The Electrocardiogram (ElectroCardioGram) is the electrical signal from the heart that can be recorded from the surface of the skin.
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).
It is also possible to record electrical signals from the heart with electrodes located inside the body (such as with a catheter for example) but this is then not called an ECG but simply an electrogram (gram = a graph; electro = electrical).
4: To record an ECG you need the following:
Three electrodes (two recording electrodes, one positive, and one negative, and a third electrode that is connected to earth)
A voltmeter to record, magnify and graph the electrical signal. The two recording electrodes and the earth electrode are connected to the voltmeter.
5: Earth electrode?
The earth electrode is necessary to discharge the body of any electrical charge, often static electricity.
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 or wrist.
The earth electrode could be located anywhere and is generally placed on the right leg or foot.
B. How is an electrical signal generated?
The voltmeter records the potential from two electrodes; one is positive and the other is negative.
When the tissue is resting, there will be no difference between the positive and the negative potential and the voltmeter will record 0 mV.
When the tissue is activated (=excited), an action potential will then 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!)
When the depolarization is located between the two electrodes, no potential will be recorded (0 mV). This is called = isoelectric (iso = same or equal).
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 (= wave) because this electrode is recording negatively.
Supose that this is an electrogram recorded from this excited tissue:
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!
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 ECG also depends on the direction of propagation of the action potential!
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).
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).
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!