How do you differentiate between Supraventricular Tachycardia (SVT) and Ventricular Tachycardia (VT) in a patient with tachycardia?

Differentiating Between SVT and VT in Tachycardia

The most reliable way to differentiate between Supraventricular Tachycardia (SVT) and Ventricular Tachycardia (VT) is through QRS complex width, with VT typically presenting with QRS >120 ms, while SVT usually has narrow QRS <120 ms, though when in doubt, wide complex tachycardia should be treated as VT until proven otherwise. 1, 2

Initial Assessment: QRS Width

• Narrow QRS Complex (<120 ms):

  • Strongly suggests SVT
  • Common mechanisms: AVNRT, AVRT, atrial tachycardia
  • Typically regular rhythm with heart rate 150-250 bpm

• Wide QRS Complex (>120 ms):

  • May be VT or SVT with aberrancy
  • If diagnosis is uncertain, treat as VT (safer approach)
  • Administering calcium channel blockers to VT can cause hemodynamic collapse 1, 2

Diagnostic Criteria for VT in Wide Complex Tachycardia

1. AV Dissociation

• Ventricular rate faster than atrial rate (visible in only 30% of VTs)
• Pathognomonic for VT when present 1, 2
• Look for:
  • Irregular cannon A waves in jugular venous pulse
  • Variability in loudness of first heart sound
  • Variability in systolic blood pressure

2. QRS Morphology Criteria (Brugada Criteria) 1

• Absence of RS complexes in all precordial leads suggests VT
• RS interval >100 ms in any precordial lead suggests VT
• QRS width >140 ms with RBBB pattern or >160 ms with LBBB pattern suggests VT
• Concordance (all precordial leads showing either all positive or all negative deflections) suggests VT

3. Vereckei Algorithm (Lead aVR) 1

• Initial R wave suggests VT
• Initial R or Q wave >40 ms suggests VT
• Notch on descending limb of predominantly negative QRS suggests VT

4. Additional Features

• Fusion complexes (merger between conducted sinus impulses and ventricular depolarization) are pathognomonic for VT 1
• R-wave peak time ≥50 ms in lead II suggests VT 1
• History of myocardial infarction, heart failure, or angina has high predictive value for VT 2

SVT with Wide QRS Complex

Three main mechanisms for wide QRS in SVT:

1. Pre-existing bundle branch block or aberrancy

   • QRS morphology may be identical to sinus rhythm with BBB
   • Rate-related aberrancy often follows a long-short sequence

2. SVT with AV conduction over accessory pathway

   • Can occur with AT, atrial flutter, AF, AVNRT, or antidromic AVRT
   • Pre-excitation pattern may be present

3. SVT with functional aberrancy

   • Due to rate-related conduction delay
   • May normalize with slowing of rate

Diagnostic Maneuvers

1. Response to Vagal Maneuvers or Adenosine

• SVT often terminates or shows transient AV block revealing atrial activity
• VT typically unaffected
• Record 12-lead ECG during maneuvers 1

2. P Wave Analysis

• In AVNRT: P waves often hidden in QRS or visible as pseudo-R in V1 or pseudo-S in inferior leads
• In AVRT: P waves follow QRS with RP interval >70 ms
• In VT: AV dissociation may be present (independent P waves) 1, 2

3. RP Interval Assessment (for Narrow Complex)

• RP < PR: Typical AVNRT or AVRT
• RP > PR: Atypical AVNRT, PJRT, or atrial tachycardia 1

Clinical Pitfalls to Avoid

1. Relying on hemodynamic stability: Stable vital signs do not differentiate between VT and SVT 1, 2

2. Misdiagnosing VT as SVT: When in doubt about wide complex tachycardia, treat as VT 2

3. Administering calcium channel blockers to potential VT: Can precipitate cardiovascular collapse 1, 2

4. Overlooking pre-excitation: Can lead to misdiagnosis and inappropriate treatment 2

5. Focusing on single ECG lead: Always obtain 12-lead ECG for accurate diagnosis 2

Management Considerations

• If hemodynamically unstable: Immediate DC cardioversion regardless of diagnosis 2

• For stable patients with wide complex tachycardia of uncertain origin: Avoid calcium channel blockers 1, 2

• When diagnosis remains uncertain: Treat wide complex tachycardia as VT until proven otherwise 1