How do you differentiate between Electrocardiogram (ECG) readings of Supraventricular Tachycardia (SVT), Paroxysmal Supraventricular Tachycardia (PSVT), and Atrial Flutter?

ECG Differentiation of SVT, PSVT, and Atrial Flutter

Understanding the Terminology First

PSVT (Paroxysmal Supraventricular Tachycardia) is not a separate entity from SVT—it is simply SVT with abrupt onset and termination. The term "paroxysmal" describes the pattern of occurrence, not a different rhythm 1, 2. Therefore, the ECG features are identical; you're differentiating the type of SVT mechanism, with atrial flutter being one specific subtype.

Step 1: Assess Rhythm Regularity

Start by determining if the ventricular rhythm is regular or irregular:

• Irregular ventricular rate → Suggests atrial fibrillation, multifocal atrial tachycardia, or atrial flutter with variable AV conduction 3
• Regular ventricular rate → Proceed to next steps to differentiate between atrial flutter, AVNRT, AVRT, or atrial tachycardia 3

Critical pitfall: Atrial fibrillation with rapid ventricular response can appear deceptively regular and be misdiagnosed as regular SVT 3. Always look carefully at R-R interval variation over multiple beats.

Step 2: Identify Atrial Activity and Rate Relationship

Atrial Flutter ECG Features:

• Atrial rate 250-350 bpm (typically 300 bpm) with characteristic "sawtooth" flutter waves best seen in leads II, III, aVF, and V1 3
• Atrial rate exceeds ventricular rate (e.g., 2:1,3:1, or 4:1 AV conduction) 3
• No isoelectric baseline between flutter waves—continuous undulating pattern 3
• Variable AV block produces different ventricular rates (e.g., alternating 2:1 and 4:1 conduction) 3

AVNRT (Most Common PSVT) ECG Features:

• Rate typically 150-250 bpm with extreme regularity after first 10-20 beats 3, 1
• P waves hidden within or at the terminal portion of QRS because atrial and ventricular activation occur nearly simultaneously 3
• Pseudo S-waves in inferior leads (II, III, aVF) and pseudo R' in V1—these are actually the terminal portion of retrograde P waves 3
• "Short RP" pattern (RP interval < PR interval) 3
• 1:1 AV relationship (rare exceptions with 2:1 block) 3

AVRT (Accessory Pathway-Mediated) ECG Features:

• Rate 150-250 bpm with regular rhythm 1, 2
• P wave visible in early ST-T segment following QRS (orthodromic AVRT) 3
• RP interval typically 70-100 msec or longer 1
• "Short RP" pattern in typical orthodromic AVRT 3
• May show pre-excitation (delta wave) on baseline sinus rhythm ECG if Wolff-Parkinson-White syndrome 3

Atrial Tachycardia ECG Features:

• Rate typically 150-250 bpm 3
• P wave morphology differs from sinus and is usually visible near end of or after T wave 3
• "Long RP" pattern (RP interval > PR interval) because rhythm is driven by atrium with normal AV conduction 3
• 1:1 AV conduction in most cases 3

Step 3: Examine P Wave Morphology and Location

The relationship between P waves and QRS complexes is the key differentiating feature:

• P waves buried in QRS → AVNRT (60% of cases have no visible P waves) 3
• P waves immediately after QRS (RP < 100 msec) → AVRT 3, 1
• P waves in ST-T segment with long RP → Atrial tachycardia or atypical AVNRT 3
• Flutter waves without discrete P waves → Atrial flutter 3

Step 4: Compare to Baseline Sinus Rhythm ECG

Always obtain a 12-lead ECG during both tachycardia and sinus rhythm 3:

• QRS identical to sinus rhythm → Confirms supraventricular origin (>90% of pediatric/young adult SVT) 3
• Pre-excitation (delta wave) on baseline ECG → Suggests accessory pathway and AVRT mechanism 3
• Different P wave morphology during tachycardia → Helps identify atrial tachycardia focus 3, 4

Critical Diagnostic Pitfalls to Avoid

Atrial fibrillation with prominent atrial activity is frequently misdiagnosed as atrial flutter—only 31% of physicians correctly identified AF with prominent atrial activity in multiple leads 5. Look for true regularity of atrial activity and consistent flutter wave morphology to confirm flutter.

Wide-complex tachycardia requires different approach: QRS >120 ms may represent ventricular tachycardia or SVT with aberrancy 3. The presence of AV dissociation or fusion complexes confirms VT 3. However, persistent aberration in SVT is exceedingly rare in infants and young patients (<3%), so wide QRS usually indicates VT 3.

Practical Algorithm Summary

1. Regular vs irregular? → Irregular suggests AF or variable-block flutter 3
2. Atrial rate > ventricular rate? → Atrial flutter or AT 3
3. Can you see discrete P waves?
   • No → Likely AVNRT or flutter waves 3
   • Yes, immediately after QRS → AVRT 3, 1
   • Yes, in ST-T segment → Atrial tachycardia 3
4. Sawtooth pattern without isoelectric baseline? → Atrial flutter 3
5. Compare with sinus ECG → Confirms mechanism and identifies pre-excitation 3, 4