Biomedical Engineering Reference
In-Depth Information
EXAMPLE PROBLEM 11.2
Ventricular fibrillation (VF) is a cardiac arrhythmia in which there are no regular QRS com-
plexes, T waves, or rhythmic contractions of the heart muscle (see Figure 11.1b). VF often leads
to sudden cardiac death, which is one of the leading causes of death in the United States. What
type of biosignal would most probably be recorded by an ECG when a heart goes into VF?
Solution
An ECG recording of a heart in ventricular fibrillation will be a random, continuous, bioelectric
signal.
11.4 SIGNAL ACQUISITION
11.4.1 Overview of Biosignal Data Acquisition
Biological signals are often very small and typically contain unwanted interference or
noise. Such interference has the detrimental effect of obscuring relevant information that
may be available in the measured signal. Noise can be extraneous in nature, arising from
sources outside the body, such as thermal noise in sensors or 60-cycle noise in the electronic
components of the acquisition system. Noise can also be intrinsic to the biological media,
meaning it can arise from adjacent tissues or organs. ECG measurements from the heart,
for instance, can be affected by bioelectric activity from adjacent muscles.
In order to extract meaningful information from biological signals sophisticated data
acquisition techniques and equipment are commonly used. High-precision low-noise
equipment is often necessary to minimize the effects of unwanted noise. Figure 11.4 shows
the basic components in a bioinstrumentation system.
Throughout the data acquisition procedure, it is critical that the information and struc-
ture of the original biological signal of interest be faithfully preserved. Since these signals
are often used to aid the diagnosis of pathological disorders, the procedures of amplifica-
tion, analog filtering, and A/D conversion should not generate misleading or untraceable
distortions. Distortions in a signal measurement could lead to an improper diagnosis.
11.4.2 Sensors, Amplifiers, and Analog Filters
Signals are first detected in the biological medium, such as a cell or on the skin's surface,
by using a sensor (see Chapter 6). A sensor converts a physical measurand into an electric
output and provides an interface between biological systems and electrical recording
instruments. The type of biosignal determines what type of sensor will be used. ECGs,
for example, are measured with electrodes that have a silver-silver chloride (Ag-AgCl)
interface attached to the body that detects the movement of ions. Arterial blood pressure
is measured with a sensor that detects changes in pressure. It is very important that the sen-
sor used to detect the biological signal of interest does not adversely affect the properties
and characteristics of the signal it is measuring.
