ECG Blog #473 — Sinus Tach & What Kind of BBB?

You are shown the ECG in Figure-1 — without the benefit of any history.

QUESTION:

This tracing was interpreted as sinus tachycardia with some kind of BBB (Bundle Branch Block).

• What kind of BBB is this?  

Figure-1: The initial ECG in today's case. What kind of BBB is this? (To improve visualization — I've digitized the original ECG using PMcardio).

ANSWER:

• There is no Bundle Branch Block — because the rhythm is not sinus tachycardia.

PEARL #1: Among my favorite truisms is the following: "12 Leads are Better than One". By this statement I mean that because a part of the QRS complex may sometimes lie on the baseline — it is possible that the QRS may appear to be more narrow than it actually is — IF — the only lead being monitored, is that lead in which part of the QRS lies on the baseline.

• We see a related issue in today's case, in which that part of the QRS in lead II that lies on the baseline — makes it seem as if the small positive deflection in this lead that is highlighted by the RED arrow is a sinus P wave. 
• Thus, one might easily be fooled from Figure-1 — into thinking that today’s rhythm is sinus tachycardia. Instead, the vertical RED line that I have added in Figure-2 shows that what "looks like" a P wave — is actually the initial part of the QRS complex!

Figure-2: Using the beginning of the QRS complex in simultaneously-recorded leads I and III  as a reference point — I've drawn in a vertical RED line that demonstrates how the QRS complex in lead II begins immediately to the right of this RED line. 

MY Assessment of the ECG in Figure-2:

Now that we know where the QRS complex begins in lead II — We are able to see that the rhythm in today's tracing is a regular WCT (Wide-Complex Tachycardia) at a rate of ~160/minute, without any indication of atrial activity. 

• The QRS complex is very wide (ie, over 0.17 second).
• QRS morphology does not resemble any known form of bundle branch block (ie, Although the all upright QRS in lead I could be consistent with LBBB — the predominantly negative QRS that is seen in all 6 chest leads is not a feature of LBBB conduction).
• There is marked fragmentation (ie, notching) of multiple QRS complexes. This diffuse fragmentation is an indication of "scar" — and suggests significant underlying heart disease (that is less likely to be seen with supraventricular rhythms).
• The QRS is almost all negative in lead V6, with a highly unusual small "double-hump" R wave (See Figure-3 below — which shows that the QRS complex in lead V6 begins just to the right of the vertical BLUE line). This bizarre QRS morphology (with this small "double-hump" R wave) — is almost never seen with supraventricular conduction. Neighboring lead V5 also looks bizarre for a supraventricular rhythm.
• Finally — the downslope of the S wave in each of the chest leads is delayed. Instead of a straighter (more rapid) S wave descent (that is typical for LBBB conduction) — this delay in initial conduction through the ventricles is a common finding with ventricular rhythms.   

Impression: 

The regular WCT rhythm in Figure-3 is almost certain to be VT (Ventricular Tachycardia).

• PEARL #2: About the only scenarios that might produce as bizarre of a QRS morphology as we see in Figure-3 without this being VT would be: i) If the patient had an identical QRS morphology in their "baseline" tracing during sinus rhythm; — or, ii) If there was some toxicity (such as hyperkalemia) that might produce bizarre QRS morphology during a supraventricular rhythm. 

Figure-3: I've added a vertical BLUE line that passes through the beginning of the QRS complex in simultaneously-recorded leads V4 and V5. The QRS complex in lead V6 begins immediately to the right of this BLUE line!

