Sunday, July 31, 2022

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).

  • 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. 

  • 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.

  • 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).

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.


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





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).

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. 



Additional Relevant ECG Blog Posts to Today’s Case:

  • ECG Blog #185 — Reviews my System for Rhythm Interpretation, using the Ps, Qs & 3R Approach.

  • 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.

  • 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

  • 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?

  • 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).

  • 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.


  1. I would also say those look like retrograde short RP P waves at terminal aspect of QRS in aVR

    1. You could be right — but I did not think those deflections were well defined enough to be certain about whether they were (or were not) retrograde P waves. But I will emphasize that lead aVR is OFTEN an excellent lead for seeing retrograde atrial activity (and I ALWAYS look there!).

  2. Lovely reading for my morning!, The way you methodically interpreted this enigmatic ECG leaves me amazed! Thank you for your labor of love, God bless you abundantly. I can see you have a "fan club" with doctors from four corners of the globe. Thank you for yet another enlightening session.