An electrocardiogram is the temporary or permanent recording of the sum of the electric cardiac activity.
Every pumping of the heart is proceded by an electric activity, starting at the sinus node and running through the cardiac excitation leading system to the muscle cells. These electrical differences in potential can be recorded on the surface of the body and written down through time. The result is a monomorphic image of the electric heart actions.
It should be noted, that the ECG can only show excitation within the heart but not the actual output. Therefore additional tests, such as the echocardiography are useful.
The potential differences can be measured in various ways. These measurements are called leads. They can be differentiated though the location of the lead or the method.
Depending on the connection of the leads, one can differentiate between unipolar and bipolar leads.
Bipolar leads register the electric voltage between two parts of the body, e.g. right arm and left arm.
The unipolar lead measures the voltage between an electrode and the electrical zero point (indifferent). To get the indifferent point, two or more electrodes are combined though a resistance.
Location:One can also differentiate between the limb leads, which measure potential differences between the limbs and the chest leads.
The limb leads show only potential differences in the frontal plane while the chest leads can also show the horizontal plane. The Nehb lead is an additional lead for special clinical questions. This is also relevant for the Frank lead in the vector-cardiography.
A conventional 12-channel ECG records the limb leads after Einthoven (3) and Goldberger (3), as well as the Wilson chest leads (6) in parallel.