Two different lead systems are commonly used for electrocardiography, limb leads examining in the frontal plane and precordial leads examining in the transverse plane (Table 13). Historically there were many other lead systems which are no longer used.
In the six frontal (coronal) plane leads, electrodes are attached to the left arm, right arm, and left (or right) leg. The three standard bipolar limb leads are: I, left arm (+) and right arm (-); II, left leg (+) and right arm (-); and III, left leg (+) and left arm (-) (Figure 11). The angles of the these leads in degrees are 0, +60, and +120, respectively, where zero degrees is arbitrarily located at the left arm.
There are also three unipolar augmented limb leads, where the negative electrode is the output of two limbs combined through 5,000 ohm resistors (Figure 12). In this arrangement the (+) electrode is at the: right arm, aVR; left arm, aVL; and left leg, aVF. The angles of these leads in degrees are -150, -30, and +90, respectively. These leads are spoken of as "augmented", because extra amplification is needed to achieve a pattern of the same magnitude as in the common limb leads I, II, and III.
Another set of 6 leads are located in the horizontal (transverse or cross-sectional) plane. These are referred to as the precordial leads (Figure 13). In this arrangement the negative electrode is the combined input from the three limbs. An (+) exploratory electrode touches the chest at six positions (V1 - V6) in a curving line from just to the right of the sternum, to left of the left ventricle, running under the left arm.
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