When left atrial enlargement occurs, it takes longer for cardiac action potentials to travel through the atrial myocardium; thus, the P wave also lengthens. Therefore, the criteria for diagnosing LAE on a 12-lead ECG is as follows:
- The length of the P wave in lead II is greater than 120 milliseconds
- The downward deflection of the P wave in lead V1 is greater than 40 milliseconds in length, with greater than 1 millimeter negative deflection (< -1 mm in amplitude)
P-mitrale occurs when the depolarization of the right atrium and left atrium are both visible in the P wave. This is seen as a notch in the P wave and occurs when the left atrium is markedly enlarged, such as in mitral valve stenosis.
Note that left atrial enlargement is not able to be diagnosed in the presence of atrial fibrillation because this rhythm is defined by erratic atrial activity and no visible P wave on the ECG. Also, LAE is a significant risk factor for developing atrial fibrillation.
1. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric, 6e
2. Surawicz B, et al. AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram. Circulation. 2009;doi:10.1161/CIRCULATIONAHA.108.191095.