Biomedical Engineering Reference
In-Depth Information
FIGURE 5-2
Waveform showing
the potential
difference across a
cell membrane as a
function of time.
conduction rate, or conduction velocity. This rate varies widely depending on the type
and diameter of the nerve fiber, but it is usually something between 20 and 140 m/s in
nerves. Propagation through heart muscle is much slower, with an average rate of only
0.2 to 0.4 m/s, and some time-delay fibers between the atria and ventricles propagate even
more slowly at between 0.03 and 0.05 m/s (Cromwell, Weibell et al., 1973).
5.2.2 Signals Characterized by Source
As can be seen from Table 5-1, a range of potentials from less than 1
V right up to
100 mV and frequencies right up to 10 kHz must be accommodated. One of the major
difficulties arises where a small amplitude signal must be examined in the presence of
noise or another much larger signal.
μ
5.2.3 Signals Characterized by Type
5.2.3.1 Static and Quasi-Static Signals
Static signals are by definition unchanging over a long period of time. Such signals are
essentially direct current (DC) levels and in isolation convey very little information. Quasi-
static signals are those that change very slowly with time, such as the long-term drift on
a sensor or the decreasing voltage on a slowly discharging battery.
5.2.3.2 Periodic and Repetitive Signals
Periodic signals repeat themselves on a regular basis, though the timescale for repetition
can be from femtoseconds up to days or even years. These include sine, square, and
sawtooth waves, and their defining nature is that each waveform is identical. Repetitive
signals are periodic in nature, but the exact shape may change slightly with time. Ultimately,
very few biological signals are truly periodic, so ECG signals and estrogen levels in the
blood are good examples of repetitive signals.
5.2.3.3 Transient and Quasi-Transient Signals
Transient signals are, by definition, one time only, whereas quasi-transient signals are
periodic but with a duration that is very short compared with the period of the waveform.
