P Wave

The P waves on an EKG represent atrial depolarization and are usually the first wave noted throughout the cardiac conduction cycle. P waves are best evaluated in leads II and V1, though lead II is the best lead overall (and is usually the main strip lead at the bottom of a 12 lead EKG). P waves should have a duration <120ms and amplitude <2.5mm; those with a longer duration or greater amplitude result from atrial enlargement. In normal sinus conduction, the right atrium depolarizes before the left atrium (sinus node sits in the right atrium). Normal P waves should be upright in lead II and inverted in aVR as the normal atrial depolarization travels inferiorly and away from the right side.

Origin of P Waves

If the P waves are not upright in lead II and biphasic in V1, then their origin is not likely to be from the sinus node. Determining their origin (assuming they are visible in all leads and are monomorphic) is based on their relationship with the QRS complex. If they are too close to the QRS complex (PR interval<120ms), then they are likely to be retrograde P waves. If they are not too close, they are from an atrial ectopic site.

Atrial Enlargement

Because the right atrium depolarizes first in a sinus rhythm, its enlargement and subsequent delayed depolarization do not make any changes to the duration of the P wave (see image above displaying its normal depolarization ends before the end of the left atrial depolarization). Its noted change to the P wave is in the amplitude of the P wave, with a taller (greater amplitude) P wave. The cutoff for this is >2.5mm. Right atrial enlargement is sometimes called P pulmonale. If the left atrium is enlarged, then the duration of the P wave is prolonged >120ms. This commonly produces a camelback appearance to the P wave as the right atrium finishes depolarization before the left atrium reaches maximum depolarization. Left atrial enlargement is sometimes called P mitrale.

Sometimes, both atrial chambers are enlarged (biatrial enlargement). When this occurs, the P waves are often tall (>2.5mm) and wide (>120ms) with a camelback appearance.