Left atrial enlargement

The left panel shows the normal pattern of atrial activation as it would appear on lead II of the electrocardiogram; activation of the right atrium (RA) occurs first, followed by left atrial (LA) activation. The right panel shows the pattern in a patient with left atrial enlargement. Delay of left atrial activation causes notching and a prolonged duration of the P wave, and the presence of left atrial hypertrophy results in increased P wave amplitude.