VT versus SVT

Regular broad complex tachycardias can be ventricular (VT) or supraventricular (SVT with aberrancy) in origin, and differentiation between the two will significantly influence your management of the patient.

Unfortunately, the electrocardiographic differentiation of VT from SVT with aberrancy is not always possible.


Differential diagnosis of regular broad complex tachycardia


ECG features increasing the likelihood of VT

Electrocardiographic features that increase the likelihood of VT include:

Absence of typical RBBB or LBBB morphology

Extreme axis deviation (“northwest axis”): QRS positive in aVR and negative in I and aVF

Very broad complexes > 160ms

AV dissociation:

  • P and QRS complexes at different rates
  • P waves are often superimposed on QRS complexes and may be difficult to discern

Capture beats: Occur when the sinoatrial node transiently “captures” the ventricles in the midst of AV dissociation, producing a QRS complex of normal duration

Fusion beats: Occur when a sinus and ventricular beat coincide to produce a hybrid complex (see Dressler beat)

AV dissociation Fusion beat capture beat VT

Positive concordance throughout the precordial leads:

ECG Positive concordance in VT
Positive concordance in VT: Leads V1-6 show entirely positive (R) complexes, with no RS complexes seen

Negative concordance throughout the precordial leads:

ECG VT Negative precordial concordance 2
Negative concordance in VT: Leads V1-6 show entirely negative (QS) complexes, with no RS complexes seen

RSR’ complexes with a taller left rabbit ear:

  • This is the most specific finding in favour of VT
  • This is in contrast to RBBB, where the right rabbit ear is taller
ECG VT Taller left rabbit ear V1
Taller left rabbit ear in V1

Brugada sign: Distance from onset of R wave to nadir of S wave is > 100ms in leads V1-6 (discussed further below)

Josephson sign: Notching/slurring near the nadir of the S wave

Lead V2 demonstrating both Josephson sign (blue) and Brugada sign (green)

Clinical factors associated with VT or SVT

The likelihood of VT is increased with:

  • Age > 35 (positive predictive value of 85%)
  • Structural heart disease
  • Ischaemic heart disease
  • Previous MI
  • Family history of sudden cardiac death (suggesting conditions such as HOCM, congenital long QT syndrome, Brugada syndrome or arrhythmogenic right ventricular dysplasia that are associated with episodes of VT)

The likelihood of SVT with aberrancy is increased if:

  • Previous ECGs show a bundle branch block pattern with identical morphology to the broad complex tachycardia
  • Previous ECGs show evidence of WPW (short PR < 120ms, broad QRS, delta wave)
  • The patient has a history of paroxysmal tachycardias that have been successfully terminated with adenosine or vagal manoeuvres

Advanced Tips for Diagnosing VT — The Brugada Criteria

  • For difficult cases, the Brugada algorithm can be used to distinguish between VT and SVT with aberrancy.
  • The algorithm is followed from top to bottom — if any of the criteria are satisfied, then VT is diagnosed.
Brugada algorithm

1. Absence of an RS complex in all precordial leads

This is essentially the same as having positive or negative concordance.

  • All precordial leads consist of either monophasic R or S waves
    • → VT is diagnosed
  • If there are any RS complexes present in V1-6
    • → go to step 2

Precordial R waves only → VT
Positive concordance in VT
Positive concordance in VT

Precordial S waves only → VT
Negative concordance in VT

RS complexes present → go to step 2
RS-complexes

2. RS interval > 100ms in one precordial lead

If RS complexes are present in V1-6 then the RS interval is measured. This is the time from the onset of the R wave to the nadir of the S wave.

  • If the RS interval is > 100 ms
    • VT
  • If the RS interval is < 100 ms
    • go to step 3
RS-interval

3. AV dissociation

  • P waves are present at a different rate to the QRS complexes
    • → AV dissociation is present and VT is diagnosed.
  • No evidence of AV dissociation can be seen
    • → go to step 4
AV-dissociation

4. Morphological Criteria for VT

Leads V1-2 and V6 are assessed for characteristic features of VT. There are two sets of morphological criteria depending on the appearance of the QRS complex in V1:

  • Dominant R wave in V1
    • → see criteria for RBBB-like morphology
  • Dominant S wave in V1  
    • → see criteria for LBBB-like morphology
RBBB-like-morphology
RBBB-like morphology
LBBB-like-morphology
LBBB-like morphology

Broad complex tachycardia with RBBB morphology

Appearance in V1-2

With a positive R wave in V1/V2, three patterns are indicative of VT:

  • Smooth monophasic R wave
  • Notched downslope to the R wave — the taller left rabbit ear
  • A qR complex (small Q wave, tall R wave) in V1

ECG VT Taller left rabbit ear V1
Notched downslope to R wave: taller left rabbit ear indicating VT

NOTE: RSR’ pattern is suggestive of SVT with RBBB
ECG Typical RSR' pattern of RBBB
Typical RSR’ pattern of RBBB

Appearance in V6

In V6, the following patterns are consistent with VT:

  • QS complex
    • Completely negative complex with no R wave
ECG VT RBBB QS-wave-V6-RBBB-VT

  • R/S ratio < 1 
    • Small R wave, deep S wave
    • Indicates VT only if LAD is also present
ECG VT V6 R-S ratio RBBB morphology 500

Broad complex tachycardia with LBBB morphology

Appearance in V1-2

With a dominant S wave in V1, the following three features are diagnostic of VT:

  • Initial R wave > 30-40 ms duration.
  • Notching or slurring of the S wave (Josephson sign)
  • RS interval (time from R wave onset to S wave nadir) > 60-70 ms
Brugada-criteria-LBBB-VT-V1-2 Wellens
Image reproduced from Wellens (2001)

Appearance in V6

With a LBBB-like pattern, the presence of Q waves in V6 is indicative of VT. 

