive deflection in lead V1 — and — with suggestion of a negative notch in each of the inferior leads). This rhythm is highly unlikely to be ATach — because if that upright deflection in lead V1 was a forward-conducting P wave — it would make for an exceedingly long PR interval. This leaves us with a reentry SVT rhythm as the probable diagnosis for ECG #2 (ie, AVNRT or AVRT).
  • ECG #3  shows a regular SVT at ~210/minute. For practical purposes, this rate is too fast to be Sinus Tachycardia. It is also too fast to be AFlutter with 2:1 AV conduction — since this would result in an atrial rate = 210 X 2 = 420/minute, which is much faster than the maximum atrial rate for flutter. AFlutter with 1:1 AV conduction is extremely uncommon, and a flutter rate of 210/minute would be uncharacteristically slow (unless the patient is on antiarrhythmic therapy). ATach is possible, but in the absence of clear sign of ectopic P waves — ATach is far less likely than a reentry SVT, which is a much more common rhythm (as per Pearl #2). While difficult to prove from this single tracing — I suspect that the RED arrows in ECG #3 (that highlight probable negative deflections in the inferior leads, with sharp angulation in lead V3) represent retrograde atrial conduction with a moderately long RP' interval that occurs well after the QRS complex (within the latter part of the ST segment).
  • ECG #4  shows a regular SVT at ~160/minute. There is a slow, gradual upslope to and beyond the ST segment, that leads up to a very deep, negative P wave (highlighted by RED arrows) in front of each of the inferior leads. This deflection before the QRS is positive in lead aVR. P wave negativity in the inferior leads rules out Sinus Tachycardia. Lack of 2:1 atrial activity rules out AFlutter. This essentially leaves us having to decide between ATach (in which case we'd have to postulate huge negative P wavesvs AVNRT, in which there is retrograde atrial conduction with a very long RP' interval.

Figure-3: For illustrative purposes — Consider the regular SVT rhythms in this Figure. RED arrows represent retrograde atrial activity. (See text).



What the RP' Interval Tells Us in Figure-3:

The KEY to the probable etiology for each of the regular SVT rhythms shown in Figure-3 lies with the relative length of the RP' interval.

  • ECG #2 in Figure-3 shows a very short RP’ interval, in which the retrograde P wave distorts the terminal portion of the QRS complex. This is virtually diagnostic of the slow-fast form of AVNRT — in which the impulse travels first down the "slow" AV nodal pathway — and back up the "fast" AV nodal pathway. It is because the impulse travels back up (ie, retrograde) over the “fast” AV nodal pathway that the RP’ interval is so short. This “slow-fast” form of AVNRT is by far the most common AVNRT form, so much so that we generally assume “slow-fast” conduction when we simply say “AVNRT”.
  • ECG #3 in Figure-3 shows a moderately long RP’ interval. This suggests that there may be an AP (Accessory Pathway) participating in the reentry pathway. Because the AP lies outside of the AV node — the time to circulate around the reentry pathway and conduct back to the atria (retrograde) is longer than when the entire reentry circuit is contained within the AV node. As a result, the retrograde P wave will often be seen to occur within the ST segment, instead of notching the terminal portion of the QRS complex. A follow-up ECG was done on this patient, and did indeed show WPW (See My Comment in the October 16, 2019 post of Dr. Smith's ECG Blog).
  • ECG #4 in Figure-1 either shows a very long RP’ interval or an Atrial Tachycardia. Although definitive distinction between an ectopic ATach vs a reentrant SVT rhythm would require EP study — I favor the diagnosis of a reentrant SVT because: i) the shape of the ST segments in ECG #4, leading up to the large negative deflection highlighted by RED arrows looks more like retrograde activity; andii) If the rhythm was ectopic ATach, we'd have to postulate huge negative P waves! Therefore  IF we are indeed seeing a very long RP' interval in ECG #4 — then this would represent the far less common "fast-slow" form of AVNRT — in which the impulse travels first down the “fast” AV nodal pathway — and back up the “slow” AV nodal pathway. Retrograde conduction back up the “slow” AV nodal pathway would account for the very long RP’ interval.


PEARL #6: Note the PVC toward the end of the long lead II rhythm strip in ECG #4. This illustrates a concept mentioned in Pearl #3 — namely, that clues to etiology will often be found when there is a "break" in the underlying SVT rhythm.

  • Note that retrograde atrial activity from "fast-slow" AV nodal reentry continues unaffected by the PVC (vertical GREEN lines in Figure-3).
  • The finding of one or more PVCs during a reentrant SVT rhythm that do not convert the rhythm, favors AVNRT over orthodromic AVRT. This is because a premature impulse arising from the ventricles is much more likely to interrupt a reentrant SVT (such as AVRT) in which a part of the cycle passes through the ventricles (via an accessory pathway) on its way back to the AV node. 
  • In contrast — the reentrant cycle with AVNRT is entirely contained within the AV node, and therefore far less likely to be "penetrated" and interrupted by a PVC.
  • Bottom Line: While EP study would be needed for definitive diagnosis — I strongly suspect the "fast-slow" form of AVNRT for the etiology of the SVT rhythm in ECG #4 instead of AVRT or ectopic ATach.

