P WAVE Represents atrial depolarization

P WAVE

· Represents atrial depolarization

· P wave amplitude is < (0.25 mV) i.e. two and a half small squares

· The duration of the P wave is < (0.12 s) i.e. should not exceed three small square

· The wave of depolarization 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.

· Positive in leads I and II

· 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.

· May be biphasic in lead V1( equal upward and downward deflection )

· 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.