When people need to go to the hospital to check the conditions of their hearts, doctors usually hook the patients up to an Electrocardiography, or ECG, machine. This piece of medical equipment monitors the electric shocks the body uses to make the heart pump blood. These electric shocks are measured through electrodes hooked up to the ECG machine. The electrodes pick up spatial information about the heart's electrical activity.
This spatial electrical activity is denoted by a lead; thus, each electrode is placed where it can intercept each of the twelve leads. The leads cover three different directions that the electrical shocks travel in the body: right to left, superior to inferior, and anterior to posterior. There are also leads that correspond to the limbs. For example, there are bipolar lead limbs on the front of the body that go from the right arm to the left arm, the right arm to the left foot, and the left arm to the left foot.
There are also augmented unipolar limb leads and unipolar chest leads, the latter being on a horizontal plane. The augmented leads go from the right arm to the left arm and foot, the left arm to the right arm and left foot, and the left foot to the right and left arms. The two chest leads go from the posterior to the anterior and from right to left. All of the leads are illustrated in Einthoven's Triangle, a graph made by the inventor of the ECG machine.
Catching the electric signals produced by the twelve leads is essential for doctors to determine the health of the heart. Therefore, the electrodes are positioned on the body at just the right locations to catch the lead signals: one on each wrist and ankle and six in a line in the middle of the chest. If the intensity of the lead goes up, the electrode corresponds with a higher wave line on the ECG machine. Likewise, if the intensity of the lead declines, the electrode produces a low wave line on the ECG machine.
Through the lines that appear on the ECG machine, doctors can tell if a muscle in the heart is not functioning correctly. This is because the lead for that part of the heart will not be present, and therefore, will not be picked up by the corresponding electrode and recorded on the ECG machine.