ECG Characteristics of Ventricular Tachycardia
Ventricular tachycardia is diagnosed on ECG by the presence of a wide QRS complex (>120 ms), a rate >100 bpm, and three or more consecutive ventricular beats, with AV dissociation being pathognomonic when present. 1, 2
Fundamental Defining Features
- Rate: VT presents with a ventricular rate greater than 100 bpm (cycle length <600 ms) 1
- QRS Duration: QRS width >120 ms in adults is the hallmark feature 1, 2
- Duration Classification: Three or more consecutive beats defines VT; nonsustained VT terminates in <30 seconds, while sustained VT lasts ≥30 seconds or requires termination due to hemodynamic compromise 1
Pathognomonic Features (100% Specific for VT)
AV Dissociation
- AV dissociation with ventricular rate faster than atrial rate is diagnostic of VT 1, 2
- P waves continue independently of QRS complexes, visible in only 30% of VT cases 1
- Physical examination may reveal irregular cannon A waves in jugular venous pulse and variable intensity of first heart sound 1
Fusion and Capture Beats
- Fusion complexes are pathognomonic of VT, representing a merger between conducted supraventricular impulses and ventricular depolarization 1, 2
- These beats occur during AV dissociation and confirm ventricular origin 1
QRS Width Criteria
- QRS >140 ms with RBBB pattern strongly favors VT 1
- QRS >160 ms with LBBB pattern strongly favors VT 1
- These criteria are less helpful when pre-existing bundle branch block, accessory pathway conduction, or antiarrhythmic drugs (Class Ia or Ic) are present 1
QRS Morphology Features in Precordial Leads (V1-V6)
Brugada Criteria
- Absence of RS complex in all precordial leads (V1-V6) implies VT 1, 2
- RS interval >100 ms (from onset of R wave to nadir of S wave) in any precordial lead is highly suggestive of VT 1, 2
Concordance Pattern
- Negative concordance (all QS complexes in V1-V6) is diagnostic for VT 1
- Positive concordance (all positive deflections in V1-V6) strongly suggests VT 1, 2
- Positive concordance does not exclude antidromic AVRT over a left posterior accessory pathway 1
Lead aVR Criteria (Vereckei Algorithm)
- Initial R wave in aVR implies VT 1, 2
- Initial R or Q wave >40 ms in aVR implies VT 1, 2
- Notch on descending limb at onset of predominantly negative QRS in aVR implies VT 1
Lead II Criteria
Additional Morphological Features
- QR complexes indicate myocardial scar and are present in ~40% of post-MI VT cases 1, 2
- QRS morphology different from patient's baseline sinus rhythm suggests VT 2
- Similar morphology between premature ventricular beats during sinus rhythm and the tachyarrhythmia strongly suggests VT 2
Important Clinical Context
- History of prior myocardial infarction with new wide-complex tachycardia strongly indicates VT 1, 2
- Hemodynamic stability does NOT rule out VT—patients can be stable with VT 3
Critical Pitfalls to Avoid
- QRS width and morphology criteria become less specific in patients taking antiarrhythmic drugs (Class Ia, Ic), those with hyperkalemia, or severe heart failure 1
- Pre-existing bundle branch block can make SVT appear as wide-complex tachycardia 3
- SVT with accessory pathway conduction (antidromic AVRT) can mimic VT with wide QRS 1, 3
- When diagnosis is uncertain, treat as VT—this is the safest approach 1, 3
Variants of VT
- Monomorphic VT: Stable single QRS morphology throughout 1
- Polymorphic VT: Changing or multiform QRS morphology with cycle length 180-600 ms 1
- Torsades de Pointes: Polymorphic VT associated with long QT/QTc, with twisting of QRS peaks around isoelectric line 1
- Bidirectional VT: Beat-to-beat alternans in QRS frontal plane axis, often associated with digitalis toxicity 1