ECG Differences Among Supraventricular Tachycardias
The key to differentiating supraventricular tachycardias on ECG is the relationship between the P wave and QRS complex: AVNRT shows P waves buried within or at the end of the QRS (RP <90 ms), AVRT displays P waves in the early ST segment (RP 90-200 ms), focal atrial tachycardia demonstrates distinct P waves with different morphology from sinus rhythm in the second half of the cycle (RP >PR, "long RP"), and atrial flutter reveals characteristic "sawtooth" flutter waves with atrial rates exceeding ventricular rates. 1
Algorithmic Approach to ECG Differentiation
Step 1: Assess Regularity and QRS Width
- Regular narrow QRS (<120 ms): Proceed to P wave analysis
- Irregular rhythm: Consider atrial fibrillation, atrial flutter with variable AV conduction, or multifocal atrial tachycardia 1
- Atrial rate > ventricular rate: Atrial flutter or atrial tachycardia 2
Step 2: Identify P Wave Location and Morphology
AVNRT (Atrioventricular Nodal Reentrant Tachycardia)
In typical AVNRT, atrial activation occurs nearly simultaneously with ventricular activation because both the fast and slow pathways are within or adjacent to the AV node. 1 This produces:
- Pseudo S waves: Narrow negative deflections in inferior leads (II, III, aVF) at the terminal portion of the QRS complex
- Pseudo r' (R prime): Slightly positive deflection at the end of QRS in lead V1
- RP interval <90 ms (measured from surface ECG) - the P wave is essentially buried in the QRS 1
- Short RP tachycardia: P wave closer to prior QRS than subsequent QRS 2
Clinical pearl: Patients with AVNRT frequently describe "neck pounding" or "shirt flapping" sensations due to cannon A waves when the right atrium contracts against a closed tricuspid valve 1, 3
AVRT (Atrioventricular Reentrant Tachycardia)
In orthodromic AVRT, the impulse travels down the AV node and returns via an accessory pathway, creating a longer circuit than AVNRT. 2 This results in:
- P waves visible in early ST-T segment - clearly separate from QRS complex
- RP interval 90-200 ms (short RP, but longer than AVNRT) 1
- P waves present in 70% of cases as distinct deflections 4
- QRS alternans occurs in 28% of AVRT cases 4
- ST-segment depression ≥2 mm or T-wave inversion present in 60% of AVRT versus only 27% in AVNRT 4
- Manifest preexcitation (delta wave) during sinus rhythm in 42% of AVRT patients 4
Important caveat: In the permanent form of junctional reciprocating tachycardia (PJRT), a rare variant of AVRT with a slowly conducting accessory pathway, the RP interval is long (RP >PR) due to delayed retrograde atrial activation 1, 2
Focal Atrial Tachycardia
Atrial tachycardia is driven by the atrium itself, not by AV nodal tissue, so P waves conduct normally to the ventricles with a physiologic PR interval. 2 Characteristics include:
- P waves in the second half of the tachycardia cycle - typically visible after the T wave 5
- Long RP tachycardia (RP >PR) - P wave closer to subsequent QRS than prior QRS 1
- P wave morphology differs from sinus rhythm - this is critical for diagnosis 2, 6
- Isoelectric baseline between P waves distinguishes focal AT from atrial flutter 5
- RP/PR ratio >1 in 90% of atrial tachycardia cases 4
- AV block can occur during tachycardia - this excludes AVRT and makes AVNRT very unlikely 5
Diagnostic pitfall: At rapid rates or with atrial conduction disturbances, P waves can be very wide without isoelectric baselines, mimicking atrial flutter 5
Atrial Flutter
Typical atrial flutter involves macroreentry around the tricuspid annulus through the cavotricuspid isthmus, producing a characteristic ECG pattern. 2 Features include:
- "Sawtooth" flutter waves - predominantly negative in leads II, III, and aVF
- Late positive deflection in V1 2
- Atrial rate typically ~300 bpm (cycle length 200 ms), though can be slower with antiarrhythmic drugs or scarring 2
- Atrial rate exceeds ventricular rate - often 2:1,3:1, or 4:1 AV conduction 2
- No isoelectric baseline between flutter waves in inferior leads 7
Reverse typical (clockwise) atrial flutter shows positive flutter waves in inferior leads and negative in V1 2
Distinguishing Features Summary Table
| Feature | AVNRT | AVRT | Focal AT | Atrial Flutter |
|---|---|---|---|---|
| RP interval | <90 ms | 90-200 ms | >PR (long RP) | Variable |
| P wave location | Buried in QRS | Early ST segment | After T wave | Continuous waves |
| P wave morphology | Pseudo S/r' | Separate, distinct | Different from sinus | Sawtooth pattern |
| Atrial:ventricular ratio | 1:1 | 1:1 | 1:1 (can have block) | Often 2:1 or greater |
| Response to AV block | Terminates | Terminates | Continues | Continues |
Additional Diagnostic Clues
Cycle Length Alternans
QRS Alternans
- Suggestive of AVRT (28% of cases) versus rare in AVNRT 4, 8
- Indicates beat-to-beat variation in ventricular activation 8
Sinus Rhythm ECG
Always compare tachycardia ECG with sinus rhythm ECG. 1 Look for:
- Delta waves (ventricular preexcitation) - highly suggestive of AVRT as the mechanism 8
- Absence of pseudo r' in V1 during sinus rhythm that appears during tachycardia confirms AVNRT 2
Common Diagnostic Pitfalls
Bundle branch block during tachycardia makes P wave identification more difficult in both AVNRT and AVRT 2
Atypical AVNRT (fast-slow or slow-slow variants) can present with long RP intervals, mimicking atrial tachycardia 5, 2
Rapid atrial fibrillation with fast ventricular response may appear regular and be misdiagnosed as regular SVT 2
Focal AT at rapid rates without clear isoelectric baseline can mimic atrial flutter 5
2:1 AV conduction in AVNRT is rare but can occur, potentially mimicking atrial flutter 2
Functional Testing for Uncertain Cases
When ECG diagnosis remains uncertain after careful analysis:
Vagal maneuvers or adenosine administration:
Observe onset and termination patterns:
The 2015 ACC/AHA/HRS guidelines emphasize that recording a 12-lead ECG during tachycardia is essential for mechanism identification and should be obtained whenever possible before terminating the arrhythmia. 1