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 Complexes

Bundle branch block can be differentiated from pathological wide QRS complexes (such as ventricular tachycardia) by assessing QRS morphology, width, AV dissociation, and specific lead patterns, with QRS duration <140ms for RBBB or <160ms for LBBB suggesting bundle branch block, while wider complexes, AV dissociation, and atypical morphologies indicate pathological causes. 1

Key Diagnostic Criteria

QRS Width Assessment

• Bundle branch block typically has:
  • QRS duration <140 ms with right bundle branch block (RBBB) pattern
  • QRS duration <160 ms with left bundle branch block (LBBB) pattern
• Pathological wide QRS (especially ventricular tachycardia) typically has:
  • QRS >140 ms with RBBB pattern
  • QRS >160 ms with LBBB pattern 1

Definitive Signs of Pathological Wide QRS (Ventricular Tachycardia)

1. AV dissociation (pathognomonic but only visible in ~30% of VT cases)
2. Fusion complexes or capture beats
3. RS interval >100 ms in any precordial lead
4. QRS concordance across precordial leads (especially negative concordance) 1, 2

Morphological Criteria by Lead

Lead V1

• Bundle Branch Block: Typical RBBB pattern (rsR') or LBBB pattern (QS or rS)
• Pathological (VT): Monophasic R wave or qR complex with RBBB pattern 1

Lead V6

• Bundle Branch Block: Typical patterns (qRs for RBBB, R wave for LBBB)
• Pathological (VT): QS or rS complex with LBBB pattern 1

Lead aVR

• Pathological (VT): Any of the following strongly suggests VT:
  • Initial R wave (R or Rs complexes)
  • Initial r or q wave >40 ms in duration
  • Notch on descending limb of predominantly negative QRS 1, 2

Advanced Diagnostic Algorithm

1. First, check for AV dissociation:

   • Look for P waves unrelated to QRS complexes
   • Check for irregular cannon A waves in jugular venous pulse
   • Variability in first heart sound loudness 1

2. Assess QRS width:

   • Measure in multiple leads (QRS may appear narrower in some leads)

   • 140 ms (RBBB) or >160 ms (LBBB) suggests pathological cause 1, 3

3. Evaluate RS interval in precordial leads:

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

4. Check for QRS concordance:

   • Especially negative concordance (all QS complexes in precordial leads) is diagnostic for VT 1

5. Compare with previous ECGs:

   • QRS complex identical to sinus rhythm suggests SVT rather than VT 1

Common Pitfalls and Caveats

• Don't rely on hemodynamic stability: Both VT and SVT can present with stable vital signs 1
• Don't rely on single criteria: No single criterion is 100% sensitive or specific 1
• Consider clinical context: History of structural heart disease increases likelihood of VT 1
• Beware of preexisting bundle branch block: SVT with pre-existing bundle branch block can mimic VT 1, 2
• Don't rely on heart rate alone: While VT can have rates of 200-500 bpm and SVT typically 260-300 bpm in infants, there is significant overlap 1
• Antiarrhythmic drugs can widen QRS: This can complicate diagnosis 2

When in Doubt

If unable to differentiate with certainty, treat as ventricular tachycardia, especially before administering calcium channel blockers which can cause hemodynamic collapse in VT 1. A test dose of intravenous adenosine can be used diagnostically in stable patients 4.