In Cardiology, What Are P Waves?

An ECG is sensitive enough to detect subtle electrical changes in the skin.
P waves are displayed on an EKG readout.
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  • Written By: Lynn Cole
  • Edited By: Angela B.
  • Last Modified Date: 20 July 2014
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In cardiology, a P wave is the graphic representation of the heart muscle’s atrial depolarization and part of a complex of electrical waves that are detected during a non-invasive test of heart function called an electrocardiogram (ECG or EKG). The other waves that follow the P wave in an ECG are the Q, R, S, and T waves. The P wave is the first deflection in the tracing of a heartbeat, typically seen as a small upward wave on an ECG. The P wave is the representation of atrial depolarization. The first part of the P wave, which looks like the rise of a small hill, indicates right atrial depolarization, while the far side of the graphic of the hill indicates left atrial depolarization.

When a heart beats, electrical polarization and depolarization occur. An ECG detects these subtle electrical changes and records them as a pattern of positive or negative waves on a monitor or on a paper readout. A physician or other healthcare professional can look at an ECG and see how the heart is functioning based on changes in the waves represented.


An ECG is sensitive enough to detect subtle electrical changes in the skin through monitoring leads placed near the heart. Depolarization of the heart muscle after a heartbeat, as it moves towards zero charge, activates the mechanisms in the heart that cause it to beat. A healthy heart will have a steady pattern of wave depolarization that can be seen on an ECG. A properly interpreted ECG can show the overall functioning of the heart and can indicate weakness or damage to the heart by watching the P wave and other waves generated during a heartbeat.

P waves are important because they precede atrial contraction, which occurs a fraction of a second after the P wave. On an ECG, all P waves should appear as small upward-reaching hills. P waves should typically be of uniform height and a short distance from the QRST complex of waves. Changes in the P wave shape, height, typical upward direction, or in the distance between the P wave and the QRST complex can indicate heart problems.

P waves should be no taller than 3 mm on a standard ECG. Taller P waves may indicate right atrial enlargement, while longer P waves that appear similar to the letter “m” may indicate left atrial enlargement. If an ECG shows multiple P waves before the QRST complex, it may indicate second- or third-degree heart blockage.


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Post 2

@NathanG - I agree that catheters are more precise; however it’s not true that the P waves are inconclusive. The article plainly states that they could indicate a right or left enlargement.

At least it will give you some idea of where to look. At any rate, these interpretations are best left up to the doctor not to the patient.

I am rarely given the readout of what my heart exam looks like, except if there is something wrong, in which case the doctor will explain what the results mean.

Post 1

Now I know how to read the ECG report from the doctor. I have seen these printouts before but all it looked like was a bunch of waves with no rhyme or reason.

I am sure there is more to the report than just what the peaked P waves indicate, but at least this can give me an idea if there is an underlying problem. However, I think that even if a P wave indicates a problem with the heart, it’s probably not as decisive as say inserting a catheter or something like that.

To me, ECG diagrams are like static noise on the radio; they may give you a faint hint of a radio signal, but they don’t let you dial in exactly on what the problem is. That’s why the real test is with using catheters to inspect the blood vessels and determine if there is a blockage.

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