How to Identify PSVT on ECG
PSVT is identified on ECG by a regular, narrow QRS complex tachycardia (rate 150-250 bpm) with abrupt onset and termination, where P waves are typically hidden within or immediately after the QRS complex. 1, 2
Core ECG Characteristics
Rate and Rhythm
- Heart rate: 150-250 beats per minute (most commonly 150-300 bpm) 2, 3
- Rhythm is extremely regular with consistent RR intervals after the first few beats 4, 5
- The regularity is described as "metronome-like" 6
QRS Complex
- Narrow QRS complex (<120 ms) is the hallmark finding 4, 2
- QRS alternans (beat-to-beat variation in QRS amplitude) may be present, particularly in AVRT (28% of cases) 7
- If QRS is wide (>120 ms), you must differentiate from ventricular tachycardia and treat as VT unless SVT can be definitively proven 4
P Wave Characteristics - The Key Differentiator
For AVNRT (most common type of PSVT):
- P waves are absent or barely visible - hidden within the QRS complex because atria and ventricles depolarize simultaneously 4, 2
- Pseudo r' wave in lead V1 - pathognomonic for AVNRT 4, 7
- Pseudo S waves in inferior leads (II, III, aVF) - pathognomonic for AVNRT 4, 7
- These pseudo waves occur in 55% and 20% of AVNRT cases respectively 7
For AVRT (accessory pathway-mediated):
- P wave always follows the QRS complex with RP interval usually >70 ms 2, 7
- P waves are separate and visible in 70% of AVRT cases 7
- During sinus rhythm, look for pre-excitation pattern (delta wave, short PR interval) which occurs in 42% of AVRT patients 7
For Atrial Tachycardia:
- P waves are separate from QRS in 80% of cases 7
- RP/PR interval ratio >1 is present in 90% of atrial tachycardia 7
Additional ECG Features
ST-Segment and T-Wave Changes
- ST-segment depression ≥2 mm or T-wave inversion is more common in AVRT (60%) than AVNRT (27%) 7
- These changes help differentiate between PSVT types 7
Cycle Length Alternans
- Cycle length alternans (beat-to-beat variation in RR intervals) occurs in 6% of AVNRT cases 7
Critical Pitfall to Avoid
Automatic ECG interpretation systems are unreliable and commonly suggest incorrect diagnoses - always perform manual interpretation of the rhythm strip 4. Do not trust the machine's reading for arrhythmia diagnosis.
When Pre-Excitation is Present
If you see a delta wave, short PR interval, and slurred QRS upstroke on the baseline ECG in a patient with paroxysmal regular palpitations, this is sufficient for presumptive diagnosis of AVRT (Wolff-Parkinson-White syndrome) 1. These patients require prompt evaluation by a heart rhythm specialist due to potential risks 6.