Hemodynamics
of
Large Arteries
105
This formula also provides a new method for obtaining true propagation
constant from measured
input impedance and apparent propagation
constant (Li, 1987).
Because the apparent phase velocity, capp, is influenced by wave
reflections in the same manner that input impedance is affected, its
frequency spectrum is similar to that of input impedance. Thus, yapp is
also influence by wave reflections. An example
of
the frequency
dependence of apparent propagation constant calculated for the femoral
arterial bed is shown in Fig. 4.3.3. They are both dependent on the
magnitude and phase of the global reflection coefficient. y, on the other
hand, is, by definition, independent
of
reflections, and the manner it
varies with frequency has been quantified by Li et al. (1980, 1981). It
can be deduced that, in the absence of reflected waves,
r=O
and
papp=p,
i.e., apparent phase velocity equals true phase velocity. This is easily
seen from Fig. 4.3.4 from data obtained by Li et al. (1980) in a
viscoelastic tube.
‘“1
12
t
Fig.
4.3.3:
Apparent propagation constant obtained as a function of frequency for the
femoral arterial bed. From Li (1 980).
Apparent phase velocities have been obtained noninvasively with
tonometers. One such example is shown in Fig.
4.3.5.
The pressure
