SOURCES OF BIOMEDICAL SIGNALS
Biomedical signals are those signals which are used primarily for extracting information on a biological system under investigation. The process of extracting information could be simple as feeling the pulse of a person on the wrist. Biomedical signals orginated from a variety of sources.
These are unique to biomedical systems.They are generated by nerve cells and muscle cells.Their basic source is the cell membrane potential which under certain conditions may be excited to generate an action potential.The electric field generated by the action of many cells constitutes the bioelectrical signals.The most common examples are ECG and EEG signals.
The measurement of acoustic signals created by many biomedical phenomena provides information about the underlying phenomena.The examples of such signals are:flow of blood in the heart,through the heart’s valves and flow of air through the upper and lower airways and in the lungs which generate typical acoustic signals.
These signals orginate from some mechanical function of the biological system.They include all types of motion and displacement signals,pressure and flow signals etc.The movement of the chest wall in accordance with the respiratory activity is an example of this type of signal.
The signals which are obtained as a result of chemical measurements from the living tissue or from samples analysed in the laboratory.The examples are measurement of partial pressure of carbondioxide and concentration of various ions in the blood.
Extremely weak magnetic fields are produced by various organs such as the brain,heart and lungs.The measurement of these signals provide information which is not available in other types of biosignals such as bioelectric signals.A typical example is that of magneto-encephalograph signal from the brain.
These signals ae generated as result of optical functions of the biological systems,occurring either naturally or induced bythe measurement process.For example,blood oxygenation may be estimated by measuring the transmitted/back scattered light from a tissue at different wavelength.
The impedance of the tissue is a source of important information concerning its composition,blood distribution and blood volume etc.The measurement of galvanic skin resistance is a typical example of this type of signal.The bio-impedance signal is also obtained by injecting sinusoidal current in the tissue and measuring the voltage drop generated by the tissue impedance.The measurement of respiration rate based on bio-impedance technique is an example of this type of signal.