What does the P wave represent?

What does the P wave represent on an ECG?

Each heartbeat starts with an electrical impulse from the heart's natural pacemaker, the sinoatrial (SA) node, in the wall of the right atrium. The impulse spreads across both atria, making their muscle cells depolarise (their electrical charge flips), which triggers the atria to contract and push blood into the ventricles below. StatPearls describes the P wave as representing the electrical depolarisation of the atria, originating at the SA node.

On the trace, it is normally a small, smooth, upward bump just before the big spike. Note that the P wave is the electrical signal: the atria contract a split second after it.

The parts of an ECG

These meanings are from StatPearls' guide to the electrocardiogram. The pause in the PR interval lets the atria finish emptying before the ventricles squeeze.

Reading an ECG: worked example

StatPearls notes that ECG paper moves at 25 mm per second, so each small square is 0.04 s and each large square (5 small squares) is 0.2 s.

• P wave length: the P wave in the diagram lasts half a large square, 2.5 small squares: 2.5 × 0.04 = 0.1 s, or 100 ms. StatPearls describes a P wave lasting more than 120 ms as abnormal.
• PR interval: from the start of P to the start of QRS is 0.8 large squares: 0.8 × 0.2 = 0.16 s, or 160 ms. StatPearls notes the P wave normally begins 120 to 200 ms before the QRS.
• Heart rate: if the R spikes of two beats are 4 large squares apart, the time per beat is 4 × 0.2 = 0.8 s, so the rate is 60 ÷ 0.8 = 75 beats per minute.

Heart rate is half of the cardiac output equation. Try this question, then explore more in our Cardiac Output Calculator:

What changes in the P wave can mean

Doctors look at the P wave's shape, size and timing. StatPearls describes, for example, long, notched P waves ("P mitrale") suggesting a problem with the left atrium, and tall P waves in certain leads ("P pulmonale") suggesting an enlarged right atrium. Reading an ECG properly takes clinical training: this guide explains the ideas, not how to diagnose.

What does a P-wave represent in an earthquake?

In earth science and physics, a P-wave is a primary wave, one of the seismic waves an earthquake sends through the Earth. The AQA GCSE specification describes P-waves as longitudinal seismic waves that travel through solids and liquids, while S-waves are transverse and travel only through solids. P-waves are the fastest, so they arrive at a seismometer first.

Because S-waves can't pass through liquid, there is a region on the far side of the Earth from an earthquake where no S-waves arrive. That is the evidence that the Earth's outer core is liquid. For more on the two kinds of wave, see our guide to transverse and longitudinal waves.

Common mistakes

• Saying the P wave is the atria contracting. It is the electrical signal that triggers the contraction.
• Mixing up the P wave and the QRS complex. P is the small first bump (atria); QRS is the big spike (ventricles).
• Calling the T wave the atria relaxing. It is the ventricles repolarising.
• Counting large squares as 0.04 s. Small squares are 0.04 s; large squares are 0.2 s.
• Mixing up the two meanings. An ECG P wave is electrical; an earthquake P-wave is a mechanical, longitudinal wave.

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