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Lesson 4 of 21 · Waves & Intervals

The P Wave

The P wave is atrial depolarization. Its presence, shape, and relationship to the QRS tell you whether the rhythm is sinus and whether the atria are enlarged.

01Normal P wave

  • Upright in I, II, aVF; inverted in aVR
  • Duration < 0.12 s (3 small boxes)
  • Height < 2.5 mm in lead II
  • Often biphasic in V1

02Abnormal morphology

P-wave morphology
Normal< 0.12 s, < 2.5 mmP mitraleLAE: wide, notchedP pulmonaleRAE: tall, peakedRetrogradeJunctional: inverted
II
P mitrale: broad, notched ('M-shaped') P ≥ 0.12 s, a sign of left atrial enlargement.
II
P pulmonale: tall, peaked P ≥ 2.5 mm in II, a sign of right atrial enlargement.

03Retrograde P waves

When the impulse starts in the AV junction, the atria depolarize from bottom to top, so the P wave is inverted in II, III, aVF. It may appear just before the QRS (short PR), be hidden inside it, or follow it.

II
Junctional rhythm with inverted P waves and a short PR.

Key takeaways

  • Sinus P: upright in II, inverted in aVR
  • Wide notched P = LAE; tall peaked P = RAE
  • Inverted P in II = retrograde / junctional