Navigating the differentiation between Wide Complex Tachycardia (WCT) and Narrow Complex Tachycardia (NCT) is an important task in the CCU or Emergency Department. While NCT is almost always supraventricular, WCT is the true diagnostic “fork in the road” where assuming Ventricular Tachycardia (VT) is the safest clinical default.
The Fundamental Distinction
The width of the QRS complex represents the efficiency of ventricular depolarization.
- NCT (QRS < 120 ms): Depolarization occurs via the rapid His-Purkinje system. It is almost exclusively SVT (AVNRT, AVRT, Atrial Flutter/Fibrillation).
- WCT (QRS ≥ 120 ms): Depolarization is sluggish, occurring through the myocardium or a compromised conduction system. 80% of WCT cases are VT.
The WCT Differential Diagnosis
When faced with a WCT, consider these four categories:
- Ventricular Tachycardia (VT): The most common and dangerous.
- SVT with Aberrancy: SVT (like AFib or Atrial Flutter) occurring alongside a fixed or rate-dependent Bundle Branch Block (RBBB/LBBB).
- SVT with Pre-excitation: Antegrade conduction over an accessory pathway (e.g., Wolff-Parkinson-White syndrome).
- Metabolic/Drug-Induced: Hyperkalemia or Class Ia/Ic antiarrhythmic toxicity.
The “Brugada” Diagnostic Roadmap
For a rapid clinical decision, the Brugada Algorithm remains the gold standard for distinguishing VT from SVT with aberrancy.
| Step | Finding | Diagnosis |
| 1. Concordance | Absence of RS complexes in all precordial leads (V1-V6)? | VT |
| 2. RS Interval | Is the RS interval > 100 ms in any precordial lead? | VT |
| 3. AV Dissociation | P-waves dissociated from QRS, capture beats, or fusion beats? | VT |
| 4. Morphology | Does the QRS meet specific VT criteria? | VT |
Morphological “Red Flags” for VT:
- Northwest Axis: QRS is negative in I and aVF (“No Man’s Land”).
- Josephson’s Sign: Notching near the end of the S-wave.
- Brugada’s Sign: The distance from the onset of the QRS to the nadir of the S-wave is > 100 ms.
Simplified Clinical Rule of Thumb
If the diagnosis is uncertain, treat as VT.
Warning: Administering Calcium Channel Blockers (Verapamil/Diltiazem) to a patient in VT can cause catastrophic cardiovascular collapse and VFib. If the patient is hemodynamically unstable (hypotension, altered mental status, chest pain), skip the algorithm and proceed to immediate synchronized cardioversion.
Summary Table
| Feature | Favoring SVT | Favouring VT |
| Regularity | Often regular (except AFib) | Often regular (can be slightly irregular) |
| QRS Width | < 120 ms (usually) | > 140 ms (RBBB) or > 160 ms (LBBB) |
| AV Dissociation | Absent | Present (highly specific) |
| Fusion/Capture Beats | Absent | Present |
| History | Previous SVT episodes | Previous MI, CHF, or Low EF |