ECG Blog #446 — What Kind of SVT?

You are shown the ECG in Figure-1 — told only that the patient had a “continuous" tachycardia.

QUESTIONS:

• How would YOU interpret the ECG in Figure-1?
• What is the differential diagnosis? — Treatment?

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

MY Thoughts on the ECG in Figure-1:

The rhythm in Figure-1 — is a regular SVT (SupraVentricular Tachycardia) at a rate just under 150/minute, without clear sign of sinus P waves (ie, there is no upright P wave in lead II).

• The QRS is narrow in all 12 leads — which is why we know this is a supraventricular rhythm.
• Although sinus P waves are absent — there is atrial activity in the form of a negative deflection at a fixed interval distance from the QRS in each of the inferior leads (BLUE arrows in Figure-2).

Figure-2: I've labeled atrial activity in ECG #1 with BLUE arrows.

PEARL #1: The 3 main considerations regarding the nature of atrial activity highlighted by the BLUE arrows in Figure-2 are: i) Do these deep negative deflections that appear before the QRS in each of the 3 inferior leads represent retrograde P waves, being conducted backward from the preceding QRS complex; — ii) Or, do these negative deflections in the inferior leads represent forward-conducting P waves from an ectopic atrial site; — or, iii) Do these deflections represent junctional tachycardia?

• Depending on which of above considerations is operative — the differential diagnosis of the rhythm in ECG #1 will be between: i) A reentry SVT rhythm with retrograde atrial conduction; ii) ATach (Atrial Tachycardia); — or — iii) Junctional Tachycardia.
• It would seem in today's case, that an automatic junctional tachycardia is the least likely of the above 3 considerations — because the PR interval of these negative deflections before the QRS in each of the inferior leads is longer than the very short PR interval that I would expect if the rhythm was junctional tachycardia.
• But — the deflections highlighted by the BLUE arrows in Figure-2 certainly could represent an ectopic ATach.

PEARL #2: Retrograde atrial conduction is recognized by P wave negativity in the inferior leads (as a result of conduction moving away from the ventricles — and back toward the atria). Such retrograde conduction typically produces a positive P wave deflection in leads aVR, aVL, V1, and sometimes in lead V2 (See BLUE arrows in each of these leads in Figure-2).

• The above said — We still can not rule out the possibility that the mechanism of the rhythm in Figure-2 is an ectopic ATach.
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• The issue of reentry SVT rhythms was discussed in detail in ECG Blog #240. In that review, I emphasized that the most common form of reentry SVT is AVNRT of the "slow-fast" type, in which forward conduction occurs over the slower AV nodal pathway — with completion of the reentry circuit by conduction back to the atria over the faster AV nodal pathway. It is because this retrograde conduction (back to the atria) occurs over the faster AV nodal pathway — that retrograde P waves occur very close to the preceding QRS complex (either hidden, because they are contained within the preceding QRS complex — or with a very short RP' interval, with the retrograde P wave notching the end of the preceding QRS).
• Alternatively — when a portion of the reentry circuit is made up of an AP (Accessory Pathway) — the retrograde P wave will typically manifest a moderately long RP' interval (that occurs well after the QRS — usually within the latter part of the ST segment). Incorporation of a portion of the AP into the reentry circuit of an SVT rhythm constitutes AVRT. The RP' interval is moderately long — because it takes more time to complete this reentry circuit that is longer than the reentry circuit in AVNRT that is contained entirely within the AV node.

What is happening in today's case is different.

• IF the BLUE arrows in Figure-2 represent retrograde atrial conduction — then the RP' interval very long (taking up well over half of the R-R interval). As a result, if the BLUE arrows represent retrograde P waves — then this rhythm would be the atypical (and much less common) form of AVNRT, which is known as the "fast-slow" type (with forward conduction over the faster AV nodal pathway — and completion of the reentry circuit by conduction back to the atria over the slower AV nodal pathway).
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• BOTTOM Line: The principal differential diagnosis of the regular SVT rhythm in today's case is between: i) Atypical AVNRT of the "fast-slow" type; — vs — ii) Ectopic ATach.

