In the healthy heart, the right atrium depolarizes first, and the left atrium follows. The action potential measured by the leads shows the characteristic P wave morphology on ECG tracings. However, in left atrial enlargement, the left atrium takes longer to depolarize due to its significant increase in mass. This presents as a bifid P wave on ECG tracing.

A hypertrophic left atrium has the following P wave characteristics due to the increased mass:

• A longer duration of activation
• P wave lasting > 0.11 seconds
• The presence of bifidity (two lobes)
  • A peak-to-peak distance of 0.04 seconds most often seen in leads I, II, and V₆ 
• A negative deflection of the P wave in leads V₁ and V₂

Normal P Wave and Bifidity

12-lead ECG Representing P Mitrale With an Inverted T Wave

12-lead ECG Representing P Mitrale With a PQ Interval of 0.16 Seconds

P mitrale with clear bifidity is commonly seen in patients with the following:

• Mitral valve stenosis
• Mitral valve incompetence of any etiology (less bifidity)
• Severe aortic valve disease
• Heart diseases with chronic left ventricular overload, such as hypertension

Studies have shown that when the ECG pattern of P mitrale is compared with the dimensions of the left atrium, the ECG pattern of P mitrale shows a high specificity of about 90%. In other words, there is a 90% probability that the patient with the characteristics of P mitrale on a 12-lead ECG does, in fact, have left atrial enlargement. However, the sensitivity is mediocre at 30%, so the absence of P mitrale characteristics on an ECG does not definitively rule out left atrial enlargement.  

Prolongation of the P wave can also be seen in the older population and is not necessarily associated with left atrial enlargement. 

P mitrale is a risk factor in developing atrial fibrillation, thrombus formation, and subsequent cerebral embolism with concomitant cerebral stroke. Approximately 70% of patients exhibiting a left atrial diameter of 60 mm or more and a bifid P wave present with atrial fibrillation.