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ECG Basics
 
 

The P Wave

 
 
 
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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.
AtriaVentricles
Atrial FibrillationIrregular
350 - 600 bpm
Irregular
100 - 180 bpm
Atrial FlutterRegular
~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.
  • Bifid P Waves
     

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.
  • Peaked P Waves
     

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)
  • P Wave Morphology Due to an Atrial Rhythm
     
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.
  • P Wave Morphology Due to an Atrial Rhythm
     
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