The P Wave
February 15th, 2021
Overview
The P wave on an ECG trace is indicative of atrial depolarisation, which may be initiated by the sinoatrial node or by an ectopic atrial focus.
Normal P Wave Size
- Duration <120ms (3mm)
- Amplitude <2.5mm
The P wave is directed inferiorly and therefore should be positive in leads I and II. It is often biphasic in lead V1.
Absence of P Waves
A lack of visible P waves preceding QRS complexes suggests a lack of sinus beats; this may occur with sinus dysfunction or in the presence of fibrillation or flutter waves. The P wave may also be hidden within the QRS complex.
| Atria | Ventricles | |
| Atrial Fibrillation | Irregular 350 - 600 bpm | Irregular 100 - 180 bpm |
| Atrial Flutter | Regular ~300bpm | Regular ~150bpm |
Causes of Absence of P Waves
- Lack of sinus beats - sinus arrest, sinoatrial axit block
- P wave hidden in the QRS complex - AV nodal reentrant tachycardia, AV reentrant tachycardia
- Fibrillation or flutter waves - atrial fibrillation, atrial flutter
Interpretation
Bradycardia
- Sinus arrest - complete lack of P waves
- Second degree sinoatrial exit block - intermittently missed PQRS complexes
- Complete sinoatrial block - indistinguishable from sinus arrest
- Junctional escape - bradycardia with QRS complexes and no P waves
Tachycardia
- Junctional tachycardia (AVNRT or AVRT) - regular narrow complex tachycardia with no visible P waves
- Atrial flutter - regular narrow complex tachycardia with regular saw-toothed baseline flutter waves
- Atrial fibrillation - irregularly irregular narrow complex tachycardia with an irregularly oscillating baseline and no discernable P waves
Bifid P Waves
Bifid P waves are also referred to as P mitrale. Their presence indicates dyssynchrony between right and left atrial depolarisation; this may be normal, or suggestive of left atrial enlargement.
Look For
- P wave with two notches.
Peaked P Waves
Peaked P waves are also referred to as P pulmonale. Their presence may be normal, or suggestive of right atrial enlargement.
Look For
- Tall P waves, with a height greater than 2.5mm.
P Wave Morphology Due to an Atrial Rhythm
Focal atrial tachycardia (FAT) - a regular narrow complex tachycardia with abnormal P wave morphology (e.g. inverted or biphasic)
Multifocal atrial tachycardia (MAT) - an irregularly irregular narrow complex tachycardia with at least three different P wave morphologies and variable PP intervals, with an isoelectric baseline.
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