PEARL #3: Distinction between atypical "fast-slow" AVNRT vs ectopic ATach can be difficult on the basis of a single ECG. Although in today's case — I thought the shape of the ST segment, and the very deep negative P waves in each of the inferior leads was more suggestive of atypical AVNRT — distinction between these 2 entities can sometimes only be made on EP testing.

• 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. This serves to immediately set up the reentry circuit, as the impulse starts down the other pathway. As a result — 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).
• Unfortunately in today's case — the onset of the regular SVT in Figure-2 was not captured, so we are not privilege to this important clue.
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• NOTE: 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).

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What Happens with "Fast-Slow" AVNRT?

To illustrate the mechanism of atypical "fast-slow" AVNRT — I show below in Figure-3 the laddergram discussed in ECG Blog #240.

• Starting with the normal sinus-conducted P wave ( = beat #3) — this is followed by a PAC ( = beat #4, which is recognized as a PAC because its P wave morphology differs from the sinus P wave that precedes beat #3).
• Mechanistically — if this PAC ( = beat #4) arrives at the AV node at just the right moment — it may encounter the slow AV nodal pathway at a point when this slow pathway is able to conduct retrograde (dotted line in the AV nodal tier of the laddergram that conducts back to the atria).
• And — if retrograde conduction over the slow pathway occurs at just the right moment — it may encounter the fast AV nodal pathway at a point when it is now able to conduct down to the ventricles. 
• In Figure-3 — these events initiate a reentry SVT of the "fast-slow" type for beats #4-thru-13. This reentry SVT stops after beat #13 — because this last beat in the run does not conduct retrograde! (ie, There is no negative P wave after beat #13).
• Sinus rhythm then resumes with beat #14 — but only lasts for 1 beat, because retrograde conduction of beat #14 is potentially able to initiate another run of SVT beginning with beat #15 (which we do not see — because the rhythm strip ends here).
• Now, if we go back to the start of this rhythm in Figure-3 — it looks as if another "fast-slow" run of AVNRT had just ended before beat #3.
• P.S. — Note that retrograde conduction from beat #4 is slightly faster (ie, with a slightly shorter RP' interval) than is retrograde conduction for all other beats on this tracing. Whether this reflects switch from one AV nodal pathway to another — or simply slowing of retrograde conduction after an initial beat is uncertain and, does not alter the finding that slow retrograde conduction (ie, with a long RP' interval) is the "substrate" for initiation and persistance of the fast-slow AVNRT in this patient.
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• PEARL #4: This less common form of "fast-slow" AVNRT that is illustrated in Figure-3 — has also been known as an "incessant" tachycardia. This name reflects that fact this atypical form of AVNRT is often difficult to treat because of its disturbing tendency of frequent recurrence and persistence once the reentry cycle is started. Presumably, the slower retrograde conduction back to the atria (that produces the very long RP' interval) — allows more time (and more opportunity) for forward conduction down the faster pathway to occur — and once this occurs, it tends to sustain the reentry cycle. (In contrast — "incessant" recurrence of reentry SVT is less likely with the typical "slow-fast" form of AVNRT, in which the shorter RP' interval allows less opportunity for the reentry cycle to be sustained).

Figure-3: Laddergram illustration of the mechanism for atypical "fast-slow" AVNRT (from ECG Blog #240). 

Editorial Comment: The intricacies of reentry SVT rhythms is a complex subject that I have simplified to facilitate practical understanding.

• For a more intricate analysis: See Katritsis and Josephson — Arrhythm Electrophys Rev 5(2):130-135, 2016.
• Additional easier-to-read links — Cadogan — Life-In-The-Fast-Lane, 2022 — and — Hafeez and Armstrong — StatPearls, 2023.

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The CASE Continues:

• The rhythm in today's case was treated with Adenosine. It finally responded to high-dose IV Verapamil.
• EP study confirmed that the mechanism of today's rhythm was AVNRT of the "fast-slow" type. Unfortunately — additional follow-up is not available. 
• Considering clinical options if this patient was ours to manage — the patient could be tried on a longterm AV nodal blocking agent (ie, oral Verapamil or Diltiazem; and/or a ß-Blocker) — but if unsuccessful (as is often the case with AVNRT of the fast-slow type) — referral to EP cardiology for consideration of ablation may become indicated.

