Datasheet

Thursday, 17 July 2003 MiniProject: Design Aspects Colin K McCord
EEE512J2 – Electronic Product Design Page 2 Chapter 2: Fundamentals
2.0. FUNDAMENTALS
2.1. Electrocardiography (ECG, EKG)
The heart is a muscular pump made up of four
chambers. The two upper chambers are called
atria, and the two lower chambers are called
ventricles.
The purpose of the atria is to act as ‘filling
chambers’ for the ventricles; the right side of the
heart is the pulmonary pump, i.e. it pumps blood
between the heart and the lungs, and the left side
of the heart is the systemic pump, i.e. it pumps
blood between the heart and the entire body.
The heart beats as a result of ‘commands’
passed in the form of bioelectric impulses and
action potentials. These action potentials result in
a series of rapid and successive patterns of depolarization and re-polarization across the cardiac muscle,
generating an electric signal. The electrical activity of the heart can be detected through the skin by small
metal discs called electrodes. The electrodes are attached to the skin on the chest, arms, and legs.
The cardiac cycle begins at the Sino-Atria node, located in the right atrium at the superior cava. The
beginning of the cycle corresponds to the contraction of the atria. Following this is a 100ms delay until the
activation of the Atria-ventricular node. This delay is important because it allows time for the ventricles to fill,
increasing the efficiency of the heart. The signal is then propagated down the ventricular septum resulting in
ventricular contraction. The signal generated over one period of the cardiac cycle is depicted in figure 2.1b
(P-Wave: Atria Depolarization, QRS-Complex: Ventricular Depolarization, T-Wave: Ventricular Re-
polarization).
P-Wave
QRS Complex
T-Wave Voltages (mV)
Time (ms)
Figure 2.1b. One period of the cardiac cycle
Note that the signal generated from the heart is extremely small (about 2mV’s in amplitude), and at a very
low frequency, having a bandwidth of about 150Hz.
The heart can be considered as an electric dipole, repetitively changing both in magnitude and direction as it
goes through the cardiac cycle. The magnitude of the dipole will be at a maximum during ventricular
contraction. This is important note, as it is quite likely that the smaller P and T waves will be lost in the effects
of noise. Therefore the theory behind detecting the cardiac signal is to place electrodes on the surface of the
body, and simply measure the different differences in potential that arise as the dipole moves through its
cycle.
The measured differences in potential are referred to as ‘leads’. Note that it is always a difference in potential
between at least two electrodes that is being measured, as there is no absolute zero reference voltage in the
Figure 2.1a. Heart Anatomy [W1]