Hemodynamics
of
Large Arteries
111
type, with arterioles being the major reflection site. Reflected pressure
and flow waves are
180"
out of phase.
This means an increase in
reflection increases pressure amplitude, but decreases flow amplitude.
Measured pressure
(P)
and flow
(Q)
waveforms measured at any site
in the vascular system can be considered as the summation of
a
forward,
or antegrade, traveling wave and a reflected, or retrograde, traveling
wave:
P=p,+p,
(4.4.1)
Q
=
QJ
+
Q,
(4.4.2)
The forward and reflected pressure components can be resolved by
means
of
the following set of equations:
Pf
(P+ Q*Z0)/2
(4.4.3)
P,
=(P-Q.Z0)/2
(4.4.4)
where
Z,
is the characteristic impedance, defined as the ratio
of
forward
pressure to forward flow, or in other words, independent
of
wave
reflections:
Z,
can be obtained from the water-hammer formula:
Pc
z
=-
a
m2
(4.4.5)
(4.4.6)
where
p
is the density
of
blood
(1.06
g/cm3), c is pulse wave velocity, 7cr2
is the cross-sectional area
of
the artery.
With the characteristic
impedance determined by a time domain method, forward and reflected
waves can also be resolved in the time domain
(Li,
1986).
