How to differentiate between regular and irregular Supraventricular Tachycardia (SVT) and preexcited Atrial Fibrillation (AF)?

Differentiating Regular vs. Irregular SVT and Preexcited AF

The key to differentiating between regular and irregular SVT and preexcited AF is to systematically analyze the 12-lead ECG, focusing on regularity of rhythm, QRS morphology, and P wave characteristics. 1

Initial ECG Assessment

Step 1: Determine Regularity

• Regular rhythm: Suggests AVNRT, AVRT, atrial flutter with fixed conduction, or atrial tachycardia
• Irregular rhythm: Suggests AF, atrial flutter with variable conduction, or MAT (multifocal atrial tachycardia) 1

Step 2: Assess QRS Width

• Narrow QRS (<120 ms): Typical of most SVTs
• Wide QRS (≥120 ms): Consider:
  • SVT with aberrancy
  • Pre-excited tachycardia (antidromic AVRT)
  • Preexcited AF
  • Ventricular tachycardia (treat as VT if uncertain) 2

Specific Diagnostic Features

Regular SVT Patterns

1. AVNRT (AV Nodal Reentrant Tachycardia):

   • Regular tachycardia (150-250 bpm)
   • P waves often hidden within QRS or appearing as pseudo S waves in inferior leads
   • Short RP interval (RP < PR)
   • RP typically <90 ms 1, 3

2. AVRT (AV Reentrant Tachycardia):

   • Regular tachycardia (150-250 bpm)
   • P waves visible in early ST segment
   • Short RP interval (RP < PR)
   • RP typically >90 ms 1, 3

3. Atrial Tachycardia:

   • Regular rhythm with 1:1 AV conduction
   • P wave morphology differs from sinus P waves
   • Long RP interval (RP > PR) 1

Preexcited AF Characteristics

• Irregularly irregular rhythm
• Wide, bizarre QRS complexes with varying morphology
• Extremely rapid ventricular rates (often >200 bpm)
• Delta waves may be present but variable
• QRS polarity changes from beat to beat 1, 4

Critical Diagnostic Pitfalls

Preexcited AF vs. VT

To differentiate preexcited AF from VT:

• Preexcited AF shows beat-to-beat variations in QRS morphology
• VT typically has more uniform QRS complexes
• In preexcited AF, extremely short R-R intervals (<250 ms) are common
• Predominantly negative QRS complexes in V4-V6 favor VT over preexcited tachycardia 4

SVT with Aberrancy vs. VT

• AV dissociation (ventricular rate faster than atrial rate) is diagnostic of VT
• Fusion beats strongly suggest VT
• QRS width >140 ms with RBBB pattern or >160 ms with LBBB pattern suggests VT
• Concordance (all precordial leads positive or negative) suggests VT or preexcitation 2

Management Considerations

Caution with Medications

• Never administer calcium channel blockers (like verapamil) to patients with preexcited AF as this can accelerate conduction through the accessory pathway and precipitate ventricular fibrillation 5
• Avoid adenosine in preexcited AF as it may increase conduction through the accessory pathway

Diagnostic Maneuvers

• Vagal maneuvers may help differentiate:
  • AVNRT/AVRT often terminate with vagal maneuvers
  • Preexcited AF shows transient slowing of ventricular rate but typically doesn't terminate 1
  • Atrial tachycardia may show transient AV block revealing underlying atrial activity

Clinical Pearls

• Patients with AVNRT more frequently describe "shirt flapping" or "neck pounding" sensations (cannon a-waves) 1
• Preexcited AF is a potentially life-threatening arrhythmia requiring urgent management 5
• When in doubt about a wide-complex tachycardia, treat as VT until proven otherwise 2
• Patients with WPW syndrome are at risk of developing preexcited AF, which can degenerate into ventricular fibrillation 5

By systematically evaluating the ECG for rhythm regularity, QRS morphology, and P wave characteristics, clinicians can accurately differentiate between regular SVTs, irregular SVTs, and the potentially dangerous preexcited AF.