Look at this ECG – how do you interpret it?
There are three main diagnostic possibilities:
- SVT with aberrant conduction due to bundle branch block
- SVT with aberrant conduction due to the Wolff-Parkinson-White syndrome
The most important distinction is whether the rhythm is ventricular (VT) or supraventricular (SVT with aberrancy), as this will significantly influence how you manage the patient. SVTs usually respond well to AV-nodal blocking drugs, whereas patients with VT may suffer precipitous haemodynamic deterioration if erroneously administered an AV-nodal blocking agent.
Unfortunately, the electrocardiographic differentiation of VT from SVT with aberrancy is not always possible.
ECG features increasing the likelihood of VT
There are several electrocardiographic features that increase the likelihood of VT:
- Absence of typical RBBB or LBBB morphology
- Extreme axis deviation (“northwest axis”) — QRS is positive in aVR and negative in I + aVF.
- Very broad complexes (>160ms)
- AV dissociation (P and QRS complexes at different rates)
- Capture beats — occur when the sinoatrial node transiently ‘captures’ the ventricles, in the midst of AV dissociation, to produce a QRS complex of normal duration.
- Fusion beats — occur when a sinus and ventricular beat coincides to produce a hybrid complex.
- Positive or negative concordance throughout the chest leads, i.e. leads V1-6 show entirely positive (R) or entirely negative (QS) complexes, with no RS complexes seen.
- Brugada’s sign – The distance from the onset of the QRS complex to the nadir of the S-wave is > 100ms
- Josephson’s sign – Notching near the nadir of the S-wave
- 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.
Examples of these ECG features of VT:
Additional 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
- Congestive heart failure
- 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.
1. Absence of an RS complex in all precordial leads
- This is essentially the same as having positive or negative concordance.
- If all the precordial leads consist of either monophasic R or S waves then VT is diagnosed.
- If there are any RS complexes present in V1-6 –> move on to the next step of the algorithm.
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 is diagnosed.
- If the RS interval is < 100 ms –> move on to step 3.
3. AV dissociation
- The ECG is scrutinised for hidden P waves; these are often superimposed on the QRS complexes and may be difficult to see.
- If P waves are present at a different rate to the QRS complexes –> AV dissociation is present and VT is diagnosed.
- If no evidence of AV dissociation can be seen –> go to step 4.
4. Morphological Criteria for VT
- If there is a dominant R wave in V1 –> see criteria for RBBB-like morphology.
- If there is a dominant S wave in V1 –> see criteria for LBBB-like morphology.
Broad complex tachycardia with RBBB morphology
Appearance in V1-2
- Smooth monophasic R wave
- Notched downslope to the R wave — the taller left rabbit ear (= Marriott’s sign)
- A qR complex (small Q wave, tall R wave) in V1
In contrast, an RSR’ pattern is suggestive of SVT with RBBB.
Appearance in V6
- QS complex — a completely negative complex with no R wave (= strongly suggestive of VT).
- R/S ratio < 1 — small R wave, deep S wave (indicates VT only if LAD is also present).
Broad complex tachycardia with LBBB morphology
Appearance in V1-2
- Initial R wave > 30-40 ms duration.
- Notching or slurring of the S wave (Josephson’s sign).
- RS interval (time from R wave onset to S wave nadir) > 60-70 ms.
Appearance in V6
- QS waves in V6 (as with RBBB-like patterns, this finding is very specific for VT).
- qR pattern = small Q wave, large R wave.
Conversely, SVT with LBBB is associated with absent Q waves in V6.
More Advanced Tips — The Vereckei Algorithm
- A dominant initial R wave in aVR is indicative of VT.
- A dominant terminal R’ wave in aVR (i.e. following a Q/S wave) is more likely SVT with aberrancy — this pattern is most commonly seen in tricyclic poisoning.
Other diagnostic algorithms
- 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…
- 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.
Read more about monomorphic VT here.
Sinus tachycardia with incomplete RBBB:
- P waves are visible before each QRS complex.
- There is a typical RBBB morphology with a RSR’ complex in V1 and wide S wave in the lateral leads I, V5-6.
- In contrast to the previous example, there is a dominant R wave in V6 (RS ratio > 1), which is much more typical of RBBB.
- QRS complexes are only slightly prolonged (110ms), making this an incomplete RBBB.
- Q waves and T-wave inversions in III and aVF suggest prior inferior infarction.
Read more about RBBB here.
- 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 here.
The patient is 5 years old.
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
Read more about WPW
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
Read more about the different types of VT by following these links:
- Wellens HJ. Electrophysiology: Ventricular tachycardia: diagnosis of broad QRS complex tachycardia. Heart. 2001 Nov;86(5):579-85. Review. PubMed PMID: 11602560; PubMed Central PMCID: PMC1729977. Full text.
- Alzand BS, Crijns HJ. Diagnostic criteria of broad QRS complex tachycardia: decades of evolution. Europace. 2011 Apr;13(4):465-72. Epub 2010 Dec 3. Review. PubMed PMID: 21131372.
- Brady WJ, Truwit JD. Critical Decisions in Emergency and Acute Care Electrocardiography
- Hampton, JR. The ECG In Practice, 6e
- Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric, 6e
- Wagner, GS. Marriott’s Practical Electrocardiography 12e
- Chan, TC. ECG in Emergency Medicine and Acute Care
- Mattu, A. ECG’s for the Emergency Physician
LITFL Further Reading
- ECG BASICS — Waves, Intervals, Segments and Clinical Interpretation
- ECG A to Z by diagnosis –alphabetical diagnostic approach to the ECG
- ECG CLINICAL CASES — ECG’s placed in clinical context with a challenging Q&A approach
- 100 ECG Quiz — Self-assessment tool for examination practice
- ECG Reference SITES and BOOKS — the best of the rest
- LITFL ECG IMAGE Database — Searchable database of LITFL ECG’s
- ECG and Cardiology Eponymous Syndromes — Cheats guide to eponymous emancipation
- ECG Exam Template — a framework for answering ECG exam questions.