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Why P wave becomes “tall” in RA enlargement and “wide” with LA enlargement ?

Posted Sep 20 2009 10:21pm

Normal P waves

normal p wave ecg rae lae

What are the components of Pwave ?

RA component: The SA node depolarises the RA first  , so the initial part of  P wave represents  RA  after about 40msec  the wave front reaches LA and it begins it’s depolarisation . LA component : By the time LA is maximally depolarised the RA  already starts its repolarisation.So there is  overlap and also a short time lag between these two wave forms . This is very important to recognise as , even if the RA conduction is prolonged in pathology the RA component of P Wave still falls within the LA wave .Hence it is not shown in the ECG and P wave is not widened in RA enlargement. This is in contrast to LA enlargement , when the terminal half of P vector delayed it stretches the P wave wide beyond the normal 110ms .Hence LAE widens the Pwave.

Why P wave becomes taller in RA enlargement ?

In classical P pulmonale , the P wave is    tall >2.5mm. It is easy to explain why it not getting wide , than   why it is getting taller !

The atrial vector has two components .The initial RA vector , is directed  anteriorly .The main reason is due to the anatomical proximity  of RA to  the chest wall . Further the  atrial  electrical dp/dt is  fast  so slope of  forward limb  is steep . Any RA voltage increases easily picked up. 

rae right atrial enlargement ecg tall p p pulmonale p tricuspidale

p wave lae left atrial enlargement bifid p wave notched p ecg

Why LAE can not produce tall P wave ?

The Left atrial vector which  follows RA vector  is mainly directed posteriorly and hence inscribe a  descending  limb of   P wave . This causes the P terminal force .  So  the direction of vectorial forces  and the anatomical locality  make a  tall & positive P  deflection highly improbable in   LAE .

*Of  course  when LAE is   huge , where a antero superovector from roof of LA may inscribe a positive wave .

What happens in bi atrial enlargement ?

It can have features of both . Tall & wide P waves  .

Can RA generate a Q wave ?

Yes . When RA assumes a huge  size  , especially if the RV is also at high pressure as  in severe PHT or valvular PS   a  q wave is generated in the lead V1 .This q wave is nothing but the intra cavitory potential of the enlarged RA.

What is the difference between atrial enlargement, atrial dilatation, atrial hypertrophy, intra atrial block and inter atrial  block  ?

The p wave morphology has no  specificity to identify the various entities. In any od the pathology of atrium the first thing that happens is a conduction delay !It is now realised the bulk of the changes we see in atrial enlargement especially in LAE is due to intra and inter atrial  blocks or more subtly conduction delay.

It is  obvious , a wide P wave can occur either  due to LAE or simple conduction delay .In elderly  hypertensive patients atrial fibrosis is more common , one can not confirm LAE  without echocardiogram .

A notched P wave  can be a very specific sign of   inter atrial block .Which is more common in severely diseased left atrium. A notch , slurred p wave is a good marker for impending AF or atrial flutter.

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