The contraction and relaxation of the heart muscle are controlled by the self-forming and impulse conduction system located in the heart, which is related to the central nervous system through external innervation.
The cardiac muscle stimulating electrical pulses start from the right atrium wall of sine knot, and getting the atrioventricular bundle through the atrial muscle wall, which is situated in the lower part of the right atrium, directly above the chamber. The atrioventricular bundle starts the atrioventricular knot starts and forks passing along both sides of the fence separating chambers. The impulse reaches to the muscle fibers through the end branches. In this way the electrical impulses of the heart muscle of the total passes through, depolarizing the cells. The repolarization also takes place exactly as specified.
It follows from the above, it described the current that flows around the heart muscle, which can be interpreted as a vector, it's magnitude and direction is constantly changing. So a measured ECG only shows the value of this vector projection of its share. The ECG curve built up by in different areas of the heart's electrical activity.
The electrical activity of the heart can be measured on the body surface thanks to the good conductivity of the human tissue. Sometimes they used to characterize the three planes: frontal, sagittal and transverse.
From these measurements to the frontal plane is the simplest. To do this the Einthoven's simplify conditions under must be used. According to which the heart's electrical activity at any moment can be described with only dipole; the leads of limb (right hand, left hand, left foot) correspond to the vertices of a regular triangle; between the heart and limb the homogeneous conductivity of tissue is located. One of the three conditions is not met, however, the distorting effect is low, compared to the simplifying assumptions can be easily adopted to standardize the measurement. Thus, sufficient to the electrodes placed on both hands and the left leg.
For the transverse plane of the measurement, the electrodes are placed on the front and on the side of the chest. They already want to attach more practice. Sagittal plane is much more circumstantial to place electrodes. The sensors are conveyed through the esophagus behind the heart. Due to the unpleasant process of what it was rarely used procedure.