B.3.3. Basic Stuff about the ECG



A. What is an ECG and how do we record it?
1.
The Electrocardiogram (ElectroCardioGram) is the electrical signal from the heart that can be recorded on the surface of the skin.
2.
It is also possible to record other signals from the skin such as the EEG (ElectroEncefaloGram = electrical signals from the brain) and the EMG (ElectroMyoGram = electrical signals from the skeletal muscles).
3.
It is also possible to record electrical signals from the heart with electrodes located inside the body (such as with a catheter or with a needle for example) but this is then not called an ECG but simply an electrogram (electro = electrical ; gram = a graph).
4.
To record an ECG you need the following:  a. Three electrodes (two recording electrodes, one positive, and one negative, and a third electrode that is connected to earth)
 b. A voltmeter to record, magnify and graph the electrical signal. The recording electrodes are connected to the voltmeter.
5. Earth electrode?
The earth electrode is necessary to discharge the body of any electrical charge, often static electricity.
( What is static electricity?) The body often contains (small) electrical charges, due to rubbing of clothes etc. These charges could seriously disturb the recording of an ECG so it is better to get rid of them during an actual recording. This is done with an earth electrode.
The shapes of the cardiac action potentials
6. Two recording electrodes:
Most often, an ECG is recorded with one electrode located on the right arm (the wrist is often used for this) and the other electrode is located on the left arm/wrist.
7.
The earth electrode could be located anywhere on the skin and is generally placed on the right leg or foot.




ECG electrodes

B. How is an electrical signal generated?

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1.
The voltmeter records the potential from two electrodes; one is positive and the other is negative.
2.
When the tissue is resting (= no action potential), there will be no difference between the positive and the negative potential and the voltmeter will record 0 mV.
3.
When the tissue is activated (=excited), an action potential will propagate through the tissue. When the depolarization reaches the positive pole, a positive deflection (=change in signal amplitude) will be detected. (In the graph; positive is up and negative is down!).
4.
When the depolarization is located between the two electrodes, no potential will be recorded (0 mV). This is called = isoelectric (iso = same or equal).
5.
A little bit later, when the depolarization reaches the negative pole, a second deflection will be recorded. This deflection is the opposite of the first deflection because this electrode is recording negatively.




Electrogram Recording

C. Some basic ECG rules:

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1.
Suppose that this is an electrogram recorded from this excited tissue:
Electrogram Recording
2.
If the polarity of the electrodes is changed (positive becomes negative and negative becomes positive) then the signal also changes its polarity (first negative and then positive). Thus, the ECG depends on the polarity of the electrodes!



Electrogram Recording
3.
If the direction of depolarization is changed; from right to left, instead of left to right, then the polarity is also changed (=reversed). Thus, the shape of the ECG also depends on the direction of propagation of the action potential!


Electrogram Recording
4.
If the amount of tissue that is excited is large (such as a thick muscle wall), then the ECG signal will also be large! Thus the ECG can detect hearts that have an abnormal thick muscle (=hypertrophy).


Electrogram Recording
5.
If the amount of tissue that is excited is small, reduced or thin (such as a thin muscle wall), then the ECG signal will be smaller then normal! Thus the ECG can detect hearts that have an abnormal thin muscle (atrophy).


Electrogram Recording
6.
This is not very common. Much more common is when a part of the muscle is no longer excitable, such as in a heart infarct. Thus, the ECG can detect infarcted tissue!

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B.3.3. Basic Stuff about the ECG


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