ECG Blog #197 — Aberrant SVT vs Fascicular VT?

You are asked to interpret the ECG in Figure-1. Unfortunately — NO clinical information on this patient is available. That said — this ECG still makes for a superb discussion.

 

QUESTION: Realizing that there is no clinical information on this case — and that I do not know what happened in this case ... Which of the following choices provides the BEST answer regarding the rhythm shown in Figure-1?

 

Answers to Choose From:

• Choice A:  The rhythm is probably SVT (SupraVentricular Tachycardia) with either aberrant conduction or preexisting RBBB (Right Bundle Branch Block).
• Choice B: The rhythm is probably VT (Ventricular Tachycardia). Immediate synchronized cardioversion is indicated.
• Choice C:  Knowing the clinical history would be of little help for determining the etiology of this rhythm.
• Choice D:  Assuming the patient was hemodynamically stable — either IV Adenosine or IV Verapamil would be initial treatments of choice.

 

 

Figure-1: What is the rhythm? Unfortunately — NO clinical information is available (See text).

 

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NOTE #1: Some readers may prefer at this point to listen to the 8-minute ECG Audio before reading My Thoughts regarding the rhythm in Figure-1. Feel free at any time to review to My Thoughts (that appear below ECG MP-14).

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Today’s ECG Media PEARL #14 (8 minutes Audio) — What is Idiopathic VT? — WHY do we care? Special attention to the 2 most common forms = RVOT (Right Ventricular Outflow Track) VT and Fascicular VT. 

• NOTE: Review of the ECG features discussed in this Audio Pearl are summarized in the ADDENDUM below in Figure-2:

 

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MY Approach to the Rhythm in Figure-1:

By the Ps, Qs & 3R Approach (which I reviewed in ECG Blog #185) — the rhythm in Figure-1 is fast and Regular — the ventricular Rate is between 190-200/minute — the QRS complex is wide — and, normal sinus P waves appear to be absent.

• I can not rule out the possibility of atrial activity in this tracing. That said — I do not think the upright deflection in front of the QRS complex in lead II is a sinus P wave because: i) A rate of 190-200/minute would be exceedingly fast for sinus tachycardia in a non-exercising adult. (In general — sinus tachycardia at rates over 170/minute are not commonly seen in a non-exercising adult); and, ii) I do not see anything even remotely suggestive of sinus P waves in any other lead (Next to lead II — lead V1 is generally best for revealing sinus P wave activity, and there is no hint at all of sinus P waves in V1).
• I also can not rule out the possibility of 1:1 VA (ie, retrograde) conduction — as a thin and very deep negative deflection appears to distort the initial part of the ST-T wave, coming right after the QRS in each of the inferior leads. 
• PEARL #1: Even if 1:1 retrograde P wave activity was present — this finding does not distinguish between VT vs a reentry SVT, because both of these rhythms may conduct retrograde with a 1:1 VA ratio.

 

IMPRESSION of the Rhythm in Figure-1:

By the Ps, Qs, 3R Approach — We have described a regular WCT ( = Wide-Complex Tachycardia) Rhythm at 190-200/minute, without sign of atrial activity (at least, without any help in our differential diagnosis from the possibility of 1:1 retrograde VA activity).

• As emphasized in ECG Blog #196 — the differential diagnosis for a regular WCT rhythm without help from atrial activity includes: i) VT (Ventricular Tachycardia); ii) SVT (SupraVentricular Tachycardia) with preexisting BBB (Bundle Branch Block); iii) SVT with aberrant conduction; or — iv) Something Else! (ie, WPW). 

 

LOOKING Further — Clinical NOTES:

Unfortunately — we do not have any clinical information on this case (ie, not even the age of the patient). It’s important to emphasize that IF this patient was hemodynamically unstable in association with the rhythm in Figure-1 — then immediate cardioversion would be indicated regardless of which of the 4 diagnostic entities listed above turned out to be the etiology. For the purpose of discussion — Let’s assume the patient is and remains hemodynamically stable with the rhythm shown in Figure-1:

