Diagnostic Algorithm for Narrow Complex Tachycardia (NCT)

 

Diagnostic Algorithm for Narrow Complex Tachycardia (NCT)

Narrow complex tachycardia refers to a regular or irregular tachyarrhythmia with QRS duration <120 ms, implying ventricular activation via the normal His–Purkinje system. A systematic ECG-based approach allows rapid and accurate diagnosis at the bedside.

---

Step 1: Confirm It Is a Narrow Complex Tachycardia

• QRS duration <120 ms
• Ventricular rate usually >100 bpm
• If QRS is borderline, consider aberrancy or pre-excited tachycardia separately

---

Step 2: Assess Regularity of RR Interval

A. Regular Narrow Complex Tachycardia

Common causes:

• AVNRT
• AVRT (orthodromic)
• Atrial tachycardia
• Atrial flutter with fixed AV conduction (usually 2:1)

B. Irregular Narrow Complex Tachycardia

Common causes:

• Atrial fibrillation
• Atrial flutter with variable AV block
• Multifocal atrial tachycardia

---

Step 3: Look for P Waves and Their Relationship to QRS

Are P waves visible?

• Clearly visible before QRS → likely atrial tachycardia or sinus tachycardia
• Absent or hidden within QRS/ST segment → suggests AVNRT or AVRT
• Saw-tooth flutter waves → atrial flutter

RP vs PR Interval (for regular rhythms)

• Short RP tachycardia (RP < PR)
  • Typical AVNRT
  • Orthodromic AVRT
• Long RP tachycardia (RP > PR)
  • Atrial tachycardia
  • Atypical AVNRT
  • Permanent junctional reciprocating tachycardia (PJRT)

---

Step 4: Evaluate Atrial Activity Pattern

1. AVNRT

• Regular, narrow QRS
• P waves absent or pseudo R′ in V1 / pseudo S in inferior leads
• Sudden onset and termination

2. AVRT (Orthodromic)

• Regular NCT
• Retrograde P waves after QRS
• History of pre-excitation (may not be visible during tachycardia)

3. Atrial Tachycardia

• Abnormal P-wave morphology
• Isoelectric baseline between P waves
• Gradual onset/termination (warm-up, cool-down)

4. Atrial Flutter

• Flutter waves at ~250–350 bpm
• Commonly 2:1 AV conduction → ventricular rate ~150 bpm
• Vagal maneuvers may unmask flutter waves

5. Atrial Fibrillation

• Irregularly irregular RR intervals
• No discrete P waves
• Fibrillatory baseline

6. Multifocal Atrial Tachycardia

• Irregular rhythm
• ≥3 different P-wave morphologies
• Variable PR intervals
• Often in COPD or metabolic illness

---

Step 5: Response to Vagal Maneuvers or Adenosine

Terminates tachycardia

• AVNRT
• AVRT

Transient AV block unmasks atrial activity

• Atrial flutter
• Atrial tachycardia

No effect

• Atrial fibrillation
• MAT

---

Simplified ECG Algorithm (Text Flow)

1. Narrow QRS (<120 ms) + HR >100
2. Regular or Irregular?
   • Irregular → AF / Flutter (variable block) / MAT
   • Regular → proceed
3. P waves visible?
   • No → AVNRT / AVRT
   • Yes → measure RP
4. RP < PR → AVNRT / AVRT
5. RP > PR → Atrial tachycardia / Atypical AVNRT

---

Practical Clinical Pearls

• A ventricular rate of exactly ~150 bpm should always raise suspicion for atrial flutter with 2:1 block
• Adenosine is diagnostic even when not therapeutic
• Always review baseline ECG for evidence of pre-excitation
• In unstable patients, synchronized cardioversion overrides diagnostic algorithms

---

Take-Home Message

A narrow complex tachycardia can be accurately diagnosed using a structured ECG algorithm focusing on regularity, P-wave analysis, RP–PR relationship, and response to AV nodal blockade. Mastery of this approach is essential for safe acute management and long-term rhythm strategy.