How do you differentiate Right Bundle Branch Block (RBBB) from Ventricular Tachycardia (VT)?

Differentiating RBBB from Ventricular Tachycardia

When faced with a wide QRS complex tachycardia showing RBBB morphology, assume VT until proven otherwise—this approach prevents the potentially fatal error of treating VT with AV nodal blockers like verapamil or diltiazem. 1

Key Diagnostic Features Favoring VT

AV Dissociation (Most Specific)

• AV dissociation with ventricular rate faster than atrial rate is pathognomonic for VT 1
• Look for independent P waves marching through the QRS complexes (visible in only 30% of VTs) 1
• Fusion beats are pathognomonic for VT—these represent a merger between conducted supraventricular impulses and ventricular depolarization 1
• On physical exam: irregular cannon A waves in jugular venous pulse, variable loudness of S1, and variable systolic blood pressure all suggest AV dissociation 1

QRS Width Criteria

• QRS duration >140 ms with RBBB morphology strongly favors VT 1
• For LBBB morphology, QRS >160 ms favors VT 1
• Note: These criteria don't apply if the patient has pre-existing bundle branch block or is on class Ia/Ic antiarrhythmics 1

QRS Morphology in Precordial Leads

• RS interval (from initial R to nadir of S) >100 ms in any precordial lead is highly suggestive of VT 1
• Negative concordance (QS complexes in all precordial leads) is diagnostic for VT 1
• Positive concordance suggests VT but doesn't exclude antidromic AVRT over a left posterior accessory pathway 1
• QR complexes indicate myocardial scar and are present in ~40% of post-MI VTs 1

Features Suggesting SVT with Aberrancy

Clinical Context

• History of prior myocardial infarction strongly indicates VT when wide QRS tachycardia occurs 1
• Younger patients without structural heart disease are more likely to have SVT with aberrancy 2
• Hemodynamic stability does NOT differentiate SVT from VT—both can be stable 1

RBBB Morphology Characteristics (Supraventricular Origin)

• Complete RBBB pattern: QRS ≥120 ms with rsr', rsR', or rSR' in V1-V2 1
• S wave duration greater than R wave or >40 ms in leads I and V6 1
• Normal R peak time in V5-V6 but >50 ms in V1 1
• QRS morphology identical to baseline ECG during sinus rhythm (if available) 1

Practical Diagnostic Algorithm

Step 1: Look for AV Dissociation

• Examine all leads carefully for independent P waves 1
• Search for fusion or capture beats 1
• If present → Diagnose VT

Step 2: Measure QRS Duration

• RBBB morphology with QRS >140 ms → Strongly favors VT 1
• Measure RS interval in all precordial leads 1
• If RS >100 ms in any lead → Diagnose VT

Step 3: Assess QRS Morphology

• Check for negative concordance in precordial leads → Diagnostic for VT 1
• Look for QR complexes suggesting scar → Favors VT 1

Step 4: Consider Clinical Context

• Prior MI + wide QRS tachycardia → Treat as VT 1
• Structural heart disease present → More likely VT 3, 2

Step 5: Use Esophageal Leads if Needed

• When P waves are not visible on surface ECG, esophageal pill electrodes can help identify atrial activity 1

Critical Management Pitfall

Never give verapamil or diltiazem for wide QRS tachycardia unless you have definitively proven it is SVT—these medications can precipitate hemodynamic collapse in VT patients 1. When in doubt, treat as VT with DC cardioversion if unstable, or procainamide/amiodarone if stable 1.

Special Considerations in ARVC

• In arrhythmogenic right ventricular cardiomyopathy, RBBB VT morphology occurs in 17% of cases, with 62% actually originating from the RV (not LV as traditionally taught) 3
• Early precordial transition (V2-V3) with superior/leftward axis suggests RV origin 3
• RBBB with positive R waves to V5-V6 and rightward axis suggests LV basal lateral origin 3