Introduction. II—Basic terminology

P wave

The sinoatrial node lies high in the wall of the right atrium and initiates atrial depolarisation, producing the P wave on the electrocardiogram. Although the atria are anatomically two distinct chambers, electrically they act almost as one. They have relatively little muscle and generate a single, small P wave. P wave amplitude rarely exceeds two and a half small squares (0.25 mV). The duration of the P wave should not exceed three small squares (0.12 s).

The wave of depolarisation is directed inferiorly and towards the left, and thus the P wave tends to be upright in leads I and II and inverted in lead aVR. Sinus P waves are usually most prominently seen in leads II and V1. A negative P wave in lead I may be due to incorrect recording of the electrocardiogram (that is, with transposition of the left and right arm electrodes), dextrocardia, or abnormal atrial rhythms.

The P wave in V1 is often biphasic. Early right atrial forces are directed anteriorly, giving rise to an initial positive deflection; these are followed by left atrial forces travelling posteriorly, producing a later negative deflection. A large negative deflection (area of more than one small square) suggests left atrial enlargement.