VT vs SVT with Aberrancy: Using the Brugada Algorithm
By the CardioLens Team
A wide-complex tachycardia on the monitor. The patient is borderline haemodynamically stable. You have about 30 seconds to decide: is this ventricular tachycardia, or supraventricular tachycardia with aberrant conduction?
Get it right, and the patient receives appropriate treatment. Get it wrong — treat VT as SVT with a calcium channel blocker — and you can precipitate cardiovascular collapse.
This is why Pedro Brugada and colleagues published their now-classic four-step algorithm in 1991. It remains one of the most practical diagnostic tools in emergency electrocardiography.
Why This Matters
Wide-complex tachycardia (QRS >120 ms) is VT until proven otherwise. Studies show VT accounts for approximately 80% of wide-complex tachycardias in the general population — and closer to 95% in patients with known structural heart disease.
Yet in emergency settings, wide-complex tachycardia is frequently misdiagnosed as SVT with aberrancy. The cost of this error can be catastrophic. Giving verapamil to a patient in VT can cause profound hypotension and cardiac arrest.
The Brugada Algorithm: 4 Steps
Step 1: Absence of RS Complex in All Precordial Leads
Look at leads V1–V6. If none of them show a typical RS complex (an R wave followed by an S wave), it's VT. Stop here. If any precordial lead shows an RS, move to Step 2.
Step 2: R-to-S Interval >100 ms
In any precordial lead that has an RS complex, measure from the onset of the R wave to the nadir (bottom) of the S wave. If this interval is >100 ms (more than 2.5 small boxes) in any lead, it's VT. Stop here. If ≤100 ms in all precordial leads, move to Step 3.
Step 3: AV Dissociation
Is there evidence of AV dissociation? Look for:
- Capture beats — a narrow QRS breaks through the wide rhythm when the atrial impulse happens to conduct through the AV node at the right moment.
- Fusion beats — a morphology in between a sinus beat and a VT beat, representing partial atrial conduction fused with ventricular depolarisation.
- Independent P waves marching through the QRS complexes at their own rate.
If any of these are present, it's VT. Stop here. If absent, move to Step 4.
Step 4: Morphology Criteria
Look at V1 and V6 and apply specific morphology rules.
Right bundle branch block pattern (positive QRS in V1):
- V1: monophasic R, QR, or RS suggests VT
- V6: R/S ratio <1, or QS, or monophasic R suggests VT
Left bundle branch block pattern (negative QRS in V1):
- V1 or V2: R >30 ms, or a notched S, or a delayed S nadir (>60 ms) suggests VT
- V6: a Q wave (any QR or QS complex) suggests VT
If any of these morphology criteria are met, it's VT. If none are met, the rhythm is SVT with aberrancy.
Other Red Flags for VT
If the patient has a history of MI, heart failure, or structural heart disease — treat wide-complex tachycardia as VT unless you have overwhelming evidence otherwise. The clinical context is often more predictive than any algorithm.
Other findings that strongly favour VT:
- Extreme axis deviation (northwest axis, −90° to −180°)
- Concordance — all precordial leads positive or all negative
- QRS width >160 ms with a left bundle pattern, or >140 ms with a right bundle pattern
- Rate >170 bpm with an extremely wide QRS
A Common Mistake
Clinicians sometimes treat unstable wide-complex tachycardia with adenosine "to see what happens." This is dangerous. While adenosine will terminate most SVTs and reveal the underlying rhythm in SVT with aberrancy, giving it to a patient in VT can cause significant haemodynamic compromise — or accelerate the VT in rare cases.
When unstable, cardiovert. Synchronised cardioversion is always appropriate regardless of whether it's VT or SVT. When stable, the Brugada algorithm gives you a defensible pathway to the right answer.
Key takeaway: Wide-complex tachycardia is VT until proven otherwise. Apply the Brugada algorithm in four steps: absence of RS in precordial leads → R-to-S interval >100 ms → AV dissociation → morphology criteria. When the patient is unstable, cardiovert. When stable, diagnose before you treat.
Practicing in CardioLens
The VT vs SVT Brugada Algorithm module inside CardioLens walks through each step interactively with paired strips showing real RBBB-pattern VT, LBBB-pattern VT, and SVT with aberrancy. The ACLS Tachycardia algorithm links directly to the relevant rhythm examples, and MEGA CODES includes scenarios where you must decide between cardioversion and medication based on your wide-complex diagnosis.
Sources
- AHA/ACC/HRS Guidelines for ECG Interpretation
- Braunwald's Heart Disease, 12th Edition
- Dubin's Rapid Interpretation of EKGs, 6th Edition
For educational purposes only — not a diagnostic tool.