There are two possible patterns of this:

  • QS waves in V6 (as with RBBB-like patterns)
    • → Very specific for VT
ECG VT RBBB QS-wave-V6-RBBB-VT

  • qR complex in V6 (small q wave, large R wave)
    • → VT
ECG VT LBBB V6 qR wave

Note: SVT with LBBB is associated with absent Q waves in V6.
ECG LBBB V6 absent Q waves
Absent Q waves in V6 with LBBB

More Advanced Tips — The Vereckei Algorithm

There is some overlap between the Vereckei and Brugada algorithms, but one of the most useful tips from the Vereckei algorithm is to examine the QRS complex in lead aVR. [PMID 18180024]

  • Dominant initial R wave in aVR 
    • indicative of VT
vereckei-aVR-VT Dominant initial R wave in aVR
  • Dominant terminal R’ wave in aVR (i.e. following a Q/S wave)
    • more likely SVT with aberrancy 
    • pattern is most commonly seen in TCA toxicity
Dominant secondary R' wave in aVR TCA Toxicity


Other diagnostic algorithms


Conclusions

  • Most of the published criteria have high specificities but very low sensitivities (e.g. 20-50%) for diagnosing VT
  • This means that even in the absence of diagnostic features for VT, there is no way to be 100% certain that the rhythm is SVT with aberrancy…
  • If in doubt, treat as VT!

Broad Complex Tachycardia Quiz

Test your skills with these broad complex rhythms…

Example 1
ECG SVT AVNRT LBBB 2
Reveal answer

AV-nodal re-entry tachycardia (AVNRT) with LBBB

  • Typical LBBB morphology with absence of Q waves in V6
  • No positive Brugada criteria
  • This patient had LBBB with identical morphology on their previous ECGs

Example 2
ECG Ventricualr tachycardia Monomorphic VT
Reveal answer

Monomorphic VT

  • This ECG is a difficult one!
  • Although there is a broad complex tachycardia (HR > 100, QRS > 120), the appearance in V1 is more suggestive of SVT with aberrancy, given that the the complexes are not that broad (< 160 ms) and the right rabbit ear is taller than the left.
  • However, on closer inspection there are signs of AV dissociation, with superimposed P waves visible in V1
  • Also, the presence of a northwest axis and an R/S ratio < 1 in V6 (tiny R wave, deep S wave) indicate that this is VT
  • This patient had a completely different QRS axis and morphology on his baseline ECG.
ECG VT AV dissociation
AV dissociation

Example 3
ECG-TCA-Doxepin-Overdose-Toxicology 1200
Reveal Answer

Tricyclic antidepressant toxicity

  • QRS complexes are very broad (~200ms) — however, unlike with VT most of the broadening is in the terminal portion of the QRS (this can be best appreciated in leads V3-V6 where narrow R waves are followed by massively broad and deep S waves)
  • There are no positive Brugada criteria — in particular, the RS interval is < 100 ms.
  • No P waves can be seen

The characteristic features of TCA toxicity are:

  • Tachycardia — this is often a sinus tachycardia with a grossly prolonged PR interval, such that the P wave is hidden in the previous T wave or QRS complex; may be difficult to differentiate from junctional tachycardia with aberrant conduction.
  • Broad QRS complexes.
  • Right axis deviation of the terminal QRS — positive R’ wave in aVR, deep S wave in lead I.

Read more about TCA toxicity


Example 4
ECG AVRT WPW 5yo boy 2
…Clue

The patient is 5 years old.

Reveal Answer

Antidromic atrioventricular re-entry tachycardia (AVRT) due to Wolff-Parkinson-White syndrome

  • This is the one rhythm that may be impossible to distinguish from VT!
  • In this case the main clue is the history — more than 95% of broad complex tachycardias in children are SVT with aberrancy

Read more about paediatric dysrhythmias


Example 5
ECG pacemaker-mediated tachycardia PMT
Reveal Answer

Rapid ventricular paced rhythm (e.g. pacemaker-mediated tachycardia)

  • There are obvious pacing spikes before each QRS complex
  • Ventricular paced rhythms have features in common with other ventricular rhythms — in this case the ECG demonstrates negative concordance in V1-6, initial R wave > 40ms in V1, RS interval > 70 ms in V1, QS complex in V6
  • Remember that the pacing spikes may not always be as obvious as this!

Read more about pacemaker dysfunctions 

Further reading

Read more about the different types of VT by following these links: 


References

LITFL Further Reading

Advanced Reading

ECG LIBRARY 700

ECG LIBRARY

Electrocardiogram

Emergency Physician in Prehospital and Retrieval Medicine in Sydney, Australia. He has a passion for ECG interpretation and medical education | ECG Library |

MBBS (UWA) CCPU (RCE, Biliary, DVT, E-FAST, AAA) Emergency Medicine Advanced Trainee in Melbourne, Australia. Special interests in diagnostic and procedural ultrasound, medical education, and ECG interpretation. Editor-in-chief of the LITFL ECG Library. Twitter: @rob_buttner

2 Comments

  1. Congratulations for one more elusive post.
    About the Brugada algorithm, you said that the first step (absence of RS) is the same as searching for concordance. But a VT may have absence of RS and still not be categorized as precordial concordance. For instance, in the presence of QR complexes. In Brugada’s original publication, figure 5 ilustrates 5 ecgs, and last one at the right is exactly the case.
    So my question is, is absence of RS really a sinonym of precordial concordance?
    Thanks a lot

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