Atrial enlargement ( hypertrophy)

To understand atrial enlargement (hypertrophy) , one should be familiar with the P wave configuration in the normal electrocardiogram ( ECG) . As can be expexted , right atrial activation (depolarization ) takes place first , since the sinus node is situated in the right atrium . soon after the intiation of right atrial depolarization , left atrial activation (depolarization ) follows . since the left atrium begins to be activated before the completion of right atrial depolarization , both atrial activation processes overlap . 

consequently , the configuration of the normal P waves in the limb leads , particularly lead II , may show slight notching . the P wave has the largest amplitude in lead II in most normal adults because the P axis is similar to the axis of lead II . however , the P wave configuration varies markedly in different limb leads depending on the P axis of a given ECG tracing . in general , the typical noormal P wave demonstrates a small ( not more than 2.5 mm in width and depth ) , round , upright configuration in lead II and an iverted configuration in lead avr .
In lead V1, a normal P wave has two components : postitive and negative . the reason for this biphasic P wave is that the right atrium is located anteriorly and to the right , whereas the left atrium is located posteriorly and to the left .thus . in lead V1, right atrial activation produces the upright (positive) components , and left atrial activation registers the inverted ( negative ) component. As described previously , the atrial repolarization process ( Ta or Tp waves) is , as a rule , not recorded on the ordinary ECG because of its frequent superimposition on the QRS complex . however, on rare occasions , Ta(TP) waves may be discernible . In this case , the ECG finding may resemble left atrial enlargement .

When right or left atrial enlargement occurs , the configuration of the P wave is altered, with increased amplitude , increased with of the the corresponding component of  the enlarged (hypertrophied) atrium or both .