• QRS morphology in Figure-1 is potentially consistent with RBBB conduction because there is an rR’ (all upright) complex in right-sided lead V1 — in association with wide terminal S waves in lateral leads I and V6. Therefore — the reason for QRS widening in Figure-1 could be either preexisting RBBB — or RBBB aberration because of the rapid rate.
• PEARL #2: Although QRS morphology in Figure-1 could be consistent with RBBB conduction — there are some atypical features. These include: i) The fact that the rR’ complex in lead V1 lacks an S wave (ie, pure RBBB conduction ideally shows a triphasic = rsR’ complex in lead V1 — in which the S wave descends below the baseline); and, ii) Although the predominant negativity with very steep descent of the S wave in lead I is consistent with LPHB (Left Posterior HemiBlock) conduction — the qR pattern (ie, small-q, tall-R) expected with LPHB is absent; iii) QRS morphology in lead III is bizarre; and, iv) QRS morphology in leads V2-thru-V6 looks strangely similar (this usually doesn’t happen with RBBB conduction). BOTTOM Line: While QRS morphology in Figure-1 clearly could still represent RBBB conduction (from either preexisting RBBB or aberrant conduction) — the fact that QRS morphology manifests several atypical features negates any positive diagnostic value that might favor a supraventricular etiology. If anything — QRS morphology would seem more suggestive of a ventricular etiology.
• PEARL #3: Finding a prior tracing on this patient could be invaluable. A history of “scar” (ie, from prior infarction or cardiomyopathy) could account for RBBB conduction with atypical features. That said, in the absence of a prior tracing for comparison — Assume VT until proven otherwise!
• PEARL #4: The patient could have Fascicular VT — and THAT could account for the RBBB-like QRS appearance with the all-upright rR’ complex in lead V1 with wide terminal S waves in lateral leads. Although the most common form of Fascicular VT manifests an RBBB-like QRS appearance plus left axis deviation (which is not present in Figure-1) — variations on this “theme” exist. Therefore — it was the fact that the ECG in Figure-1 resembles RBBB but has several atypical features — that led me to suspect Fascicular VT as the diagnosis.
• PEARL #5: As per the “theme” of today’s Blog post — Fascicular VT is one of the 2 most common forms of Idiopathic VT, which is the term used to describe the approximately 10% of all VT rhythms in which the patient has VT in the absence of underlying structural heart disease. As a result — learning the age of this patient and the clinical History would be of invaluable assistance for assessing the likelihood of your rhythm diagnosis. For example — IF the patient in today’s case was a previously healthy young adult who suddenly developed this arrhythmia during strenuous exercise — the likelihood of Fascicular VT would significantly increase (For more on Idiopathic VT — See our ECG Media Pearl #14 [above] and our Summary on this topic [below] in Figure-2).

Final PEARL: Assuming the patient is hemodynamically stable — knowing the most likely diagnosis for the rhythm in Figure-1 is essential for selecting optimal initial treatment. On the basis of the above discussion — I favor Choice D as the “best” answer regarding this rhythm ( = Either IV Adenosine or IV Verapamil would be initial treatments of choice).

• The first 3 choices (A, B & C) are all suboptimal because — the atypical features for RBBB (from Pearl #2) significantly reduce the likelihood that the rhythm is an SVT — immediate cardioversion is not necessarily needed for VT if the patient is stable — and, Pearl #5 illustrates how knowing some clinical history in this patient could be of invaluable assistance for determining the likely rhythm diagnosis.
• BOTTOM Line: QRS morphology in leads I, V1 and V6 looks like a Fascicular VT (albeit without the left axis deviation that is usually seen with this rhythm). IF the clinical history was similar to that proposed in Pearl #5 (ie, a previously healthy young adult who suddenly developed this arrhythmia during strenuous exercise) — I’d probably initiate treatment with IV Verapamil, with the thought that this medication has the highest chance for success when the rhythm is Fascicular VT (plus the fact that Verapamil is also usually effective for reentry SVTs). But without this type of clinical history, and without any known history for coronary disease — I’d probably initiate treatment with IV Adenosine, with the thought that this drug is usually safe (because of its ultra-short IV half-life) — and because Adenosine will regularly convert reentry SVTs (plus it might also work if the RBBB-like pattern seen Figure-1 represented a form of idiopathic VT). 
• CAUTION — Prudence dictates remaining at the bedside during treatment, ready to cardiovert at any moment if the patient's condition deteriorates.
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• P.S. — I don’t have follow-up, and I fully acknowledge other answers are possible. YOUR comments are welcome!

 

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Acknowledgment: My appreciation to Nurul Syafika Syazwani (from Temerloh, Malaysia) for the case and this tracing.

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ADDENDUM (2/22/2021):

I summarize KEY features regarding Idiopathic VT in Figure-2.

Figure-2: Review of KEY features regarding Idiopathic VT (See text).

 

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Additional Relevant ECG Blog Posts to Today’s Case:

• ECG Blog #38 and Blog #85 — Review of Fascicular VT.
• ECG Blog #278 — Another case of a regular WCT rhythm in a younger adult.
• ECG Blog #35 — Review of RVOT VT.
• ECG Blog #42 — Comprehensive review of criteria for distinguishing VT vs Aberration.