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A. The components of the ECG:


In order to understand the ECG, you need to know and remember the normal conduction of the impulse in the heart:

SA-node -> Atria -> AV-node -> Bundle of His -> Purkinje tissues -> Ventricles.

(see: B.3.2. Cardiac Excitation)

A typical ECG consists of a sequence of waves. The normal waves are labelled the P wave, the QRS wave and the T wave.
  1. The P-wave is a reflection (= represents) the depolarization of the atria.
  2. The QRS-complex represents the depolarization of the ventricles.
  3. The T-wave represents the repolarization of the ventricles.

Because the atria are depolarized before the ventricles, the P-wave will occur before the QRS-complex.


Remember that the impulse was delayed in the AV-node (between the atria and the ventricles). This is visible as a delay in the ECG; there is therefore some time between the end of the P-wave and the beginning of the QRS complex; the PQ-interval.


There is a second isoelectric segment in the ECG; between the end of the S and the beginning of the T-wave. During this period, all the cells in the ventricles are depolarized.

( isoelectric = same or equal potential at both recording electrodes).
B. Difficult Stuff !!
  1. Where is the atrial repolarization on the ECG?

It is not there! There is no visible atrial repolarization on the ECG. This is because the amplitude of the ECG signal depends on the speed and the strength of the electrical signal in the heart. Something that is very fast and excites a lot of tissue will show a strong signal on the ECG. A good example of this is the QRS-complex because it represents fast depolarization of the thick ventricular wall.

2. Weak signals are not visible on the ECG:


A very weak signal is not visible on the ECG. For example, the action potentials propagating in the Sinus Node (which occurs before the P-wave) and in the AV-node (which occurs between the P-wave and the QRS-complex) are too slow and too weak to be visible on the ECG.

3. Speed of excitation in the atria:

In the atrium, the depolarization of the atrium is quite fast, and this creates the P-wave. But the repolarization of the atria is much slower (TT), and hence is too weak to be seen on the ECG.

(Remember the slow triangular shape of the repolarization in the atria?)

4. Speed of excitation in the ventricles:

Something similar also happens in the ventricles. There, the depolarization is very fast (one of the fastest in the heart) but the repolarization is slower (although not as slow as in the atrium). That is the reason why the T-wave is weaker and more spread out than the QRS-complex.


The T-wave is usually bigger than the P-wave. This is because much more tissue is being repolarized (in the ventricles) then depolarized (in the atria).


By the way, we call the QRS a “complex” and not a wave because it actually consists of three separate waves: “Q” “R” and “S”.
C. Some Pathophysiology:

The reason why the ECG is so popular is because it tells so well what is going on in the sick heart.


For example, you may have noticed that the AV-node is quite a weak link between the atria and the ventricles. What would happen if the impulse took longer than normal to propagate through the AV-node and reach the ventricles?

Answer: the PQ-interval would be longer than normal. And that is exactly what a cardiologist does when analyzing an ECG; the analysis consists of measuring intervals such as this one.


In some diseases, the left ventricle is thicker than normal (left ventricular hypertrophy). This would cause a stronger depolarization and therefore a higher QRS amplitude.

By the way, there is a mistake in this tracing! Can you spot it?

(link: mistaken T-wave)


In other diseases, the opposite happens and the ventricles, or part of it, die (such as during a myocardial infarction). This will lead to a weaker QRS complex and a lower amplitude.

(Notice that the T-wave is now also smaller).


Another reason why the ECG is so popular is because it also describes very well the rhythm and the rhythm disorders in the heart. When the heart beats too fast (= tachycardia) or too slow (= bradycardia), this is easily visible in the ECG. Especially the tachycardias are very important because these can degenerate into arrhythmias (= rhythm disturbances), which may cause sudden death.


mistaken T-wave:

Slides B.3.4. The Electrocardiogram
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