The Electrocardiogram (ElectroCardioGram) is the electrical signal from the heart that can be recorded on 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).
To record an ECG, you need the following:
5. Earth electrode?
The earth electrode is necessary to discharge the body of any potential electrical charge, often static electricity. (Footnote: what is 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 on the skin and is generally placed on the right leg or foot.
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 (m = milli Volt; 1/1000 of a volt).
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 upand 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 because this electrode is recording negatively.
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).