Wide Complex Tachycardia, VT Versus SVT With Aberrancy
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Direct answer
A wide complex tachycardia has a QRS duration of 120 ms or more and is treated as ventricular tachycardia until proven otherwise. Ventricular tachycardia and supraventricular tachycardia with aberrant conduction look alike on ECG, so the Brugada algorithm, AV dissociation and capture or fusion beats are used to separate them.
What you must remember
- Definition: a wide complex tachycardia has a QRS duration of 120 ms or more at a rate above 100 beats per minute.
- Default rule: treat as VT until the Brugada algorithm excludes it, since misdiagnosis followed by verapamil can cause haemodynamic collapse.
- AV dissociation: P waves that march independently of the QRS complexes are specific for VT, though they are not always visible.
- Capture and fusion beats: a capture beat is a narrow QRS inside the tachycardia, and a fusion beat is a hybrid complex, and both are specific for VT.
- Concordance: a QRS that is entirely positive or entirely negative across all precordial leads favours VT.
- Axis: a northwest, extreme right axis favours VT.
- Prior ECG: a known bundle branch block on an old sinus ECG with the same morphology supports aberrancy.
- Stable VT: IV amiodarone or procainamide is used, with synchronised DC cardioversion if the patient becomes unstable.
The Brugada algorithm, step by step
Each step is applied in order, and a positive finding at any of the first three steps establishes VT. The morphology steps are reached only when the first three are negative.
- Concordance: if no RS complex appears in any precordial lead from V1 to V6, the rhythm is VT.
- RS interval: an RS interval longer than 100 ms in any precordial lead indicates VT.
- AV dissociation: atrial activity that is independent of the QRS complexes establishes VT.
- Morphology in V1 and V2: a monophasic R or qR pattern, or in LBBB-like complexes a broad initial R wave, notched S downstroke or RS interval over 60 ms, favours VT, whereas a typical RBBB-like triphasic rsR' pattern favours aberrancy.
- Morphology in V6: a QS, rS or QR pattern in lead V6 favours VT.
Because the first three steps are decisive, most rhythms are classified before morphology is considered. Steps 4 and 5 refine the decision only when the earlier criteria are negative.
Traps in wide complex vignettes
Wide complex questions are scored on the default rule, so the trap is usually a shortcut taken before the algorithm is applied.
- Trap: choosing SVT with aberrancy because the patient is young and well Correct: VT occurs in young patients with structural or channel disease, so the algorithm must be applied before the rhythm is labelled.
- Trap: giving verapamil to terminate a regular wide complex tachycardia Correct: verapamil can cause collapse if the rhythm is VT, so amiodarone or procainamide is preferred in a stable patient.
- Trap: accepting a past history of bundle branch block as proof of SVT Correct: a prior ECG helps only if its morphology matches the current tracing, since VT can coexist with bundle branch block.
Frequently asked questions
What is the Brugada algorithm used for?
The Brugada algorithm separates ventricular tachycardia from supraventricular tachycardia with aberrancy in a regular wide complex tachycardia. It applies criteria in order, starting with concordance, RS interval and AV dissociation, and then QRS morphology in V1, V2 and V6.
Why is AV dissociation diagnostic of ventricular tachycardia?
AV dissociation means atrial activity runs independently of the ventricular complexes, so the ventricles are being driven by a focus other than the atria. This finding points to VT, because supraventricular tachycardia with aberrancy keeps the atria and ventricles linked.
What are capture and fusion beats?
A capture beat is a narrow QRS complex that appears within the wide tachycardia, showing that an atrial impulse occasionally reached the ventricles normally. A fusion beat is a hybrid complex formed when normal and ventricular activation merge, and both beats are specific for ventricular tachycardia.
Why is verapamil avoided in undiagnosed wide complex tachycardia?
Verapamil depresses myocardial contractility and vascular tone, so it can cause hypotension or cardiac arrest when the rhythm is ventricular tachycardia. For this reason, it is avoided until the rhythm has been confirmed as supraventricular.