What are the criteria for diagnosing ventricular tachycardia?

Diagnostic Criteria for Ventricular Tachycardia

Ventricular tachycardia is defined as a cardiac arrhythmia of 3 or more consecutive ventricular complexes at a rate >100 beats/min (cycle length <600 ms) that emanates from the ventricles. 1

Core Electrocardiographic Criteria

Basic Definition

• Three or more consecutive ventricular complexes at a rate exceeding 100 beats/min is the fundamental criterion 1
• QRS duration typically >120 ms in adults distinguishes VT from narrow-complex supraventricular rhythms 1, 2
• In infants, VT may present with QRS <90 ms but with morphology different from sinus QRS 2

Duration-Based Classification

• Sustained VT: Duration >30 seconds and/or requires termination due to hemodynamic compromise in <30 seconds 1
• Nonsustained VT: Three or more consecutive beats that self-terminate in <30 seconds 1

Pathognomonic ECG Features

AV Dissociation (Most Specific)

• Presence of AV dissociation with ventricular rate faster than atrial rate is pathognomonic for VT 1, 2, 3
• Independent P waves continuing unrelated to QRS complexes during tachycardia strongly indicate VT 2
• Look for irregular cannon A waves in jugular venous pulse and variability in first heart sound loudness on physical examination 1

Fusion and Capture Beats

• Fusion beats (combination of supraventricular and ventricular complexes) are diagnostic of VT 1, 2, 3
• These represent merger between conducted sinus impulses and ventricular depolarization 3

QRS Morphology Criteria

Precordial Lead Analysis (Brugada Criteria)

• Absence of any R-S complexes in leads V1-V6 implies VT 1, 2, 3
• R-S interval (onset of R wave to nadir of S wave) >100 ms in any precordial lead strongly suggests VT 1, 2, 3
• QRS concordance (all precordial leads showing either positive or negative deflections) is diagnostic for VT 1, 2, 3

Lead aVR Analysis (Vereckei Algorithm)

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

QRS Width Criteria

• QRS width >140 ms with right bundle branch block pattern favors VT 1, 3
• QRS width >160 ms with left bundle branch block pattern favors VT 1, 3

Lead II Analysis

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

Additional Morphological Features

• QR complexes indicate myocardial scar and are present in approximately 40% of post-MI VT patients 1, 2
• Similar morphology between premature ventricular beats during sinus rhythm and the tachyarrhythmia strongly suggests VT 2

Clinical Context That Strengthens VT Diagnosis

Patient History

• Patients with prior myocardial infarction who develop wide QRS tachycardia are highly likely to have VT 1, 2
• First occurrence of wide QRS-complex tachycardia after an infarct strongly indicates VT 1

Critical Pitfalls to Avoid

Common Diagnostic Errors

• Never assume hemodynamic stability rules out VT - stable vital signs do not distinguish between SVT and VT 3
• Relying solely on QRS width can be misleading when patients have pre-existing bundle branch blocks or are taking class Ia or Ic antiarrhythmic drugs 1, 2
• Electrolyte abnormalities (especially hyperkalemia) or metabolic disorders can cause wide QRS complexes mimicking VT 1, 2
• Pre-existing bundle branch block can make SVT appear as wide complex tachycardia 3
• SVT with accessory pathway conduction (antidromic AVRT) can present with wide QRS 3

Management Principle

• When the diagnosis of wide QRS-complex tachycardia is uncertain, treat as VT rather than SVT 1, 2, 3
• Never use calcium channel blockers in undiagnosed wide complex tachycardias as they can precipitate hemodynamic collapse in VT 3

Diagnostic Approach Algorithm

1. Confirm wide QRS complex (>120 ms) at rate >100 bpm 1, 2
2. Search for AV dissociation or fusion beats - if present, diagnosis is VT 1, 2, 3
3. Apply Brugada criteria: Check for absence of R-S complexes or R-S interval >100 ms in precordial leads 1, 2, 3
4. Apply Vereckei algorithm: Examine lead aVR for initial R wave, Q wave >40 ms, or notching 1, 2, 3
5. Check for QRS concordance in precordial leads 1, 2, 3
6. Assess QRS width: >140 ms (RBBB) or >160 ms (LBBB) favors VT 1, 3
7. Consider clinical context: Prior MI strongly suggests VT 1, 2