Differential diagnosis of wide complex tachycardias – Part I

There seems to be a lot of confusion with regard to wide complex tachycardias.

For some reason, health care providers of all ranks and stripes forget the most basic rule.

If it's a wide complex rhythm (fast or slow) it's ventricular until proven otherwise!

Reasonable people can disagree as to what constitutes "proof" but you need to start somewhere, and the place to start is not "it's SVT with aberrancy (or a bundle branch block) until proven otherwise."

That's how patients get killed.

A little bit of knowledge can be a dangerous thing, and I'm not a huge fan of Wellens' Criteria or Brugada's Criteria. In the wrongs hands, these algorithms do more harm than good.

When they "rule in" VT, they're fine. When they (wrongly) suggest SVT, they're dangerous.

There are several reasons these algorithms might wrongly suggest SVT. No algorithm is perfect. In addition, mistakes are made when using the algorithms, and the algorithms are sometimes used in ways they were not designed to be used.

For example, with modified leads MCL-1 and MCL-6.

Wellens' Critera and Brugada's Criteria are not the same, and the former is part of the latter, so I don't mean to oversimplify here. But in my experience, a far more valuable tool is to recognize the typical patterns. In other words, typical RBBB, typical LBBB, typical bifascicular block (RBBB/LAFB), and typical bifascicular block (RBBB/LPFB).

Having said that, just because the ECG shows a typical pattern, that doesn't mean the ECG shows SVT!

VT can mimic virtually all typical patterns on the 12 lead ECG.

I remember when I was a cardiac monitoring technician on the Critical Care Stepdown (CCSD) unit of a medium-sized (about 400 beds) community hospital with 2 cath labs and an open heart program on the west side of Cleveland, Ohio.

Our unit could handle 32 patients, and we were always full. That translates into 4 monitors, each with 8 different heart rhythms, and it was my job to watch them.

We used Zymed's EASI lead system, which allowed us to view a "derived" 12 lead ECG from 4 modified electrodes (3 leads: Channel 1, Channel 2 and Channel 3).

The technology has since been licensed to Philips Healthcare.

The system is ideal for stepdown units (where hooking a patient up to continuous 12 lead ECG monitoring isn't practical), holter monitors (where the morphology of an arrhythmia might help the cardiologist), NASA astronauts, and so on.

Much of what I know about 12 lead ECGs stems from my experience comparing the "derived" 12 lead ECG to the standard 12 lead ECG in the patient's chart, and asking cardiologists lots of questions.

The EASI lead system has many advantages, but it has a couple of drawbacks. It is very dependent upon proper lead placement. I mean no disrespect to my nurse colleagues (they were very competent and I enjoyed working with them), so let's just say their self esteem did not depend on lead placement precision.

The other main drawback (for a stepdown unit) is simple. Where is the surgical wound for an open heart patient? Right down the middle of the sternum. Where do you have to place the electrodes for Channel 1 in the EASI lead system? On the top and bottom of the sternum. Since that's not possible for an open heart patient, you have to:

1.) place the electrodes off to the side (which is what we did 99.9% of the time),
2.) use standard lead placement, or
3.) use some type of modified lead placement.

The whole advantage of the EASI lead system is that it gives you the opportunity to view arrhythmias in 12 leads. It's a lot easier to figure out what those FLBs (funny little beats) are when you can look at the limb leads and the precordial leads. So what's the next best thing?

If we had a set of leads that allowed the limb leads plus one precordial lead, that would have been a good alternative. But since we didn't, I decided to place a post-CABG patient in MCL-1 and MCL-6.

I had read (and this was validated by a discussion with a cardiologist) that you could differentiate wide complex tachycardias using these leads, since they mimic leads V1 and V6, and those are the relevant leads for Wellen's Criteria.

Wouldn't you know it? The patient cooperated and experienced a wide complex tachycardia!

The arrhythmia lasted about 12 seconds. The patient was totally asymptomatic.

