P wave (electrocardiography)
During normal atrial depolarization, the main electrical vector is directed from the SA node towards the AV node, and spreads from the right atrium to the left atrium. This turns into the P wave on the ECG.
The P wave is upright in II, III, and aVF (since the general electrical activity is going toward the positive electrode in those leads), and inverted in aVR (since it is going away from the positive electrode for that lead).
A P wave must be upright in leads II and aVF and inverted in lead aVR to designate a cardiac rhythm as sinus rhythm.
Clinical significance
The relationship between P waves and QRS complexes helps distinguish various cardiac arrhythmias:
- A prolonged P wave may indicate left atrial enlargement.[1] Left atrial enlargement can accompany mitral stenosis.[2]
- A P wave with increased amplitude can indicate hypokalemia.[3] It can also indicate right atrial enlargement.[4]
- A P wave with decreased amplitude can indicate hyperkalemia.
- Absence of the P wave may indicate atrial fibrillation.
- A saw tooth formed P wave may indicate atrial flutter.