Comment:

I do not have follow-up of this case — as it is a tracing from my files of many years ago during my days as a hospital Attending. That said — I find it an insightful example of how we can be fooled into thinking this rhythm is sinus tachycardia, if we lose sight of basic principles:

• Statistically — at least 80% of regular WCT rhythms without clear sign of atrial activity will turn out to be VT. So even before we look at the ECG — we should presume that a regular WCT rhythm is likely to be VT.
• This figure increases substantially when: i) The QRS is extremely wide (as it is in today's tracing); — ii) There is marked fragmentation (suggesting significant underlying heart disease) — iii) The QRS does not resemble any known form of conduction defect; — and, iv) The QRS is "ugly" (suggesting an origin outside of the ventricular conduction system).
• Therefore — the "onus of proof" is to demonstrate that the WCT is not VT, rather than the other way around. Before concluding that the rhythm is supraventricular — we need to either prove that QRS widening is the result of aberrant conduction or demonstrate in a prior or post-conversion tracing that the same abnormally widened QRS morphology is also present during sinus rhythm.
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• Regarding today's case (assuming there is no underlying toxicity such as hyperkalemia) — it should take no more than seconds for us to presume VT and treat the patient accordingly until proven otherwise.

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Additional Relevant ECG Blog Posts to Today’s Case:

• ECG Blog #185 — Reviews my System for Rhythm Interpretation — with use of 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. 
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• ECG Blog #220 — Review of the approach to the regular WCT ( = Wide-Complex Tachycardia).
• ECG Blog #196 — Another Case with a regular WCT.
• ECG Blog #263 and Blog #283 — Blog #361 — Blog #384 — and Blog #460 — and Blog #468 — More WCT Rhythms ...
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• ECG Blog #197 — Reviews the concept of Idiopathic VT, of which Fascicular VT is one of the 2 most common types. 
• ECG Blog #346 — Reviews a case of LVOT VT (a less common idiopathic form of VT).
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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.
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• ECG Blog #211 — WHY does Aberrant Conduction occur?
• ECG Blog #301 — Reviews a WCT that is SupraVentricular! (with LOTS on Aberrant Conduction).
• ECG Blog #445 and Blog #361 — Another regular WCT rhythm ...
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• ECG Blog #323 — Review of Fascicular VT.
• 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 — Criteria to distinguish VT vs Aberration.
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• ECG Blog #133 and ECG Blog #151— for examples in which AV dissociation confirmed the diagnosis of VT.
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• Working through a case of a regular WCT Rhythm in this 80-something woman — See My Comment in the May 5, 2020 post on Dr. Smith’s ECG Blog. 
• Another case of a regular WCT Rhythm in a 60-something woman — See My Comment at the bottom of the page in the April 15, 2020 post on Dr. Smith’s ECG Blog. 
• A series of 3 challenging tracings with QRS widening (See My Comment at the bottom of the page in the March 6, 2025 post on Dr. Smith's ECG Blog).
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• Review of the Idiopathic VTs (ie, Fascicular VT; RVOT and LVOT VT) — See My Comment at the bottom of the page in the September 7, 2020 post on Dr. Smith’s ECG Blog.
• Review of a different kind of VT (Pleomorphic VT) — See My Comment in the June 1, 2020 post on Dr. Smith’s ECG Blog.

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ADDENDUM (3/14/2025):

• I've reproduced below from ECG Blog #361 — a number of helpful figures and my Audio Pearl on assessment of the regular WCT rhythm.

Figure-4 : My LIST #1 = Causes of a Regular WCT (Wide-Complex Tachycardia) of uncertain Etiology (ie, when there is no clear sign of sinus P waves).

Figure-5 Use of the "3-Simple Rules" for distinction between SVT vs VT.

Figure-6: Use of Lead V1 for assessing QRS morphology during a WCT rhythm.

—  —

ECG Media PEARL #13a (12:20 minutes Audio) — reviews “My Take” on assessing the regular WCT (Wide-Complex Tachycardia), when sinus P waves are absent — with tips for distinguishing between VT vs SVT with either preexisting BBB or aberrant conduction.

ECG Media PEARL #28 (4:45 minutes Video) — Reviews WHY some early beats and some SVT rhythms are conducted with Aberration (and why the most common form of aberrant conduction manifests RBBB morphology).

• CLICK HERE — to download a PDF of this 6-page file on Aberrant Conduction.