How do you differentiate between rate-related right bundle branch block (RBBB) and ventricular tachycardia (VT) in a patient presenting with a wide complex tachycardia?

Differentiating Rate-Related RBBB from Ventricular Tachycardia

When confronted with a wide complex tachycardia showing RBBB morphology, you must presume ventricular tachycardia until proven otherwise and treat accordingly, as misdiagnosing VT as SVT and administering calcium channel blockers can cause hemodynamic collapse and death. 1, 2, 3

Critical Clinical Context First

Before analyzing the ECG, obtain these specific historical details that fundamentally alter your diagnostic approach:

• History of prior myocardial infarction has >95% positive predictive value for VT and is the single most important clinical factor—if present, you are dealing with VT until definitively proven otherwise 1
• First occurrence of wide QRS tachycardia after MI strongly indicates VT 1, 2
• Absence of structural heart disease shifts probability toward SVT with aberrancy, though this does NOT rule out VT 1

Algorithmic ECG Differentiation

Step 1: Look for Pathognomonic VT Features (If Present, Diagnosis is Made)

• AV dissociation with ventricular rate faster than atrial rate proves VT, though visible in only 30% of cases 4, 1, 2, 3
• Fusion complexes (merger of conducted supraventricular impulses with ventricular depolarization) are pathognomonic for VT 4, 1, 3
• Look for these on physical exam as well: irregular cannon A waves in jugular venous pulse, variable loudness of S1, and variable systolic blood pressure all indicate AV dissociation 4

Step 2: Apply QRS Width Criteria

• QRS width >140 ms with RBBB pattern favors VT (for rate-related RBBB in SVT, width is typically <140 ms) 4, 2, 3
• Rate-related bundle branch block in SVT typically occurs due to rapid rate rendering one bundle refractory, but the QRS morphology should still resemble typical RBBB with width <140 ms 4, 2

Step 3: Analyze Lead V6 R/S Ratio (Particularly Useful for RBBB Pattern)

• In RBBB pattern tachycardia with R/S ratio <1.0 in V6, an RS/QRS ratio >0.41 in lead V6 indicates VT with 97% sensitivity and 90% specificity 5
• The RS/QRS ratio is calculated as: (interval from QRS onset to nadir of S wave) ÷ (total QRS width) in lead V6 5
• This criterion is particularly valuable for differentiating fascicular VT from SVT with RBBB 5

Step 4: Apply Additional VT Criteria

• RS interval >100 ms in any precordial lead is highly suggestive of VT 2, 3
• Negative concordance pattern (all precordial leads negative) is diagnostic for VT 2
• R-wave peak time ≥50 ms in lead II suggests VT (though this is less reliable with pre-existing left BBB) 3, 6

Rate-Related RBBB Characteristics (SVT with Aberrancy)

Rate-related BBB has these distinguishing features:

• Typically initiated by long-short sequence, not just rapid rate alone 4
• QRS morphology resembles typical RBBB pattern with rsR' in V1 and qRS in V6 4
• QRS width usually <140 ms 4, 2
• If rate-related RBBB develops during orthodromic AVRT, tachycardia rate may slow if BBB is ipsilateral to bypass tract location 4

The Most Dangerous Pitfall

Never administer verapamil or diltiazem for wide complex tachycardia of uncertain etiology—this is the most lethal error, as these agents precipitate hemodynamic collapse in VT patients 4, 1, 2, 3. The guideline is unequivocal: if you cannot definitively prove SVT, treat as VT 4, 1, 2, 3.

Treatment Algorithm Based on Diagnosis

If Hemodynamically Unstable (Either Diagnosis)

• Immediate synchronized DC cardioversion is the only appropriate intervention 1, 2

If Stable and Diagnosis Uncertain

• Treat as VT with IV procainamide, sotalol, or amiodarone 2, 3
• Never use calcium channel blockers 4, 1, 2, 3

If Definitively Proven SVT with Rate-Related RBBB

• Vagal maneuvers followed by adenosine are appropriate 1
• Calcium channel blockers can be considered only after definitive diagnosis 1

Important Nuance Regarding RBBB Pattern Without Ischemic Heart Disease

One retrospective study found that in patients with RBBB pattern wide complex tachycardia without underlying ischemic heart disease, calcium channel blockers successfully converted 22 of 23 patients (96%), including 14 cases ultimately diagnosed as VT 7. However, this contradicts guideline recommendations and should not change your approach—guidelines explicitly warn against this practice because the risk of hemodynamic collapse outweighs potential benefit 4, 1, 2, 3.

Post-Conversion Management

• All patients with wide QRS tachycardia of uncertain etiology require referral to electrophysiology 2
• VT in setting of prior MI mandates ICU admission, immediate cardiology consultation, and consideration for ICD placement and EP study 1