P wave (electrocardiography)

The P wave in the ECG represents atrial depolarization, which results in atrial contraction.

The P wave is a summation wave generated by the depolarization front as it transits the atria. Normally the right atrium depolarizes slightly earlier than left atrium since the depolarization wave originates in the sinoatrial node, in the high right atrium and then travels to and through the left atrium. The depolarization front is carried through the atria along semi-specialized conduction pathways including Bachmann's bundle resulting in uniform shaped waves. Depolarization originating elsewhere in the atria (atrial ectopics) result in P waves with a different morphology from normal.

Peaked P waves (>2.5mm) suggest right atrial enlargement, cor pulmonale, (P pulmonale rhythm), but have a low predictive value (~20%).

A P wave with increased amplitude can indicate hypokalemia. It can also indicate right atrial enlargement.

A P wave with decreased amplitude can indicate hyperkalemia.

Bifid P waves (known as P mitrale) indicate left-atrial abnormality - e.g. dilatation or hypertrophy.

If at least three different shaped P waves can be seen in a given ECG lead tracing, this implies that even if one of them arises from the SA node, at least two others are arising elsewhere. This is taken as evidence of multiple (i.e. at least two) ectopic foci, and is called multifocal (or more correctly, multiform) atrial rhythm if the rate is ≤100) or multifocal atrial tachycardia if the rate is over 100. This appears particularly commonly in exacerbations of chronic obstructive lung disease.

If the baseline has totally an irregular form this suggests fibrillatory waves of atrial fibrillation or possibly artefact; a saw tooth shaped baseline suggests the flutter waves of atrial flutter. With either of these rhythms, if the ventricular rate is fast, the fibrillatory or flutter waves can easily be misinterpreted as P waves.

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