Friday, November 18, 2022

ECG Blog #345 — Forward - Backward Conduction?

The ECG in Figure-1 — was obtained from a previously healthy young man who presented with “rapid heart beat”. He was hemodynamically stable at the time this ECG was recorded.
  • How would YOU interpret this ECG?
  • Is there any atrial activity?
  • Are AFlutter (Atrial Flutter) and Sinus Tachycardia in your differential diagnosis?

Figure-1: The initial ECG in today’s case — obtained from a previously healthy young adult who presented with “rapid heart beat” (To improve visualization — I've digitized the original ECG using PMcardio).

MY Thoughts on the ECG in Figure-1:
As always — I favor beginning my interpretation of a 12-lead ECG with assessment of the cardiac rhythm — before addressing the 12-lead ECG. As per ECG Blog #185 — I favor the Ps, Qs, 3R Approach, beginning with whichever of these 5 parameters is easiest to assess for the tracing in front of me:
  • The rhythm in Figure-1 is fast and Regular.
  • The QRS is narrow in all 12 leads (ie, The QRS is not more than half a large box in duration = not more than 0.10 second). Therefore — the rhythm is supraventricular!
  • Normal sinus P waves are absent (ie, there is no regularly-occurring, upright P wave in lead II).
  • The Rate of the rhythm is ~190/minute

PEARL #1: In my experience — the easiest way to accurately estimate heart rate when the rhythm is rapid and regular — is by the Every-Other-Beat Method (See ECG Blog #210). The reason this is clinically relevant in today’s case — is that accurate estimation of the heart rate significantly narrows our differential diagnosis for determining the cardiac rhythm.
  • To apply the method — Find a part of a QRS complex in any of the 12-leads, that begins or ends on a heavy ECG grid line. In Figure-2 — the peak of the R wave in lead III begins just after the 1st vertical RED line.
  • As shown by the RED numbers — the R wave of the 3rd QRS complex in lead III (ie, the R-R interval of every-other-beat) occurs just over 3 large boxes later.
  • Therefore — the amount of time needed to record 2 beats (BLUE numbers in Figure-2) — is just over 3 large boxes. This means that HALF the rate is a little bit less than 300 ÷ 3 or ~95/minute. The actual heart rate is TWICE this amount (ie, ~95 X 2  = 190/minute)

Figure-2: Use of the Every-Other-Beat Method — for rapid estimation of heart rate (See text).

PEARL #2: From the parameters that we’ve just assessed — the rhythm in Figure-2 is a Regular SVT (SupraVentricular Tachycardia) at a rate of ~190/minute, but without clear sign of sinus P waves (ie, without a definite upright P wave in lead II). This description should prompt consideration of the followling differential diagnosis:
  • 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.

PEARL #3: 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 or performing another form of active exercise). 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 clearly over this rate range — then the rhythm will usually not be sinus tachycardia. 
  • NOTE: All bets are off in children — in whom sinus tachycardia at a rate 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 much less likely, because this rate would be faster-than-expected for 2:1 AV conduction, and too slow for 1:1 AV conduction. 
  • KEY Point: 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 a slower ventricular response) for AFlutter.

  • NOTE: Keep in mind 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). As noted in PEARL #2 — I include ATach in the above Regular SVT differential diagnosis LIST for completeness — but in a non-EP-referral practice, ATach will not be seen nearly as often as AFlutter and the reentry SVTs.

  • SUMMARY: 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 from PEARL #2 could be present ( = Sinus Tach — AVNRT-AVRT — ATach — AFlutter).

Regarding Today's CASE:
As derived above — today's case features a completely regular SVT rhythm without sinus P waves — at a rate of ~190/minute
  • Given this rapid a rate — Sinus tachycardia and AFlutter are both unlikely (See PEARL #3). 
  • Statistically — the completely regular SVT rhythm that is seen in Figure-2 will less often be the result of ATach. 
  • This leaves a reentry SVT (ie, AVNRT or AVRT) — as the most likely diagnosis.

  • The "Good News" — is that regardless of whether the rhythm in today's case is AVNRT or AVRT — both rhythms usually respond to either Adenosine or use of some other AV nodal blocking agent (ie, Verapamil, Diltiazem, a ß-blocker).

  • Although sinus P waves are absent in ECG #1 — Could there be other evidence of atrial activity?

  • HINT: What might the RED arrows in Figure-3 be pointing to?

Figure-3: What might the RED arrows be pointing to?

What Might the RED Arrows Be?
Subtle signs of atrial activity during a regular SVT rhythm may provide an important clue to the etiology of the arrhythmia.
  • Retrograde atrial activity during an SVT rhythm 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.
  • Other leads that I have found most helpful for recognizing retrograde atrial activity are lead aVR and lead V1. When seen in these leads — retrograde atrial activity appears as a positive deflection.

  • 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) in which the reentry circuit is entirely contained within the AV node becomes more likely! (González-Torrecilla et al: Ann Noninvasive Electrocardiol 16(1):85-95, 2011).
  • In contrast, when the RP' interval is somewhat longer — then AVRT (AtrioVentricular Reciprocating Tachycardia) in which the reentry circuit involves both the AV node and passage outside of the AV node to involve an AP (Accessory Pathway) becomes more likely.
  • 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 (See ECG Blog #240 for detailed discussion of these RP' concepts).

  • Regarding Today's Case: While difficult to be certain — I suspected that the deepest part of the negative T waves in the inferior leads and the peak of the T waves in leads aVR and V1 were more pointed than expected (RED arrows in Figure-3). IF these peaked deflections did indeed represent retrograde atrial activity — then the RP' interval would be relatively long. This would support the likelihood of orthodromic AVRT (ie, with the impulse initially traveling down the normal AV nodal pathway to enter the ventricles — followed by retrograde passage of the reentry circuit returning to the atria via conduction over an AP).

