Application to biomedical signals - Practical Biomedical Signal Analysis Using MATLAB - page 170

FIGURE 4.26: The typical leads used in electrocardiography. At left—limb leads;
top right—chest leads. Bottom-right: the heart vector and its projection on the
Einthoven triangle.

4.2.1.2 Physiological background and clinical applications

The transmission of electrical heart activity is reflected in the features of the ECG
evolution. When depolarization propagates toward the given electrode the detected
voltage is seen as positive and is represented by an upward deflection in the ECG.
Let us consider the potential recorded by the lead II during the heart cycle. The ac-
tivation starts in the SA node and propagates toward AV, the depolarization of atria
follows with heart vector pointing down and left 9 which is reflected in the positive
P wave ( Figure 4.28) . Then there is a delay at the AV node corresponding to the
flat portion of the ECG. The septal depolarization with vector pointing to the right
corresponds to the Q wave. The period between the P wave and the QRS complex
corresponds to the time between the atrial and ventricular depolarization when ECG
is on isoelectric level. This period serves as a reference for ECG amplitude estima-
tion. The prominent QRS peak corresponds to the depolarization of ventricles with a
heart vector pointing to the left and down. The width of the QRS complex depends
on conduction speed through the ventricle. S-T segment corresponds to the pause be-
tween ventricular depolarization and repolarization. Next, a phase of repolarization
follows, during which both the direction of propagation and the electrical polarity
are changed which results in a positive T wave. U wave, which can be observed by
means of high resolution electrocardiography, and which is also connected with re-
polarization, follows the T wave. In case of a short heart cycle it might be impossible
to register the U wave.

9 The left and right concerns subjects' left and right.