P Wave Characteristics in Supraventricular Tachycardia (SVT)
In supraventricular tachycardia (SVT), P waves are present in approximately 60% of cases but typically have a different morphology from sinus P waves and may be hidden within the QRS complex. 1
P Wave Visibility and Characteristics in Different SVT Types
AVNRT (AV Nodal Reentrant Tachycardia)
- Most common form of SVT (140-250 bpm) 2
- P waves are often hidden within the QRS complex 1
- When visible, P waves may appear as:
- Pseudo-R wave in lead V1
- Pseudo-S wave in inferior leads (II, III, aVF) 1
- These findings are pathognomonic for AVNRT when present
AVRT (AV Reciprocating Tachycardia)
- P waves are typically visible after the QRS complex
- RP interval usually exceeds 70 ms 3
- P wave morphology differs from sinus P waves 1
Atrial Tachycardia
- P waves are usually visible but with abnormal morphology
- P waves precede each QRS complex
Diagnostic Approach to SVT
ECG Criteria for Narrow QRS Tachycardia
- If no P waves are apparent and RR interval is regular, AVNRT is most likely 1
- If P wave is present in ST segment and separated from QRS by 70 ms, AVRT is most likely 1
- In tachycardias with RP longer than PR, consider:
- Atypical AVNRT
- Permanent form of junctional reciprocating tachycardia (PJRT)
- Atrial tachycardia 1
Differential Diagnosis
- Narrow QRS complex (< 120 ms) almost always indicates SVT 1
- Regular rhythm with rates typically 150-250 bpm 3
- P waves may be:
- Hidden within QRS complex (especially in AVNRT)
- Visible but with abnormal morphology
- Following QRS complex (in AVRT) 4
Clinical Implications
Diagnostic Challenges
- P waves may be difficult to identify during tachycardia
- Consider using:
Common Pitfalls
- Misdiagnosing wide-complex tachycardia as SVT when it could be ventricular tachycardia 2
- Failing to obtain a 12-lead ECG during tachycardia 1
- Overlooking subtle P waves that may be partially hidden within the QRS complex
- Misinterpreting artifact as P waves
Management Considerations
- Vagal maneuvers are first-line for acute termination and may help reveal P waves 2
- Adenosine administration can terminate most SVTs and may transiently block AV conduction, revealing underlying atrial activity 2
- Beta-blockers or calcium channel blockers are recommended as first-line pharmacologic therapy 2
- Electrophysiology study should be considered for recurrent symptomatic episodes 2
In summary, while P waves are present in most SVTs, they may be difficult to visualize on standard ECG due to their timing relative to the QRS complex or their abnormal morphology. Their presence, location, and morphology provide valuable diagnostic information about the specific type of SVT mechanism.