How do you differentiate between pathological wide QRS (Quantitative Rhythm Strip) complex and right bundle branch block (RBBB)?

Differentiating Pathological Wide QRS from Right Bundle Branch Block

The most reliable approach to differentiate pathological wide QRS complexes from right bundle branch block (RBBB) is to look for AV dissociation, fusion complexes, and specific QRS morphology features, as these are pathognomonic for ventricular tachycardia (VT). 1

Key Diagnostic Criteria

QRS Width Assessment

• RBBB typically has QRS duration <140 ms 2, 1
• QRS width >140 ms with RBBB pattern strongly suggests pathological origin (VT) 2, 1
• Normal RBBB morphology includes:
  • rsr', rsR', or rSR' pattern in leads V1 or V2
  • S wave duration greater than R wave or >40 ms in leads I and V6
  • Normal R peak time in leads V5 and V6 but >50 ms in lead V1 2

RS Interval and QRS Morphology

• RS interval >100 ms in any precordial lead is highly suggestive of VT 2, 1
• In V1, monophasic R wave or qR complex suggests VT with RBBB pattern 1
• In V6, QS or rS complex suggests VT with RBBB pattern 1
• RS/QRS ratio >0.41 in lead V6 is a reliable indicator of VT in RBBB pattern with reversed R/S complex 3

QRS Concordance

• QRS pattern with negative concordance across precordial leads is diagnostic for VT 2
• Positive concordance does not exclude antidromic AVRT over a left posterior accessory pathway 2

Evidence of AV Dissociation

• AV dissociation with ventricular rate faster than atrial rate is pathognomonic for VT 2, 1
• Look for:
  • Irregular cannon A waves in jugular venous pulse
  • Variability in first heart sound loudness
  • Variability in systolic blood pressure 2
• Fusion complexes (merger between conducted sinus impulses and ventricular depolarization) are pathognomonic of VT 2
• P waves not necessary for tachycardia maintenance strongly suggests VT 2

Additional Distinguishing Features

• Presence of QR complexes indicates myocardial scar (present in ~40% of post-MI VTs) 2
• Initial R wave in aVR or initial R/Q wave >40 ms in aVR suggests VT 1
• Notch on descending limb at onset of predominantly negative QRS suggests VT 1
• Presence of ventricular fusion beats indicates ventricular origin 2

Clinical Context Considerations

• History of previous myocardial infarction and first occurrence of wide QRS tachycardia after infarct strongly indicates VT 2
• RBBB can be benign in the absence of underlying cardiac disease but may have poor prognosis in coronary artery disease 4
• Incomplete RBBB (QRS duration between 110-119 ms) may be a normal variant, especially in athletes 5

Common Pitfalls

• Misdiagnosing VT as SVT with aberrancy can lead to inappropriate treatment with calcium channel blockers, potentially causing hemodynamic collapse 1
• When in doubt, treat as ventricular tachycardia, especially before administering calcium channel blockers 1
• Don't rely on hemodynamic stability to distinguish SVT from VT, as stable vital signs can occur in both conditions 1
• SVT with pre-existing bundle branch block can be mistaken for VT 1

By systematically applying these criteria, clinicians can more accurately differentiate between pathological wide QRS complexes and right bundle branch block, leading to appropriate treatment decisions and improved patient outcomes.