  • To Emphasize — Whether or not an AP was participating in the reentry circuit of the SVT rhythm in today's case should not affect clinical decision-making. This is because AV nodal blocking agents should work regardless of the mechanism of this reentry SVT rhythm.

Conclusion of Today's CASE:
The patient in today's case was treated with IV Adenosine. The result of this treatment is shown in Figure-4.
  • What happened?

Figure-4: The repeat ECG after administration of IV Adenosine. (To improve visualization — I've digitized the original ECG using PMcardio).

The Result of Treatment:
Treatment with IV Adenosine resulted in prompt conversion to sinus rhythm (Figure-4). Although fairly normal sinus P waves are now seen in lead II — the PR interval is clearly shortened in each of the chest leads. The QRS complex is wide — and a distinct delta wave is seen in each of these chest leads (BLUE arrows).
  • This post-conversion tracing in Figure-4 reveals that today's patient has WPW (Wolff-Parkinson-White) Syndrome. Because delta waves were not seen during the tachycardia — this patient's AP was "concealed" during the SVT rhythm.
  • In retrospect — the RED arrows I highlighted in Figure-3 did in fact represent the result of retrograde atrial activity returning to the atria over the AP (thereby producing the relatively long RP' interval).

  • Final POINT: The patient in today's case may not necessarily need ablation of the AP. Whether or not this patient should be referred to EP Cardiology should depend on: i) What the patient wants to do! — andii) How often his SVT episodes occur — how long episodes last — and how difficult it is (or is not) to control episodes by attention to potential precipitating factors and/or with medication (See ECG Blog #155 for discussion of the management of newly discovered WPW).

Acknowledgment: My appreciation to an anonymous sender (from Malaysia) for the case and this tracing.

ADDENDUM (11/18/2022):
  • I have previously presented the material in the 2 Audio Pearls below — but this content bears repeating for easy reference. Appreciation of these concepts should be automatic for assessment of the patient who presents with a regular SVT rhythm.

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. 

Today's ECG Media PEARL #64 (10:50 minutes Audio) — Reviews my LIST #2: Common Causes of a Regular SVT Rhythm.


Additional Relevant Material to Today's Case:
  • See ECG Blog #185 — for review of the Systematic Ps, Qs, 3R Approach to rhythm interpretation.

  • See ECG Blog #240 — for Review on the ECG assessment of the patient with a regular SVT rhythm (including distinction between the various types of SVT reentry).
  • See ECG Blog #250 — for Review of another case of a regular SVT with ST depression.

  • ECG Blog #210 — reviews the Every-Other-Beat Method for rapid estimation of heart rate.

  • ECG Blog #220 — reviews my LIST #1: Causes of a Regular WCT and — HOW to assess Hemodynamic Stability (Listen to Audio Pearl #37 in this post).

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

  • González-Torrecilla et al: Ann Noninvasive Electrocardiol 16(1):85-95, 2011 — Reviews distinction between AVNRT vs AVRT and other regular SVT rhythms in patients without WPW.


  1. If there is an orthodromic re-entry tachcardia then there is no delta wave even if there is manifest conduction over the accessory pathway. So not only in concealed pathway I think?

    1. Hi. THANK YOU for your question (and sorry for my delay in answering). If the reentry “circuit” consists of the normal AV nodaly pathway WITH connection to an accessory pathway ( = AP) — then the direction of travel will determine IF you do (or do not) see a delta wave! Many patients with a reentry SVT have an “occult” AP. By this I mean that the AP ONLY allows retrograde conduction! That is — the impulse travels DOWN the normal AV nodal pathway (and continues on to the ventricles) — with the impulse also traveling “upward” (ie, back or retrograde to the atria via the AP). In this case — the rhythm is an “orthodromic AVRT”.

      There also are a number of patients with AVRT in which conduction can be EITHER forward down the AP ( = antidromic) — OR — forward down the AV node and back up the AP (which is the “orthodromic” AVRT). Sometimes with an AVRT reentry SVT — the QRS may be narrow during the SVT — but then AFTER conversion, you may see sinus rhythm with delta waves (because conduction may change to going down the AP). And then rarely — AVRT may be antidromic, in which case the QRS during the SVT is wide (and looks like VT) because you are first traveling down the AP.

      The above possibilities are different than simple “AVNRT” — in which the reentry circuit is entirely contained WITHIN the AV Node.

      Lots of possibilities. Hope this helps — :)

  2. Hello dr. Ken! Thank you for this wonderfull blog. It save lifes. I have one question here. How and when can we suspect and what are the criteria for junctional tachicardia, which is also narrow?Thank you for your answer.

    1. Hi. THANK YOU for your question (and sorry for my delay in answering). Recognition of junctional tachycardia can sometimes be difficult to distinguish from a reentry SVT rhythm (AVNRT or orthodromic AVRT) — since for all of these rhythms, the QRS is narrow and sinus P waves are absent. That said — the following may be helpful.

      — Junctional tach is NOT a common rhythm. AVRT and AVNRT are much more common in practice.

      — Accelerated junctional rhythms or junctional tachycardia tend to occur in certain select situations (ie, congenital heart disase; post cardiac surgery; acute inferior MI; “sick patient” who may be in shock or have other multisystem disease; digoxin toxicity). That is not to say that you cannot have other situations — but they are much less common.

      — The rate of junctional tach tends to be slower (100-130/minute range) than the faster rates that are more common with reentry SVT rhythms.

      — Seeing what happened before and/or after the tachycardia may reveal the etiology!

      That said — sometimes it is difficult to distinguish junctional tach from reentry SVT rhythms.

    2. Dr. Ken, thank you very much for your answer and this blog