How to Diagnose Ventricular Tachycardia on ECG
When faced with a wide QRS complex tachycardia (>120 ms), treat it as VT until proven otherwise—this approach prioritizes patient safety given VT's higher mortality risk. 1, 2, 3
Initial ECG Assessment: QRS Width is Your First Decision Point
- Narrow QRS (<120 ms): Almost always supraventricular tachycardia (SVT), not VT 2, 3
- Wide QRS (≥120 ms): Could be VT, SVT with bundle branch block, or SVT with accessory pathway conduction—proceed with systematic evaluation below 1, 2
Pathognomonic Features That Prove VT (Look for These First)
These findings definitively diagnose VT when present:
AV dissociation with ventricular rate faster than atrial rate—visible in only 30% of VT cases but diagnostic when present 1, 2, 3
Fusion beats: QRS complexes that represent a merger between a conducted supraventricular impulse and ventricular depolarization—pathognomonic for VT 1, 2, 3
Capture beats: Occasional narrow QRS complexes during wide complex tachycardia, representing conducted supraventricular beats 4
QRS Duration Criteria
- QRS >140 ms with RBBB pattern or >160 ms with LBBB pattern strongly favors VT 1, 3
- However, patients on class Ia or Ic antiarrhythmics, those with hyperkalemia, or severe heart failure can have wider QRS with SVT, reducing specificity 1
Morphologic Criteria in Precordial Leads (V1-V6)
Scan all precordial leads systematically:
- Negative concordance (all QRS complexes negative/QS pattern in V1-V6): Diagnostic for VT 1
- Positive concordance (all QRS complexes positive in V1-V6): Suggests VT, but doesn't exclude antidromic AVRT over left posterior accessory pathway 1
- RS interval >100 ms (from onset of R wave to nadir of S wave) in any precordial lead: Highly suggestive of VT 1, 2, 3
- QR complexes in V2-V6: Indicate myocardial scar, present in ~40% of post-MI VT 1, 2
Lead aVR Criteria (Often Overlooked but Powerful)
- Initial R wave in aVR: Suggests VT 2, 3
- Initial R or Q wave >40 ms in aVR: Implies VT 2, 3
- Notch on descending limb of predominantly negative QRS in aVR: Part of Vereckei algorithm, implies VT 3
Lead II Criteria
Clinical Context That Increases VT Probability
- Prior myocardial infarction with first wide QRS tachycardia after the infarct: Strongly indicates VT 1, 2
- Structural heart disease (89% of VT patients in one series had old MI) 5
- Similar morphology between premature ventricular beats during sinus rhythm and the tachycardia: Strongly suggests VT 2
Common Pitfalls to Avoid
- Never assume hemodynamic stability means SVT—VT can be well-tolerated, especially in younger patients 3, 4
- Never give calcium channel blockers (verapamil/diltiazem) to undiagnosed wide complex tachycardia—they can precipitate hemodynamic collapse in VT 1, 3
- Don't rely solely on QRS width when patient has pre-existing bundle branch block or takes antiarrhythmics 1, 3
- Beware of SVT with accessory pathway conduction (antidromic AVRT)—can mimic VT with very wide QRS 1, 3
- Automatic ECG interpretations are unreliable—always interpret manually 4
Systematic Approach When Diagnosis is Unclear
Step 1: Check for predominantly negative QRS in V4-V6—favors VT 5
Step 2: Look for QR complex in any of V2-V6—favors VT 5
Step 3: Assess AV relationship—more QRS than P waves proves VT 5
This three-step approach achieved 75% sensitivity and 100% specificity for VT diagnosis in coronary artery disease patients 5
Diagnostic Maneuvers During Tachycardia
Vagal maneuvers or adenosine during continuous ECG recording can help differentiate mechanisms 1, 4
Esophageal electrodes can help visualize P waves when not visible on surface ECG 1
When in Doubt: The Golden Rule
If you cannot definitively prove SVT, treat as VT—this is the safest approach given VT's higher morbidity and mortality risk. 1, 3, 4