I was thrilled! To an ECG dork, the opportunity to view a brand new wide complex tachycardia in MCL-1 and MCL-6 is quite a treat.

It didn't take me long to decide that the "right bunny ear" was taller than the "left bunny ear" in lead MCL-1 (surrogate for lead V1). Yup, this was typical RBBB. SVT with aberrancy all the way!

The cardiologist agreed, and wrote down in the chart that the patient experienced a short run of atrial fibrillation with "rate activated RBBB".

We were both wrong.

No, the patient didn't die, or experience an adverse outcome. At least, I don't think s/he did. But then again, I don't know for sure. It wasn't until years later that I realized this ECG shows VT.

Let me prove it to you.

Take a good look at the circled QRS complex. I didn't notice it right away (okay, until years later) but it's wider than the previous QRS complexes. There is also a change in amplitde.

There's something different about this QRS complex, all right. It's a fusion complex. In other words, it's a hybrid.

A "fusion" QRS complex is the result of ventricular depolarization happening through two different mechanisms at the same time. In this case, "normal" ventricular depolarization through the AV node/His/Purkinje system, and spontaneous ventricular depolarization from a PVC (most likely triggering the run of VT that follows).

Still aren't sure?

Look at the PR interval of the underlying rhythm, and the PR interval of the fusion complex. The PR interval of the fusion complex is slightly shorter (thank you Tom G.).

If this evidence isn't compelling enough (and I think it is), look at the following image.

If you march out the P waves, AV dissociation is present.

Point, game and match.

I know what you're thinking. Brugada's Criteria would have caught this run of VT! Sure. If you are skilled enough to identify AV dissociation at the point of patient contact and decision making.

My point is simple. Your default diagnosis should be VT anyway! In my opinion, QRS morphology is insufficient evidence that a wide complex tachycardia is SVT with aberrancy.

That's all I wanted to establish for Part I. :)

See also:

Differential diagnosis of wide complex tachycardias – Part I

Differential diagnosis of wide complex tachycardias – Part II

Differential diagnosis of wide complex tachycardias – Part III

Differential diagnosis of wide complex tachycardias – Part IV

Differential diagnosis of wide complex tachycardias – Part V

Differential diagnosis of wide complex tachycardias – Part VI

*** UPDATE ***

I recently discovered this article from the March 2006 issue of Emergency Medical Services. In it, the author states:

With the introduction of new pharmacological interventions that target specific areas of the cardiac conduction system, it has become increasingly important for EMS providers to make an accurate interpretation of an ECG. Though most paramedics have no difficulty distinguishing VT from narrow complex supraventricular tachycardia (SVT), some might fall victim to the "wide + fast = VT" trap when looking at SVT with aberrant conduction. Although VT and SVT with aberrant conduction look similar, they vary greatly in terms of origin, pathophysiology and treatment. Mislabeling dysrhythmias can have severe consequences. Improper identification of VT could place a patient in grave danger by delaying indicated pharmacological and electrical interventions.

Listen carefully.

"Wide and fast = VT" is not a trap! It's a rule of thumb that exists to protect you and your patient!

The author continues:

A common aphorism among advanced practitioners is, "When in doubt whether a WCT is VT or SVT, treat patients as if they are experiencing VTs." This stems from a statistic showing that approximately 80% of all WCTs are VT. Though this aphorism is generally a good rule of thumb, it is also important to acknowledge that one in five WCTs is not VT and therefore requires different treatment regimens. One must possess the proper diagnostic tools and knowledge to decide whether a WCT is VT or SVT with aberrant conduction. EMS providers should be able to differentiate VT and SVT with aberrant conduction with confidence and a high degree of certainty.

They do not require different treatment regimens!

Unstable SVT is treated the same as unstable VT! Anyone disagree?

It's debatable whether or not a stable wide complex tachycardia should be treated in the field at all, but if you do reach for an antiarrhythmic, it better be one that works for SVT and VT (i.e., amiodarone or procainamide).

