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 plus Junctional 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.
Dr.S.Venkatesan MD 