What are the key features to identify ventricular tachycardia (VT) on an electrocardiogram (ECG)?

Key Features to Identify Ventricular Tachycardia on ECG

When evaluating a wide-complex tachycardia, VT should be your presumed diagnosis until proven otherwise, as it carries higher mortality risk and requires immediate recognition. 1, 2

Primary Diagnostic Features

QRS Complex Characteristics

• Wide QRS duration >120 ms is the fundamental feature of VT in adults, though this alone is not diagnostic as SVT with aberrancy can also present with wide complexes 1, 3, 2
• QRS width >140 ms with RBBB pattern or >160 ms with LBBB pattern strongly favors VT over SVT 1, 2
• R-S interval >100 ms (measured from onset of R wave to nadir of S wave) in any precordial lead is highly suggestive of VT—this is part of the Brugada criteria 1, 3, 2

Pathognomonic Findings

• AV dissociation (ventricular rate faster than atrial rate with independent P waves) is diagnostic of VT when present, though visible in only 30% of cases 1, 3, 2
• Fusion beats (merger of conducted sinus impulses with ventricular depolarization) are pathognomonic for VT 1, 3, 2
• Capture beats during tachycardia confirm ventricular origin 1

Precordial Lead Analysis (V1-V6)

• Concordance: All precordial leads showing either positive or all negative QRS deflections strongly suggests VT 1, 3, 2
• Absence of RS complexes in all precordial leads implies VT per Brugada criteria 1, 2
• QR complexes in leads V2-V6 indicate myocardial scar and are present in ~40% of post-MI VT 1, 3, 4

Lead-Specific Criteria

Lead aVR (Vereckei Algorithm)

• Initial R wave in aVR suggests VT 1, 3, 2
• Initial R or Q wave >40 ms in aVR implies VT 1, 3, 2
• Notch on descending limb at onset of predominantly negative QRS implies VT 1, 2

Lead II

• R-wave peak time ≥50 ms in lead II suggests VT 1, 3, 2

LBBB Morphology-Specific Criteria

When the tachycardia has LBBB morphology, look for:

• R wave >30 ms duration in V1 or V2 5
• Any Q wave in V6 5
• Duration >60 ms from QRS onset to nadir of S wave in V1 or V2 5
• Notching on downstroke of S wave in V1 or V2 5

Clinical Context Clues

• Prior myocardial infarction with new wide-complex tachycardia strongly indicates VT 1, 3
• Similar morphology between premature ventricular beats during sinus rhythm and the tachyarrhythmia strongly suggests VT 1, 3
• On physical exam: irregular cannon A waves in jugular venous pulse and variable intensity of first heart sound suggest AV dissociation and VT 1

Critical Pitfalls to Avoid

• Never assume hemodynamic stability rules out VT—patients can be stable with VT 2
• Pre-existing bundle branch block makes QRS width criteria less reliable, as SVT can appear wide 1, 2
• Antiarrhythmic drugs (Class Ia, Ic) and electrolyte abnormalities (hyperkalemia) can widen QRS in SVT, mimicking VT 1, 3
• SVT with accessory pathway conduction (antidromic AVRT) presents with wide QRS and can be misdiagnosed 1, 2
• When diagnosis is uncertain, always treat as VT—this is the safest approach given the mortality implications 1, 2

Practical Approach

Start by assessing QRS width: if >120 ms, systematically evaluate for AV dissociation, fusion beats, concordance, and apply Brugada/Vereckei criteria 1, 3, 2. Compare the tachycardia morphology to any available sinus rhythm ECG—if the QRS during tachycardia differs significantly from baseline, suspect VT 1, 3. In infants, note that VT may have QRS <90 ms but will still differ from the sinus QRS morphology 1, 3.