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3.4 ECG leads
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3.4 ECG leads

In order to evaluate general picture of the heart electrical activity, there are some electrodes at different places, because the electrical impulses from the heart conductivity system reach the body surface in many directions. The ECG registries the information from different viewpoints, which are named, leads. A lead represents a particular view of the heart’s electrical activity between two points. They can help identify the location and determine the extent of possible myocardial damage. There are two types of leads – unipolar and bipolar – are widely used in current clinical practice.

 

Figure 9 – Einthoven’s triangle (this diagram can be converted to the following sequence)

In 1913 Einthoven considered that the right and the left extremities and the midpoint of left leg, when joined together, make up an equilateral triangle, and the heart occupy the middle of this triangle.

More extended (especially in emergency practice) are the bipolar (standard, limb) leads – leads I, II and III. This leads called limb leads because at one time, the subjects of electrocardiography had to accurately put their extremities in the basins of salt water that record impulses for string galvanometer. The three bipolar (or Einthoven's) leads are: lead I – dipole connecting the negative (red) electrode on the right arm and the positive (yellow) electrode on the left arm; lead II – dipole connecting the negative (red) electrode on the right arm and the positive (green) electrode on the left leg and lead III – dipole with the negative (yellow) electrode on the left arm and the positive (green) electrode on the left leg. The right foot is called grounding (black).

Also, another 9 leads, called unipolar, necessary to use in day to day clinical practice. The direction of unipolar leads is from the "center" of the heart radially outward and includes the chest leads and limb leads — VL, VR, & VF. In 1942 Goldberger intended augmented unipolar limb leads. They are originated from the same three electrodes as leads I, II, and III. But, they view the heart from different angles vectors because the negative electrode for these leads is a variation of Wilson's central terminal, which is derived by adding, leads I, II, and III together and plugging up them into the negative terminal of the ECG. For example, aVL lead formed by positive electrode from left arm and negative part resulting impulses from foot and right arm.

 

The resulting vectors of augmented unipolar limb leads show follow picture:

If the two leads systems (standard and augmented limbs) are located on top of each other, the following picture is produced:

 

Together with leads I, II, and III, unipolar limb leads aVR, aVL, and aVF outline the basis of the hexaxial reference system, which is used to estimate the heart's electrical axis in the frontal plane. The importance of this system is given below.

If the exploring electrodes are located over the heart region, they are called unipolar chest leads. Generally six chest leads are applied in everyday general practitioner work – V1, V2, V3, V4, V5 and V6. The following diagram shows the placement of these leads:

 

V1 – 4th intercostal space, at the right border of the sternum.

V2 – 4th intercostal space, at the left border of the sternum.

V3 – Midpoint between V2 and V4 in a line jointing these two points

V4 – 5th intercostal space, at the midclavicular line.

V5 – 5th intercostal space, at anterior axillary line.

V6 – 5th intercostal space, at midaxillary line.

Every lead relatively corresponds with anatomy of the heart. Therefore conventionally are distinguished «left» leads: leads I, II, aVL, V4, V5 and V6; and «right»: leads III, aVF, V1 and V2.  Lead V3 is a transition zone between right and left parts of the heart.

Consequently, there are twelve ordinarily leads, three of them (I, II & III) are standard and the rest of them are unipolar.



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