ECG Blog #323 — WCT with RBBB Morphology

The ECG in Figure-1 — was obtained from a man in his 50s, who presented to the ED (Emergency Department) with "palpitations".

• How would YOU interpret the ECG in Figure-1?
• Doesn't this look like RBBB (Right Bundle Branch Block)?

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:

There is an obviously fast tachycardia in Figure-1. Usually I begin by assessing the rhythm in the long lead II rhythm strip that appears at the bottom of the tracing. Unfortunately — small amplitude of the artifact-laden, nearly isoelectric QRS complex in lead II renders such assessment problematic. That said — virtually every other lead provides the overview that follows. Using the Ps, Qs, 3R Approach (See ECG Blog #185):

• The rhythm in ECG #1 is fast and Regular. I estimate the Rate to be ~210/minute.
• I see no sign of atrial activity (ie, No P waves). 
• The QRS complex during the tachycardia is wide (I measure ~0.12 second = 3 little boxes in duration).

IMPRESSION: The above parameters lead to description of the rhythm in Figure-1 as being a regular WCT ( = Wide-Complex Tachycardia) at ~210/minute, without clear sign of atrial activity.

• As emphasized in many of my prior ECG Blogs (especially in ECG Blog #220) — the finding of a regular WCT rhythm without clear sign of atrial activity should always be assumed to be VT until proven otherwise (statistical odds ~90% when such individuals are older adults with underlying heart disease).

PEARL #1: 90% is not 100%! Although we need to assume VT for any regular WCT rhythm without P waves until proven otherwise — sometimes the rhythm will be supraventricular!

• Assessment of QRS morphology helps greatly to narrow down the likelihood that a given WCT rhythm is either VT or an SVT (SupraVentricular Tachycardia) with preexisting BBB (Bundle Branch Block) or aberrant conduction (See ECG Blog #196 for details).
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• The chances of a WCT rhythm being supraventricular are greatly increased IF — QRS morphology is consistent with one of the known forms of conduction block (ie, RBBB; LBBB; LAHB or LPHB; or RBBB with one of the hemiblocks).

Take another LOOK at the ECG in Figure-1:

• Doesn't QRS morphology look like RBBB conduction?
• Is the rhythm in this tracing VT? — or SVT with RBBB conduction?

Figure-2: I've labeled the initial ECG in today's case (See text).

ECG #1: Is this VT? — or SVT with RBBB Conduction?

As emphasized in Pearl #1 — the chance that a regular WCT rhythm will turn out to be supraventricular is increased IF — QRS morphology is consistent with some known form of conduction defect.

• As emphasized in ECG Blog #204 — the 3 KEY leads for rapid determination of BBB are right-sided lead V1 — and the 2 left-sided leads I and V6. I've enclosed a QRS complex from each of these 3 leads within a RED rectangle in Figure-2. QRS morphology is perfectly consistent with RBBB conduction because: i) There is an rsR' complex in lead V1 (with an s wave that descends below the baseline — and a taller right "rabbit ear" R' wave); and, ii) There are upright R waves with wide terminal S waves in lateral leads I and V6.

PEARL #2: The other (more subtle) factor I favor for assisting in determination of the likelihood that a regular WCT rhythm is VT — is whether any of the other 9 leads show findings that look atypical for a particular conduction defect.

• None of the "rules" for assessing QRS morphology when assessing a regular WCT rhythm are perfect. Exceptions always exist. For example — QRS morphology may be dramatically altered in a "baseline" ECG in patients who have significant underlying heart disease. In such cases — QRS morphology will not look "typical" when heart rate increases.
• That said — I've enclosed within a BLUE rectangle in Figure-2, a representative QRS complex in 5 leads that looks unusual for "typical" RBBB conduction. Specifically — i) The QRS in lead II looks bizarre. It's tiny amplitude, biphasic shape looks "out-of-place" between the RS complex in lead I and the rSR' complex in lead III; and, ii) QRS morphology in chest leads V2,V3,V4 and V5 also looks highly unusual for RBBB conduction — because of multiphasic (overly fragmented) complexes that look "out-of-place" following the highly characteristic rsR' complex in lead V1.

BOTTOM Line: While impossible to rule out an SVT with RBBB conduction for the regular WCT rhythm in Figure-2 — the unusual appearance of the above 5 leads suggests that this rhythm is VT.

• Since QRS morphology in the 3 KEY leads (I,V1,V6) resembles RBBB conduction — a Fascicular VT should be presumed until proven otherwise. 
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• PEARL #3: As discussed in ECG Blog #197 — 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. Recognition of Fascicular VT is therefore very relevant clinically — because the course, prognosis and treatment of this arrhythmia is different from that of ischemic or scar-related VT, that makes up the other 90% of VT rhythms.
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• NOTE: By way of a reminder — I've reproduced below in the ADDENDUM the Summary Sheet and Audio Pearl on Idiopathic VT from Blog #197.

CASE Follow-Up: 

The initial ECG in today's case was recognized as Fascicular VT — and treated accordingly. Since the patient was hemodynamically stable — 5 mg of IV Verapamil was given. The result of this treatment is shown in ECG #2 (See Figure-3).

Figure-3: Comparison of the initial ECG — with the post-conversion tracing, obtained after giving 5 mg IV Verapamil. (To improve visualization — I've digitized the original ECG using PMcardio).

Case CONCLUSION:

Prompt conversion to sinus rhythm is the typical response of Fascicular VT to IV Verapamil in a patient with no underlying heart disease.

• EP study confirmed that the patient had a left posterior fascicular VT. This was successfully ablated.

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Acknowledgment: My appreciation to Mubarak Al-Hatemi (from Qatar) for the case and this tracing.

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ADDENDUM (7/31/2022):

I summarize KEY features regarding Idiopathic VT in Figure-4.

Figure-4: Review of KEY features regarding Idiopathic VT (See text).

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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. 

 

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Additional Relevant ECG Blog Posts to Today’s Case:

• ECG Blog #185 — Reviews my System for Rhythm Interpretation, using the Ps, Qs & 3R Approach.
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• ECG Blog #210 — Reviews the Every-Other-Beat (or Every-Third-Beat) Method for estimation of fast heart rates — and discusses another case of a regular WCT rhythm.
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• ECG Blog #220 — Review of the approach to the Regular WCT (Wide-Complex Tachycardia).
• ECG Blog #196 — Reviews another Case with a Regular WCT Rhythm. 
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• ECG Blog #204 — Reviews the ECG diagnosis of the Bundle Branch Blocks (RBBB/LBBB/IVCD). 
• ECG Blog #203 — Reviews ECG diagnosis of Axis and the Hemiblocks. For review of QRS morphology with the Bifascicular Blocks (RBBB/LAHB; RBBB/LPHB) — See the Video Pearl in this blog post.
• ECG Blog #211 — WHY does Aberrant Conduction occur?
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• ECG Blog #197 — Review of Fascicular VT (including Audio Pearl and Summary sheet on Idiopathic VT).
• ECG Blog #301 — Reviews a WCT that is SupraVentricular! (with LOTS on Aberrant Conduction).
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• 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.