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the ECG.
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.
A typical ECG consists of a sequence of
waves. The normal waves are labeled P, QRS
and T
Because the atria are depolarized before the
ventricles, the P-wave will occur before the
a. The P-wave is a reflection ( =
represents) the depolarization of the
b. The QRS-complex is a reflection of
the depolarization of the ventricles.
c. The T-wave represents the
repolarization of the ventricles.
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 segment.
There is a second isoelectric segment in the
ECG; between the end of the S and the
beginning of the T-wave. At this moment in
time, all the cells in the ventricles are
page 1/4
B. Difficult Stuff !!
1. Where is the atrial repolarization on
the ECG?
2. Weak signals are not visible on the
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.
A very weak signal is not visible on the
ECG. For example, the impulse from the
Sinus Node (which occurs before the Pwave) and 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:
4: Speed of excitation in the ventricles:
In the atrium, the depolarization of the
atrium is quite fast, and this creates the Pwave. But the repolarization of the atria is
much slower, and hence is too weak to be
seen on the ECG.
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 then the Pwave. 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”.
page 2/4
C. Some Pathophysiology:
The reason why the ECG is so popular is because it tells so well what is going on in the sick
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
Answer: the PQ-time 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?
In other diseases, the opposite happens and
the ventricles, or part of it, dies out (such as
during a myocardial infarction). This will
lead to a weaker QRS complex and a lower
(notice that the T-wave is now also smaller).
page 3/4
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.
page 4/4