Narrow Complex Tachycardia: ECG-Based Diagnostic Approach

 

Narrow Complex Tachycardia: ECG-Based Diagnostic Approach

Narrow complex tachycardia (NCT) refers to a tachyarrhythmia with a heart rate >100 bpm and a QRS duration <120 ms. A narrow QRS indicates that ventricular activation occurs via the normal His–Purkinje system, implying a supraventricular origin of the rhythm.

Systematic ECG analysis allows rapid and accurate diagnosis, which is crucial for appropriate management.

---

Definition and ECG Criteria

• Heart rate >100 beats per minute

• QRS duration <120 ms

• Usually supraventricular in origin

---

Step 1: Assess Rhythm Regularity

The first and most important step is to determine whether the rhythm is regular or irregular.

---

Regular Narrow Complex Tachycardia

Common causes include:

1. Sinus tachycardia

• Gradual onset and offset

• Normal P waves preceding each QRS

• PR interval constant

2. Atrial flutter with regular block (e.g., 2:1 block)

• Ventricular rate often ~150 bpm

• Flutter waves may be hidden

3. AVNRT (Atrioventricular Nodal Re-entrant Tachycardia)

• Sudden onset/termination

• P waves absent or retrograde (pseudo R′ in V1 or pseudo S in inferior leads)

4. AVRT (Orthodromic)

• Re-entry using an accessory pathway

• QRS narrow because antegrade conduction is via AV node

5. Atrial tachycardia (unifocal)

• Abnormal P-wave morphology

• Isoelectric baseline between P waves

6. Junctional tachycardia

• P waves absent or retrograde

• Usually due to enhanced AV junction automaticity

---

Irregular Narrow Complex Tachycardia

Common causes include:

1. Atrial fibrillation

• Irregularly irregular rhythm

• No discrete P waves

2. Atrial flutter with variable block

• Flutter waves with changing conduction ratios

3. Multifocal atrial tachycardia

• At least three different P-wave morphologies

• Irregular rhythm, often in COPD or hypoxia

---

Step 2: Analyze RP and PR Intervals (in Regular NCT)

In regular narrow complex tachycardia, RP–PR relationship helps narrow the diagnosis further.

---

Short RP Tachycardia (RP < PR)

1. Typical AVNRT

• Most common SVT

• Retrograde P waves buried in or just after QRS

2. AVRT (most cases)

• Retrograde P waves shortly after QRS

---

Long RP Tachycardia (RP > PR)

1. Atypical AVNRT

• Retrograde P waves clearly visible after QRS

2. Atrial tachycardia

• P waves precede QRS but RP interval longer

3. Atrial flutter

• Flutter waves may mimic long RP pattern

4. Junctional tachycardia (most cases)

• Retrograde atrial activation

5. Sinus tachycardia

• Physiologic long RP with normal P-wave axis

---

Clinical Importance

• Rapid differentiation guides acute therapy

• AV-nodal–dependent tachycardias respond to vagal maneuvers and adenosine

• Atrial arrhythmias may not terminate with AV nodal blockade

• Prevents misdiagnosis and inappropriate treatment

---

Key Takeaway

A structured ECG approach—starting with rhythm regularity and followed by RP–PR analysis—provides a powerful framework for diagnosing narrow complex tachycardias accurately and efficiently at the bedside.

For more cardiology articles and ECG infographics visit:

drmusmanjaved.com