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[Feature] P wave [Pathology] The p-wave is typically upright in most leads except for aVR; an unusual p-wave axis (inverted in other leads) can indicate an ectopic atrial pacemaker. If the p wave is of unusually long duration, it may represent atrial enlargement. Typically a large right atrium gives a tall, peaked p-wave while a large left atrium gives a two-humped bifid p-wave. [Duration] <80 ms [Description] 
The p-wave represents depolarization of the atria. Atrial depolarization spreads from the SA node towards the AV node, and from the right atrium to the left atrium.
Electrocardiography - Wikipedia, the free encyclopedia