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I did receive the 2 short rhythm strips in Figure-4 — that were recorded after IV Adenosine was given to today's patient.

QUESTION:

• What do YOU see?
• Are these rhythm strips consistent with the mechanism for fast-slow AVNRT that I illustrate in Figure-3?

Figure-4: Rhythm strips recorded after administration of IV Adenosine to today's patient.

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ANSWER = Figure-5:

I am not sure how long it was after IV Adenosine was given until the rhythm strips in Figure-5 were recorded. IDEALLY: A continuous rhythm strip should always be recorded for the 60-90 seconds after administration of IV Adenosine (as the BEST way to capture the critical diagnostic period prompted by this medication).

• The first 9 beats in Panel A of Figure-5 show a reentry SVT rhythm at ~135/minute, with a fairly long RP' interval (YELLOW arrows following these first 9 beats). Note slight slowing of the SVT with beats #8 and 9.
• Beat #10 is a PVC — after which a brief pause occurs.
• Beat #11 is a supraventricular escape beat — since it is not preceded by a sinus P wave. There is however a retrograde P wave (YELLOW arrow) following beat #11 — but note that the RP' interval following this beat is shorter than it was during the 9-beat run of AVNRT (That the RP' interval is decreasing is best appreciated by comparing the obviously longer RP' intervals following beats #1 and 2 — with this shorter RP' interval following beat #11).
• Beat #12 is another supraventricular escape beat. Note that no retrograde P wave follows this beat.
• Sinus conduction resumes with beats #13 and 14 (RED arrow) — with both of these sinus-conducted beats followed by retrograde P waves with a shorter RP' interval than was seen at the beginning of the AVNRT run for beats #1 and 2.
• 
  
• In Panel B — We now see normal sinus rhythm at a more appropriate rate, but now without retrograde conduction!
• Beat #7 is a PAC (occurring earlier-than-expected — and preceded by a different morphology P wave, as shown by the BLUE arrow). As was shown in Figure-3 — the onset of reentry SVT rhythms is often precipitated by one or more PACs that predispose to potential for the retrograde conduction that initiates and perpetuates the reentry cycle.

Putting It All Together:

These 2 short rhythm strips tell the story of how IV Adenosine successfully converted today's fast-slow AVNRT to sinus rhythm:

• The first 9 beats show the beginning of a slowing of this patient's AVNRT (that had been sustained as shown in Figure-1 at a rate closer to 150/minute). In additional to slowing of the reentry SVT rate — you'll often see introduction of slight irregularity into this previously regular rhythm (as we see in Panel A).
• You'll often see some ventricular beats (as per beat #10) as Adenosine is working. A series of escape beats often follows — as sinus rhythm resumes.
• As previously emphasized — it is the long RP' interval that characterizes (and sustains) this atypical fast-slow form of AVNRT. These 2 rhythm strips show the effect of Adenosine in reducing the RP' interval, with ultimate cessation of retrograde conduction.
• Normal sinus rhythm has returned in Panel B. The PAC (beat #7) no longer initiates a run of reentry SVT — because the AV node is no longer allowing retrograde conduction.

Figure-5: I've labeled Figure-4.

CASE Conclusion: 

As shown in Figure-5 — Adenosine converted the rhythm in today's case. But because of the ultra-short half life of Adenosine — the "incessant" fast-slow AVNRT rhythm kept returning until repeated IV Verapamil dosing succeeded in sustaining sinus rhythm.

• Figure-6 — shows that with the recording of ECG #3 — sinus rhythm has returned. Colored arrows highlight the change in P wave morphology between the 2 tracings.

Figure-6: Conversion to sinus rhythm following treatment.

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Acknowledgment: My appreciation to Kenneth Khoo (from Malasia) 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 #188 — Reviews the essentials for reading (and/or drawing) Laddergrams, 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.

• González-Torrecilla et al review distinction between AVNRT vs AVRT — in Ann Noninvasive Electrocardiol 16(1):85-95, 2011 .

Other sites where I've discussed similar cases:

• 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. 
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• 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).