Application to biomedical signals - Practical Biomedical Signal Analysis Using MATLAB - page 169

4.2 Heart signals
4.2.1 Electrocardiogram
Electrocardiogram (ECG) is a record of electrical activity of a heart. The contrac-
tion activity of a heart is driven by the electric pacemaker—the assembly of cardiac
cells that possess the property of automatic generation of action potentials. Such cells
are located in the sino-atrial node (SA), the atrio-ventricular node (AV), and in the
specialized conduction systems within the atria and ventricles. Myocardial cells have
the unique property of transmitting action potentials from one cell to an adjacent cell
by means of direct current spread.

4.2.1.1 Measurement standards

Einthoven—the pioneer in the field of electrocardiology—as a model of electrical
activity of the heart muscle, proposed a two-dimensional dipole placed in a homoge-
nous conducting volume of thorax in the middle of a triangle with vertexes defined
by the electrodes placed on the arms and left leg. Einthoven standard leads I, II, III
measured the potential differences: I- between the left arm (LA) and the right arm
(RA), II - (between the left leg (LL) and the RA), III- ( between the LL and the LA).
The propagation of the electrical activity throughout the cardiac cycle can be repre-
sented by an instantaneous electrical heart vector (Figure 4.26) . In spite of the crude
approximation of the body geometry, the projections of heart vector on the arms of
the Einthoven triangle became a standard in electrocardiography.
The typical leads used in electrocardiography are shown in Figure 4.26. They in-
clude three standard ECG leads: I, II, III, and the so-called augmented limb leads:
aVR, aVL, aVF corresponding to the potential recorded between a given limb (corre-
spondingly: right arm, left arm, left foot) with respect to the average of the potentials
of the other two limbs. In standard clinical applications signals from six leads placed
on torso are also used. The potentials measured between electrodes placed on the
limbs correspond to the projections on the frontal plane. The electrodes located on
a torso supply the information about ECG vector projection on the horizontal plane
(Figure 4.26).
In a three dimensional system proposed by Frank electrodes are placed according
to Cartesian XYZ axes ( Figure 4.27) . In principle three orthogonal projections are
sufficient to reconstruct the vector; however in case of ECG the redundancy in the
information is beneficial since the projection axes are non-orthogonal; the body is
not a homogenous sphere and the vector changes in time. Besides some particular
features of ECG are more visible in specific derivations.
Sampling frequency of ECG has to be chosen according to the aim of the analysis.
Commonly accepted range of ECG is 0.05-100 Hz. However, when ventricular late
potentials are considered the upper frequency range reaches 500 Hz.