Note: With the 2010 AHA ECC Guidelines adenosine is once again an option for undifferentiated regular wide complex tachycardia.

If you give a calcium channel blocker to a wide complex tachycardia without knowing with 100% certainty that it's SVT with BBB (or aberrancy) you are a fool.

The article also contains outright errors. Here's one of the most disturbing.

SVTs with aberrancy will produce either a right or left axis deviation. If the aberrancy is conducted in a RBBB pattern, right axis deviation will be present. If the aberrancy is conducted in a LBBB pattern, left axis deviation will be present. In almost all VT, the axis will be in the extreme right quadrant.

This is nonsense!

In the first place, RBBB and LBBB aberrancy can both show a normal axis. RBBB aberrancy in particular can show a normal axis, right axis deviation, or left axis deviation (bifascicular patterns).

Most cases of VT present with an other-than-extreme axis.

See what I mean about a little bit of knowledge being dangerous?

*** End update ***

See also:

The Prehospital 12-Lead ECG blog receives its first hate mail


  • SoCal Medic says:

    Tom,With all the education you do through your blog, it is a shame you can’t issue CE’s for it.. Another great piece and thanks again for the time you spend on this.

  • Tom B says:

    That’s an interesting idea, Christopher! I’ll have to chew on that one…. Thanks for the positive feedback!

  • walma says:

    There is no place in prehospital settings for differential diagnosis between VT and SVT with aberrancy or RBBB/LBBB, Wide complex tachycardia is ALWAYS VT.There is even no place for such differential in “safe” ER. The best place for such difficult diagnosis is electrophysiology office. Forget Wellens and Brugada’s Criteria. They are no good for us.Few studies for example:Failure to Agree on the Electrocardiographic Diagnosis of Ventricular Tachycardia . Annals of Emergency Medicine , Volume 27 , Issue 1 , Pages 35 – 38 M . Herbert , S . Votey , M . Morgan , P . Cameron , L . Dziukas Study objective: To determine the extent of interobserver agreement in the ECG diagnosis of ventricular tachycardia (VT) by using a four-step algorithm and three observers. Methods: Simulated emergency department setting from records of an urban university teaching hospital. All ECGs taken in the ED during a 2-year period that showed a QRS duration of more than 120 msec and a heart rate faster than 110 beats per minute were reviewed. ECGs were categorized as demonstrating sinus rhythm (SR), irregular broad-complex tachycardia (I-BCT), or regular broad-complex tachycardia (BCT). Copies of the BCT ECGs and short clinical histories were given to each of three emergency physicians, who used a published, four-step algorithm (the Brugada algorithm) to categorize the BCT ECGs as indicating VT, indicating supraventricular tachycardia with aberrancy (SVT-A), or indeterminate. Interobserver agreement was assessed with the K-statistic. Results: The records contained 178 ECGs, 88 of which were SR, 63 I-BCT, and 27 BCT. The 27 BCT ECGs were selected for review. The emergency physicians disagreed with each other 22% of the time in differentiating VT from SVT-A (K=.58). Conclusion: Application of the algorithm to actual clinical practice in the ED would probably result in the misdiagnosis of a substantial minority of patients having BCT, with potentially serious adverse consequences.And… Isenhour JL, Craig S, Gibbs M, et al. Wide-complex tachycardia: continued evaluation of diagnostic criteria. Acad Emerg Med 2000;7:769-73. 5.Two emergency physicians and two cardiologists, 157 ecgs with wide complex tachycardias. They apply Brugada’s Criteria.Final diagnosis confirmed in electrophysiology lab.Emergency physicians sensitive in 83% and sensitive in 79%, Interobserver agreement in 82% of cases. This mean one of them missed 17% of VT patient and second one missed 21% of patient with true VT!Cardiologists sensitive in 85% and sensitive in 91%, Interobserver agreement in 81 % of casesThis mean one of them missed 15% of VT patient and second one missed 9% of patient with true VT! They were much better then ER docs but still… Interobserver agreement means that they cant agree between each other to establish final diagnosis.So now you can make the decision…

