How do you differentiate a bundle branch block from a pathological wide QRS (quantitative electrocardiogram measurement) complex?

Differentiating Bundle Branch Block from Pathological Wide QRS Complex

The most reliable way to differentiate bundle branch block from pathological wide QRS complexes is to look for AV dissociation, fusion complexes, and specific QRS morphology features that are pathognomonic for ventricular tachycardia. 1, 2

Key Diagnostic Features

Evidence of AV Dissociation

• AV dissociation with ventricular rate faster than atrial rate is pathognomonic for ventricular tachycardia (VT), though only visible in about 30% of VT cases 1
• Look for:
  • Irregular cannon A waves in jugular venous pulse
  • Variability in first heart sound loudness
  • Variability in systolic blood pressure
  • P waves that are dissociated from QRS complexes

Fusion and Capture Beats

• Fusion complexes (merger between conducted sinus impulses and ventricular depolarization) are pathognomonic for VT 1
• Capture beats (normal QRS complexes that appear intermittently during tachycardia) indicate VT

QRS Width and Morphology

• QRS width criteria:

  • QRS >140 ms with right bundle branch block (RBBB) pattern suggests VT 1
  • QRS >160 ms with left bundle branch block (LBBB) pattern suggests VT 1
  • Normal bundle branch block typically has QRS duration <140 ms (RBBB) or <160 ms (LBBB)

• RS interval >100 ms in any precordial lead strongly suggests VT 1, 2

• QRS concordance across precordial leads (especially negative concordance) is diagnostic for VT 1

Morphological Clues in Specific Leads

Lead V1 and V6 Analysis

• In V1, monophasic R wave or qR complex suggests VT with RBBB pattern
• In V1, rS complex with R/S ratio <1 suggests true RBBB
• In V6, QS or rS complex suggests VT with LBBB pattern
• In V6, monophasic R wave suggests true LBBB

Lead aVR Analysis

• Initial R wave in aVR suggests VT 2
• Initial q or r wave >40 ms in aVR suggests VT 2
• Notch on descending limb of predominantly negative QRS suggests VT 2

Clinical Context Considerations

• Pre-existing bundle branch block:

  • Compare current QRS morphology with previous ECGs
  • Different QRS morphology during tachycardia compared to baseline pre-existing bundle branch block suggests VT 3
  • QRS identical to sinus rhythm suggests SVT with pre-existing BBB 2

• In wide QRS with ventricular pacing or bundle branch block, apply this formula for QTc assessment:

  • Wide QRS complex adjusted QTc = QTc + [QRS - 120 ms] 1

Pitfalls to Avoid

• Relying on hemodynamic stability to differentiate - both VT and SVT can present with stable vital signs 2
• Misdiagnosing VT as SVT with aberrancy can lead to inappropriate treatment with calcium channel blockers, potentially causing hemodynamic collapse 2
• Overreliance on single criteria - no single criterion is 100% sensitive or specific 2

Algorithm for Differentiation

1. First, look for AV dissociation or fusion beats (pathognomonic for VT)
2. Check QRS width (>140 ms RBBB or >160 ms LBBB suggests VT)
3. Examine RS interval in precordial leads (>100 ms suggests VT)
4. Analyze QRS morphology in V1, V6, and aVR
5. Look for QRS concordance across precordial leads
6. Compare with previous ECGs if available

When in doubt, treat as ventricular tachycardia, especially before administering calcium channel blockers which can cause hemodynamic collapse in VT. 1, 2