What are the characteristics of ventricular tachycardia (VT) on an electrocardiogram (ECG)?

ECG Characteristics of Ventricular Tachycardia

Ventricular tachycardia on ECG is characterized by a wide QRS complex (>120 ms), a rate exceeding 120 beats/min, and when present, AV dissociation is pathognomonic for the diagnosis. 1, 2, 3

Core Diagnostic Features

QRS Complex Width and Rate

• QRS duration >120 ms is the fundamental criterion, with VT favored when QRS width exceeds 140 ms in RBBB pattern or 160 ms in LBBB pattern 1, 2, 3
• Heart rate typically >120 beats/min, though in infants VT may present with rates of 200-500 beats/min 2
• The QRS morphology differs from the patient's baseline sinus rhythm QRS configuration 2

Pathognomonic Signs (When Present, Diagnostic of VT)

AV dissociation is the gold standard finding—when the ventricular rate exceeds the atrial rate with independent P waves marching through the QRS complexes, VT is confirmed 1, 2, 4. However, this is clearly visible in only 30% of VT cases 1.

Fusion beats represent a merger between conducted supraventricular impulses and ventricular depolarization, and are pathognomonic when identified 1, 2, 4.

Capture beats (intermittent normally conducted supraventricular beats during the tachycardia) also confirm VT 1.

Morphological Criteria in Precordial Leads

Critical Measurements

• RS interval >100 ms (measured from onset of R wave to nadir of S wave) in any precordial lead is highly suggestive of VT 1, 2, 4
• Negative concordance (all QS complexes across precordial leads V1-V6) is diagnostic for VT 1
• Positive concordance (all positive deflections in precordial leads) strongly suggests VT, though it doesn't exclude antidromic AVRT over a left posterior accessory pathway 1, 4

Lead-Specific Findings

• Initial R wave in lead aVR suggests VT 2, 4
• Initial R or Q wave >40 ms in aVR implies VT 2, 4
• R-wave peak time ≥50 ms in lead II suggests VT 2, 4
• QR complexes indicate myocardial scar and are present in approximately 40% of post-MI VT cases 1, 2

Clinical Context Clues

Historical Features

• Prior myocardial infarction with first occurrence of wide QRS tachycardia after the infarct strongly indicates VT 1, 2
• Similar morphology between premature ventricular beats during sinus rhythm and the tachyarrhythmia strongly suggests VT 2

Physical Examination Findings

When P waves are not visible on ECG, look for evidence of AV dissociation on physical exam 1:

• Irregular cannon A waves in the jugular venous pulse
• Variability in the loudness of the first heart sound
• Beat-to-beat variability in systolic blood pressure

Critical Pitfalls to Avoid

Never assume hemodynamic stability rules out VT—patients can maintain adequate blood pressure during VT, and this does not distinguish between SVT and VT 4.

Limitations of morphological criteria occur in several scenarios 1:

• Patients taking class Ia or Ic antiarrhythmic drugs (can widen QRS in SVT)
• Pre-existing bundle branch block (reduces diagnostic value of QRS morphology)
• Hyperkalemia or severe heart failure (alters QRS characteristics)
• Electrolyte abnormalities or metabolic disorders 4

When diagnosis is uncertain, treat as VT—this is the safest approach given the higher morbidity and mortality risk 1, 4, 5. Never use calcium channel blockers in undiagnosed wide complex tachycardias, as they can precipitate hemodynamic collapse in VT 4.

Diagnostic Algorithm Approach

1. Confirm wide QRS (>120 ms) and rate >120 bpm 2, 3
2. Search for AV dissociation, fusion beats, or capture beats—if present, diagnosis is VT 1, 2, 4
3. Measure RS interval in all precordial leads—if >100 ms in any lead, diagnose VT 1, 2, 4
4. Assess for concordance—negative concordance is diagnostic for VT 1, 2, 4
5. Evaluate lead aVR—initial R wave or Q >40 ms suggests VT 2, 4
6. Consider clinical context—prior MI with new wide complex tachycardia strongly favors VT 1, 2
7. If uncertain after systematic evaluation, treat as VT 1, 4, 5

Adjunctive Diagnostic Tools

Esophageal pill electrodes can be useful when P waves are not visible on surface ECG to better assess AV relationship 1.

Comparison with baseline ECG during sinus rhythm is valuable—if available, a QRS conduction delay during sinus rhythm reduces the value of morphology analysis 1.