The P Wave | ECG Basics - MedSchool
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ECG Basics
 
 
ECG Basics
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.
 

The P Wave

 
 

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.
    • Absence of P Waves
    • 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

    • 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
       
Last updated on December 1st, 2019
 
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