How do I differentiate broad QRS complex tachycardias?

Differentiating Broad QRS Complex Tachycardias

When encountering a broad QRS complex tachycardia (>120 ms), always assume it is ventricular tachycardia (VT) until proven otherwise, as misdiagnosis and inappropriate treatment can lead to significant morbidity and mortality. 1, 2

Initial Assessment

Clinical Context

• History of myocardial infarction or structural heart disease strongly suggests VT
• First occurrence of wide QRS tachycardia after infarction is highly indicative of VT 1
• Hemodynamic stability does NOT reliably differentiate SVT from VT

Key ECG Criteria for VT

1. AV Dissociation:

   • Ventricular rate faster than atrial rate (pathognomonic but visible in only 30% of VTs)
   • Look for irregular cannon A waves in jugular venous pulse
   • Variable loudness of first heart sound and systolic blood pressure

2. QRS Morphology:

   • RS interval >100 ms in any precordial lead (highly suggestive of VT)
   • Negative concordance in precordial leads (all precordial leads show QS complexes)
   • Presence of fusion or capture beats (diagnostic for VT)
   • QR complexes (present in ~40% of post-MI VTs)

3. QRS Width:

   • 140 ms with RBBB pattern or >160 ms with LBBB pattern

Lead-Specific Criteria

V1 Positive QRS Complexes

• Monophasic or biphasic QRS in V1
• Deep S wave in V6
• QRS width >140 ms
• Superior QRS axis

V1 Negative QRS Complexes

• QRS width >160 ms
• Right-sided QRS axis
• Broad R peak (>40 ms) in V1/V2
• Slurred S downstroke in V1/V2
• Any Q wave in V6

Diagnostic Algorithm

1. Is the tachycardia regular or irregular?

   • Irregular: Likely atrial fibrillation with aberrancy or pre-excitation
   • Regular: Continue evaluation

2. Look for AV dissociation:

   • If present → VT
   • If absent or unclear → Continue evaluation

3. Examine QRS morphology:

   • Negative concordance across precordial leads → VT
   • RS interval >100 ms in any precordial lead → VT
   • Fusion/capture beats → VT
   • QR complexes → VT

4. Compare with baseline ECG (if available):

   • Different QRS morphology than baseline BBB → Suggests VT
   • Identical to baseline BBB → Suggests SVT with pre-existing BBB

5. Response to adenosine:

   • No response → Likely VT
   • Gradual slowing then acceleration → Likely VT
   • Sudden termination → Likely SVT
   • Transient AV block revealing atrial activity → SVT

Important Caveats

• Morphological criteria are less reliable in patients taking certain antiarrhythmic drugs, those with hyperkalemia, or severe heart failure 1
• Wide QRS tachycardias are frequently misdiagnosed despite ECG criteria
• If diagnosis remains uncertain after evaluation, treat as VT 1, 2
• Avoid calcium channel blockers in undiagnosed wide QRS tachycardias as they may cause hemodynamic collapse in VT 1
• For hemodynamically unstable patients, immediate DC cardioversion is indicated regardless of the presumed mechanism 2

Special Considerations

• SVT with wide QRS can occur due to:

  1. Pre-existing bundle branch block
  2. Rate-related aberrant conduction
  3. Anterograde conduction over an accessory pathway (pre-excitation)

• Comparison with baseline ECG can be valuable in differentiating VT from SVT with pre-existing BBB 3

Remember that accurate differentiation is critical for appropriate treatment and patient outcomes, but when in doubt, treating as VT is the safest approach.