When the electrical impulse passes through the heart which are picked up and graphed as an electrocardiogram or ECG, which is simply a recording of the flow of the electrical current through the heart. This is a common test used to detect problems in the heart.
An ECG is performed by placing electrodes on the skin overlying the heart. As the electrical impulse moves from the atria, which are the top two chambers, to the ventricles down below, the voltage measurement between the electrodes varies, and this produces a graph of how your heart is performing. This provides the person running the test with valuable information based on the intensity of the heart's contractions and the time intervals between those contractions.
A typical ECG recording of one heartbeat consists of a P wave, a QRS complex, a T wave, and in some cases also a U wave. Usually more than two electrodes are used for measuring the ECG. The most simple is the 3-lead ECG with one electrode on the left arm (LA), one on the right arm (RA) and one on the left leg (LL). These electrodes are often used to make three bipolar leads, being lead I (LA+RA), lead II (LA+LL) and lead III (RA+LL). A more elaborate ECG recording is the 12-leads ECG, in which also the so called precordial leads are measured.