The next step in the orderly approach for 12-lead ECG interpretation is determination of the morphology of the ECG complexes.
Morphology of complexes
We have previously identified the various components of the ECG complex: P, QRS, ST, T, U. But now in order to make proper ECG interpretation, we need to understand the normal morphology of these complexes.
P wave
How should we evaluate the normal P wave morphology? Lead I will always be positive, since atrial depolarization is right to left. Since both right and left atrial depolarization go from superior to inferior, then both the initial and terminal part of the P wave in lead II will always be positive, with perhaps a slight notch differentiating the 2 atria. Lead aVR’s P wave will always be negative for the same reason.
Lead V1 is an anterior-posterior lead. Since the right atrium is depolarized first, and in an anterior direction, the initial part of the P wave will be positive. Since the left atrium depolarizes later, and in a posterior direction, the terminal part of the P wave will be negative. Hence, the P wave in V1 is usually diphasic (positive and then negative) but sometimes only a portion of atrial depolarization may be seen in V1, and only a positive or negative P might be noted.
Leads II and V1 are the best leads to analyze the atrial rhythm because they typically have the best defined P waves.
P wave criteria
This table shows criteria for two of the most common P wave abnormalities in leads II and V1. Note that a P wave 0.12 second in lead II (particularly if notched) and/or a terminal negative component of the P that is at least 0.1 mV in depth and 0.04 seconds in duration (i.e., a little box wide and deep) is consistent with a left atrial abnormality. A P wave > 0.25 mV in amplitude in lead II or an initial positive deflection of the P wave in lead V1 that is > 0.15mV in amplitude is consistent with a right atrial abnormality.