Editorial Comment: Normal limits of ECG measurements related to atrial activity using a modified limb lead system

Atrial repolarization is rarely seen clearly on a standard 12-lead electrocardiogram on account of the fact that it is largely obscured by the QRS complex. In the main, during sinus rhythm at a relatively normal heart rate, only 50 ms of an atrial repolarization wave might be seen at the terminal potion of the PR segment. In the vast majority of ECGs, this part of the PR interval is at the same potential as the P wave onset. On the other hand, depression or reciprocal elevation of the terminal part of the PR interval relative to P wave onset is often regarded as being due to pericarditis (1) or atrial infarction (2).