P and T wave analysis in ECG signals using Bayesian methods

1.3 - Electrocardiography 15
the standard and augmented limb leads gives a 360 ◦ circle divided into twelve 30 ◦ sections that
is called the hexaxial reference system. Lead I (pointing toward the left arm) is taken as the
reference for 0 ◦ lead orientation, with increasing positive values in a clockwise direction to 180 ◦
and increasing negative values in a counterclockwise direction such that −180 ◦ is the same
orientation as +180 ◦ . Although Lead II, with an orientation of about 60 ◦ is the most typical
orientation of an axis pointing from atria to ventricles, it is not abnormal for heart orientation
to be in the range of −30 ◦ to +110 ◦ which can make Leads aVL, aVF, or III rather than Lead
II the primary indicator of normal cardiac depolarization.
Precordial leads
The 360 ◦ hexaxial reference system defines what is called the frontal plane of ECG monitoring
- a view of the heart from the front. To provide a view through the horizontal plane - viewing
the heart as if the body was sliced horizontally through the middle of the chest - six additional
“unipolar” precordial (chest) leads (designated V1 to V6) are placed on positions of the left
rib cage forming a quarter-circle around the heart (see Fig. 1.6). The ECG represents the
difference between each of these electrodes (V1-V6) and the central terminal.
Figure 1.6: The six standard precordial leads. Image adapted from [CAM06].
1.3.2 ECG interpretation
Of all the 12 leads, the first six are derived from the same three measurement points. Therefore,
any two of these six leads include exactly the same information as the other four. Over 90%
of the heart’s electric activity can be explained with a dipole source model. To evaluate this