It is done on a millimeter paper calibrated at 10 mm / mV in the vertical plane and a speed of 25 mm / s in the horizontal plane, so that horizontally 1 mm corresponds to 0.04 seconds, 5 mm to 0.2 seconds and 25 mm to 1 second. Vertically, 1 mm corresponds to 0.1 mV (Guadalajara, 2006).

Gallardo, L. B. (2018). Record of amplitude and time [illustration].

It is a line segment with direction and sense; its graphic form consists of an arrow where the tip represents a positive charge (+) and the tail a negative (-). In this way, depolarization vectors can be recorded and calculated to graphically display on paper with positive or negative deflections, depending on whether they move away from or approach the scanning electrode. Therefore, the difference in charges between two points is called the dipole theory (Guadalajara, 2006).

Every vector has a positive and a negative pole, as well as a middle or central point that has a value of zero or zero. If a registration electrode faces the positive charge of the vector, a positive deflection will be inscribed, and if it faces the negative charge, the deflection will be negative. If the recording electrode has a situation perpendicular to the vector, an isodiphase deflection will be registered, whose net voltage is zero (Guyton y Hall, 2011).

Gallardo, L. B. (2018). Dipole [illustration].

There are 12 derivations, described by Einthoven, as the vertices of an equilateral triangle, with the heart in the center of it. Scanning electrodes placed on specific parts of the body record the depolarization vectors. As the heart can not be seen from only one place, as is a three-dimensional organ, it has to be seen from different places to assess the electrical activity (Guadalajara, 2006). The main planes used are the horizontal and the frontal, depending on the place where the electrode is placed and will be the electrical characteristics that we will appreciate. The derivations can be divided into:

• Bipolar (DI, DII, DIII)
• Monopolar (aVR, aVL, aVF)
• Precordial or horizontal plane (V1,V2, V3, V4, V5 y V6)

Gallardo, L. B. (2018). Axis Derivations [illustration].

It indicates the average direction toward where the process of depolarization or repolarization of cardiac cells is going. Your information is valuable for clinical practice, since it may indicate, in the first instance, alterations in the disposition of the heart and not necessarily indicate a pathology (Guadalajara, 2006). In itself, the electrical axis is no more than the direction of the total vectors of depolarization of the ventricles. Its calculation is very simple, we only look at whether the QRS of the DI and aVF leads is positive or negative. With these data, we can determine if the cardiac axis is normal or is deviated.

• If the QRS in DI and aVF is positive, the axis is normal.
• If the QRS in DI and aVF is negative, the axis has an extreme excursion.
• If the QRS is negative and aVF is positive, the axis is deviated to the right.
• If QRS is positive in DI and negative in aVF, it is necessary to evaluate derivation II: if it is positive in DII, the axis is normal; if it is negative in DII, the axis is deviated to the left.

Gallardo, L. B. (2018). Cardiac Electrical Axis [illustration].