P wave

The P wave is the first small bump on each heartbeat of an ECG, and it marks the moment the two upper chambers of the heart — the atria — are electrically activated. If you picture the heartbeat as a wave passing through the muscle, the P wave is the ripple reaching the atria, just before they squeeze blood down into the ventricles.

Electrically, the P wave reflects atrial depolarization: the spread of the impulse from the heart's main pacemaker across both atria. A normal P wave is small, smooth, and rounded, and in normal sinus rhythm one P wave precedes each QRS complex. Its shape and timing tell the reader whether the beat started in the right place and whether the atria are enlarged or strained.

When P waves are absent, irregular, or oddly shaped, it often points to an atrial rhythm problem. In atrial fibrillation, for example, organized P waves disappear and are replaced by a chaotic, quivering baseline. Reading the P waves — present or not, normal or abnormal — is therefore one of the first steps in interpreting any rhythm.