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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.

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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.One thing you probably noticed is that the P wave is much smaller than the QRS wave. You can use this fact to help remember what’s happening during the waves. If you recall, the atria are the smaller two chambers of the heart, and they don’t contract quite as strong; therefore, the P wave is smaller. In contrast, the ventricles are the muscular pumps of the heart, and the QRS wave represents their depolarization – it is much larger of a wave.

T Wave

After the QRS complex, we see another small peak, and this is called the T wave.

The T wave represents the repolarization of the ventricles. We know that repolarization is the opposite of depolarization and refers to the change of the membrane potential that returns the membrane back to a negative value.We might want to think about this as a return back to the resting state or relaxation phase of the ventricles.

We learned previously that that’s called diastole. This means that the ventricles are relaxed and under less pressure. This causes the semilunar valves to close, so the second heart sound, ‘dub,’ happens shortly after the T wave appears on the ECG.

Lesson Summary

Let’s review. The electrocardiogram or ECG is a test that records the electrical activity of your heart. It can detect the electrical impulses that move through the heart, providing a graphic tracing of how your heart is performing.In a normal ECG, there’s three distinct waves. The first wave is the P wave, which represents the depolarization of the atria. This happens right before the atria contract and push blood into the ventricles.

The next wave is called the QRS wave. This wave is much larger than the P wave, and it represents the depolarization of the ventricles. This, in turn, stimulates contraction of the ventricles and ejection of blood from the ventricles to the large arteries leaving the heart. So you can think of the QRS wave as representing ventricular systole. We also see that the AV valves close during the QRS wave, and this causes the first heart sound, ‘lub.

‘The T wave comes next, and this represents the repolarization of the ventricles. This returns the ventricles to a resting state, or diastole. At this time, the semilunar valves close. This causes the second heart sound, ‘dub.’

Learning Outcome

After seeing this video lesson, you will be able to:

  • Identify the purpose of an ECG
  • Recognize the three wave types
  • Describe the purpose of each wave on the ECG

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