How to differentiate types of narrow QRS (Quadrature Amplitude Modulation R-wave to S-wave) tachycardia on an electrocardiogram (ECG)?

Differentiation of Narrow QRS Tachycardia Types Based on ECG

The key to differentiating narrow QRS tachycardia types is identifying the relationship between P waves and QRS complexes, with particular attention to RP intervals, P wave morphology, and response to vagal maneuvers or adenosine. 1

Primary Classification of Narrow QRS Tachycardias

Narrow QRS tachycardias (QRS duration <120 ms) are almost always supraventricular in origin. The initial assessment should follow this algorithm:

1. Regular vs. Irregular Rhythm

   • Irregular rhythm: Suggests atrial fibrillation, multifocal atrial tachycardia (MAT), or atrial flutter/tachycardia with variable AV conduction
   • Regular rhythm: Proceed to P wave analysis 1

2. P Wave Identification

   • P waves visible: Analyze RP relationship
   • P waves not visible: Most likely AVNRT 1

Detailed Differentiation Based on P Wave Relationship

For Regular Tachycardias with Visible P Waves:

1. Short RP Interval (RP < PR)

   • RP < 90 ms: Typical AVNRT

     • Look for pseudo r' in V1 and pseudo S waves in inferior leads (II, III, aVF) 1
     • These findings are pathognomonic for AVNRT 1

   • RP > 90 ms: Orthodromic AVRT

     • P wave visible in early ST segment, separated from QRS by ~70 ms 1
     • Often associated with QRS alternans (28% of cases) 2, 3
     • May show ST-segment depression ≥2 mm or T-wave inversion (60% of cases) 2
     • Pre-excitation during sinus rhythm supports this diagnosis (42-45% of cases) 2, 3

2. Long RP Interval (RP > PR)

   • Atrial Tachycardia:

     • P wave morphology differs from sinus P wave 1
     • RP/PR ratio > 1 in 89-90% of cases 2, 3

   • Atypical AVNRT: Uncommon "fast-slow" variant 1

   • PJRT (Permanent form of junctional reciprocating tachycardia):

     • A form of AVRT with a slowly conducting accessory pathway 1

Response to Interventions

1. Vagal Maneuvers/Adenosine Response:
   • AVNRT: Usually terminates abruptly
   • AVRT: Usually terminates abruptly
   • Atrial Tachycardia: May cause transient AV block revealing continuing atrial activity
   • Atrial Flutter: Reveals flutter waves during transient AV block 1

Additional Diagnostic Features

• QRS Alternans: More common in AVRT (27-28% of cases) 2, 3
• Cycle Length Alternans: More common in AVNRT (6% of cases) 2
• P Wave Separation from QRS: More common in AVRT (68-70%) and atrial tachycardia (75-80%) 2, 3

Common Pitfalls to Avoid

1. Misidentification of P Waves:

   • Use multiple leads to identify P waves
   • Consider esophageal pill electrodes if P waves are not visible on surface ECG 1

2. Mistaking Wide QRS Tachycardia for SVT:

   • Always obtain a 12-lead ECG to confirm narrow QRS complex
   • Ventricular tachycardia can masquerade as SVT if only a single lead is examined 4

3. Overlooking AV Wenckebach Phenomenon:

   • "Grouped beating" pattern is critical for differentiating AT from AVRT and AVNRT 5

4. Relying Solely on ECG for Definitive Diagnosis:

   • Despite these criteria, approximately 20% of tachycardias may be incorrectly classified based on ECG alone 3
   • If diagnosis is uncertain and patient is unstable, treat as ventricular tachycardia 1

Multivariate Predictors of Tachycardia Type

Independent predictors of tachycardia mechanism include:

• Presence of P wave separate from QRS complex
• Pseudo r' deflection in lead V1
• QRS alternans during tachycardia
• Presence of pre-excitation during sinus rhythm 2, 3

These criteria correctly identify approximately 86% of AVNRT and 81% of AVRT cases 3.