Help from the Post-Conversion Tracing:

It's important to always repeat the ECG after conversion to sinus rhythm (Figure-4). This is because definitive determination of SVT etiology may only be possible after comparison of the ECG during and after the tachycadia.

  • PEARL #7: The BEST way to prove that suspicious deflections during the SVT rhythm were indeed reflective of retrograde atrial conduction — is to obtain a post-Conversion tracing. Note that retrograde atrial activity during the SVT (RED arrows in Figure-4) — is no longer present after conversion to sinus rhythm (BLUE arrows now showing smooth transition from the end of the QRS to the beginning of the ST segment). The only possible explanation is that the RED arrows represented retrograde P waves from a reentrant cycle with a very short RP' interval — such that the rhythm in ECG #2 was AVNRT.

Figure-4: Comparison of the SVT rhythm in ECG #2 with the post-Conversion tracing proves that the SVT rhythm was AVNRT (See text).


The Mechanism of the "Fast-Slow" form of AVNRT:

Finally — Consider the rhythm shown in Figure-5.

  • QUESTION: Can you explain what is happening in Figure-5?


Figure-5: A final example of a regular SVT rhythm. How does this SVT begin? (See text).




The laddergram in Figure-6 reveals the mechanism operative for the SVT rhythm in ECG #5. This illustrates another concept mentioned in Pearl #3 — namely, that the onset and/or termination of the SVT rhythm may provide an invaluable clue to etiology.

  • Note that we see the onset of the tachycardia in Figure-6. After a sinus beat (ie, beat #3 in Figure-6) — a PAC (ie, beat #4) initiates the process.
  • This PAC presumably finds the slow AV nodal pathway amenable to retrograde conduction at the moment it is traveling toward the ventricles (ie, dotted, slanted lines in the AV Nodal Tier of Figure-6). IF the timing is “just right” (as shown in this laddergram) — then a reentry circuit utilizing the slow AV nodal pathway (which produces the very long RP’ interval) can be set up.
  • I’m uncertain why this “fast-slow” AVNRT terminates after beat #13 (perhaps because the R-R interval of beats #12-13 is a little bit shorter than preceding R-R intervals during the tachycardia?).
  • Following 1 sinus beat ( = beat #14) — it looks like the “fast-slow” AVNRT is about to begin again.


PEARL #8: ATach typically begins with gradual acceleration of an ectopic atrial focus (ie, "warm-up" phenomenon). It often terminates with gradual deceleration (ie, "cool-down" phenomenon). This is not what we see in Figure-6!

  • Reentrant SVT rhythms typically begin with one or more PACs, that serendipitously find one of the AV nodal pathways amenable to retrograde conduction. This serves to initiate the reentrant cycle.
  • There is no "warm-up" in Figure-6. After one retrograde beat — a constant reentrant R-R interval is set up for the 8 beats in the run of SVT that follows (ie, beats #6-thru-13).
  • There is no "cool-down". Instead — we see abrupt conversion to sinus rhythm with beat #14 (albeit the very next beat seems about to initiate another run).
  • A similarly abrupt termination (ie, without "cool-down") is observed at the beginning of ECG #5 (prompt cessation of SVT after beat #2).
  • Bottom Line: The sequence for "onset" and "offset" of the SVT rhythm in Figure-6 is typical for the uncommon "fast-slow" reentrant form of AVNRT.

Figure-6: Laddergram illustration of the mechanism for the SVT rhythm in Figure-5.



Related ECG Blog Posts to Today’s Case: 

  • ECG Blog #185 — Reviews the Ps, Qs & 3Rs Approach to systematic rhythm interpretation.
  • ECG Blog #188 — Reviews the essentials for reading (and/or drawingLaddergrams, with LINKS to numerous Laddergrams I’ve drawn and discussed in detail in other blog posts. 
  • ECG Blog #229 — Why is AFlutter so commonly overlooked
  • ECG Blog #137 — AFlutter with an unusual conduction ratio. 
  • ECG Blog #138 — AFlutter vs Atrial Tachycardia
  • ECG Blog #40 — Another regular SVT that turned out to be AFlutter.


Other sites where I've discussed some of the cases shown in today's blog:

  • The March 6, 2020 post in Dr. Smith's ECG Blog.
  • The October 31, 2016 post in Dr. Smith's ECG Blog.
  • The October 16, 2019 post in Dr. Smith's ECG Blog. 

  • Please check out the November 12, 2019 post in Dr. Smith's ECG Blog — in which I reviewed the case of a different kind of regular SVT Rhythm (AFlutter).


  1. Thank you so much! Would you please clarify the pearl about pvc? Did you mean that if there were PVCs during the svt it is more possible to be AVNRT ?

  2. If in the middle of a regular SVT rhythm you see a PVC — but the regular SVT continues despite the PVC — then the mechanism is more likely to be AVNRT — because the reentry circuit is entirely contained WITHIN the AV Node (therefore LESS likely to be interrupted by a premature ventricular beat!).

    On the other hand — IF after a PVC, a regular SVT rhythm is interrupted (and sinus rhythm is restored) — then the mechanism is more likely to be AVRT — because part of the reentry circuit involved the accessory pathway, which is located OUTSIDE of the AV Node (and therefore is more susceptible to be interrupted by an early ventricular beat).