  • Tom B says:

    Maciek wrote:"There is no place in prehospital settings for differential diagnosis between VT and SVT with aberrancy or RBBB/LBBB, Wide complex tachycardia is ALWAYS VT.">snip<I appreciate the sentiment, Maciek. However, sometimes you can accurately identify wide complex tachycardia as SVT in the field (and I'm using the broadest possible definition of wide complex tachycardia).For example, two abnormalities that go hand-in-hand with heart failure are atrial fibrillation and bundle branch block.When I'm training paramedics with the heart rhythm simulator, I like to give at least one scenario with AF and LBBB (or nonspecific IVCD) with a rate of 160 (or even higher). But I give a pressure of 160/110.This may seem like a far fetched example, but I'm just trying to show that sometimes, "wide and fast" is not VT, and it’s acceptable to treat it like it’s not VT, provided that we start with the default assumption that it is VT, and then go about proving that it’s not VT in a responsible manner.The main problem I have is using QRS morphology as the main tool to differentiate between VT and SVT.Tom

  • PQRST says:

    All I have to say right now is: Thanks a lot for this and so many other great articles that I have found here in your blog. Thanks for great education! You should write a book!:-)Happy new year and all the best from Norway!

  • Tom B says:

    I’m glad you’re enjoying it, Klaus! It would be difficult to say which of us is the bigger ECG dork! :D

  • PQRST says:

    I just came home from the ER with a bunch of 12 leads and rhythm strips showing a broad complexed rhythm that I’m not sure about. Therefore, I turned to your blog for help. I’ve started at part 1 one now. I just want to applaude the way you write. It’s both entertaining and educating. Great posts!

  • PQRST says:

    By the way, and this might be a stupid question, but what does the shortened PR interval before the fusion complex indicate? I mean, why is this compelling evidence? I’m sure it is, I just didn’t catch on here..-klaus

  • Tom B says:

    Thanks for the kind words, Klaus.The shortened PR interval indicates earlier than expected ventricular depolarization due to a mechanism other than the normal AV conduction.In other words, it helps prove that a PVC hit just prior to the regularly scheduled QRS complex, creating a so-called “fusion” complex. That’s why the QRS morphology is different.

  • PQRST says:

    Ah.. ok. I see. But can you really determine anything from the PR interval when there is AV dissociation?

  • Tom B says:

    Ah, but when does the AV dissociation begin? Not until the onset of VT. The shortened PR interval reveals the moment the triggering PVC hit.

  • Brian T says:

    Hey Tom-I’m wondering what your experience is with the accuracy of computerized interpretive statements with regard to WCT of unknown origin. Also, are you aware of what criteria the computer uses to call a WCT VT vs SVT? Thanks, Brian

  • Tom B says:

    Brian – My anecdotal experience is that the computerized interpretive statement should not be trusted to accurately diagnose cardiac rhythms.Sometimes the computer gets the measurements wrong, and when that happens, all bets are off!Tom

  • Dodge says:

    I agree Tom write a book the best one i had a read of years ago was a book written by Mike Taigman, titled taigmans advanced cardiology i still dust it of these days it was simple and concise shame it has never been updated HINT HINT 

9 Trackbacks

Leave a Reply

Your email address will not be published. Required fields are marked *

EMS 12-Lead

Cardiac Rhythm Analysis, 12-Lead ECG Interpretation, Resuscitation

STEMI Expert?

  • Click here to find out!
  • 12-Lead ECG Challenge app


    The 12-Lead ECG Challenge app is now available for $4.99!

    (Apple iOS) (Android) (Amazon)
    (Web Based)

    See the product review from FRN-TV here!

    Interested in resuscitation?


    FireEMS Blogs eNewsletter

    Sign-up to receive our free monthly eNewsletter

    Visitor Map / Stats

    Locations of visitors to this page