Electrocardiogram (ECG): Definition & Wave Types

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  • 0:06 Intrinsic Conduction System
  • 0:36 Electrocardiogram (ECG)
  • 1:49 P Wave
  • 2:30 QRS Wave
  • 4:00 T Wave
  • 4:46 Lesson Summary
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Lesson Transcript
Rebecca Gillaspy

Dr. Gillaspy has taught health science at University of Phoenix and Ashford University and has a degree from Palmer College of Chiropractic.

Expert Contributor
Maria Airth

Maria has a Doctorate of Education and over 20 years of experience teaching psychology and math related courses at the university level.

Do you know how problems in the heart are detected? In this lesson, you will learn how to read an electrocardiogram (ECG) and discover what is happening in the heart during the P, QRS, and T waves.

Intrinsic Conduction System

Your heart is a very unique muscle because it has the ability to create electrical impulses on its own without any outside influences. This ability is thanks to the built-in regulating system called the intrinsic conduction system. We previously learned that the electrical impulse begins at the pacemaker of your heart, which we call the SA node. Then the impulse travels through the atria to the AV node, and then down through the ventricles, causing the heart to beat in a rhythmic and predictable way.

Electrocardiogram (ECG)

When this electrical impulse passes through your heart, electrical currents are created that spread through your body and reach the surface of your skin. Now, you don't feel these impulses, but they can be picked up and graphed as an electrocardiogram or ECG, which is simply a recording of the flow of the electrical current through the heart. This is a common test used to detect problems in the heart. You've probably seen one if you've ever watched a television drama based in a hospital, because when one of your favorite characters 'flatlined,' that meant that the ECG was no longer detecting an electrical charge, and you heard that familiar flatline 'beeeeep' of the ECG monitor.

Electrodes placed on the chest read the electrical currents produced by the heart
how ECG is performed

An ECG is performed by placing electrodes on the skin overlying the heart. As the electrical impulse moves from the atria, which are the top two chambers, to the ventricles down below, the voltage measurement between the electrodes varies, and this produces a graph of how your heart is performing. This provides the person running the test with valuable information based on the intensity of the heart's contractions and the time intervals between those contractions.

P Wave

In a normal ECG, there's three distinct waves. Together these waves represent one heartbeat. Looked at separately, the waves tell us what's happening in the heart at a certain time. The first wave is called the P wave. You can see from this picture that it's a relatively small wave. It represents the depolarization of the atria. What does that mean? Well, we remember that depolarization is defined as the change in the cell's membrane potential to a more positive state. It's this change that generates the electrical impulse that starts the heart's contraction. So we can associate the P wave of an ECG with the contraction of the atria.

QRS Wave

If we move along the graph of the ECG, we see a small dip followed by a large spike and another dip. This series is usually considered together, and it's called the QRS wave. You'll notice that this puts the waves in alphabetical order. The QRS wave is sometimes called the QRS complex, and it represents the depolarization of the ventricles. This quickly leads to the contraction of the ventricles and ejection of blood out of the heart and into the large arteries exiting the heart.

The QRS Wave is the largest spike on the ECG graph and is associated with ventricle contraction
QRS Wave

So if we think about this further, because the QRS wave is associated with the contraction of the ventricles, then we see that it's also associated with the beginning of systole, because systole is the phase of the cardiac cycle where the ventricles contract. We remember that the contraction of the ventricles causes the AV valves to close, and this causes the first heart sound, 'lub.' So the first heart sound is also associated with the QRS wave.

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Additional Activities

Make Your Own ECG

In this activity students will create their own electrocardiogram waves and then analyse the waves to identify each wave type.


  • Shallow rectangular baking tray or box
  • White paper
  • Tape (optional)
  • Marble (or other small sphere)
  • Paint
  • Small bowl


  • Cover the bottom of the baking tray in white paper (use tape to secure the paper if desired).
  • Pour some paint into a bowl.
  • Completely cover the marble in paint.
  • Place the marble in the middle of the paper on the short end of the tray.
  • Gently tilt the tray away from you so that gravity will cause the marble to roll towards the other side of the tray.
    • Make subtle movements with your wrists to cause the marble to move backwards and forwards in a wave fashion as it progresses towards the other side.
    • Try to get some small movements and some large movements of the marble with your tilting. The aim is to recreate an ECG.
  • If the marble runs out of paint before reaching the other side of the paper, dip it in the paint again and start the marble rolling again at the spot where the paint ran out.
  • After creating your mock ECG, analyse it to the best of your ability.
    • Can you see a P Wave?
    • Were you able to make the marble move enough to create obvious QRS waves?
    • Is there a T Wave in your model?
  • Repeat the process until you are able to create a clear ECG pattern with identifiable parts.
    • Label the parts of the ECG.
    • Include a brief paragraph explaining what each part of the ECG is and what it says about the heart's functions.

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