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What is the difference between isoelectric P waves and absent p waves ?

Posted Nov 04 2009 10:01pm

P waves represent atrial depolarisation. The p wave height  and width depends not only the size of the RA and LA but also the site of  origin of atrial  impulse .A normal SA nodal origin of P wave produce the normal shaped p waves.

We know  ectopic  p waves can have a wide variation of morphology.(Fully inverted, partially inverted, slurred, bi phasic, notched, rounded , deformed, etc. The morphology is dictated by the direction of p wave vector and thus it is quite variable in different leads. Further  it is also determined by the inter atrial and intra atrial conduction.So in summary , an ectopic p wave can have any morphology we can think off !

What is isoelctric P waves

It is rather a surprise we have not thought about so long,   like a low voltage QRS ,  a  p wave can also be very low amplitude and it may be entirely isoelectric , which could actually mean the p waves are as good as absent.This can happen in all leads or in few leads. .Atria gets electrically activated but fails to inscribe a p wave .This is termed as isoelectric p waves

The importance of isoelectric p waves

It  can  happen , both  in sinus rhythm  and in ectopic atrial rhythm . Absent p waves should be differentiated form isoelectric p waves. It is typically described in focal atrial rhythm arising from the right side  of  the  inter atrial septal near the   perinodal  tissue.The atrial tachycardias arising from this site are classically have isoelectric p waves in most of the leads especially  V1 .

Other causes of absent p waves

  • Atrial fibrillation

The classical example .in fact here p waves are replaced by fine or coarse fibillatory waves

  • Sinus arrest  plusJunctional rhythm with retrograde VA block

Not all junctional rhythm result in absent p waves .Many record inverted retrograde p if there is VA                            conduction.

  • Sino ventricular conduction . P waves appears  absent in surface ECG. It occurs in hyperkalemia /renal failure is due to high levels of pottassium   which suppress the atrial activity sort of atrial electrical paralysis but still impulse originates in SA node traverses  the inter atrial pathway and reach ventricles.typically P waves are absent or can be termed isolectric.
  • Atrial  stunning following cardioversion

Long standing atrial tacycardias may fail to resume it’s mechanical (or even electrical ) activity after  cardiversion  .If it is electrical stunning the p waves do not immediately appear  but occurs later .In fact this could be termed as failed cardioversion.

* Note  p waves are failed to identified in many of the VTs AVNRTs

Final message

Absent p waves ,  isoelectric p waves , hidden p waves, merged p waves , low voltage p waves , unrecorded p waves,  selective absence of p waves in some leads all can happen in clinical cardiology practice.

One should realise the importance  differentiating   absence of   p waves in the given strip of ECG from failure of p waves to  get recorded by the  ECG machine .This has diagnostic significance.

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