96
Dynamics
of
the Vascular System
Pressure and flow waveforms do not necessarily contain the exact
number
of
significant harmonics. When the flow harmonic becomes very
small, the ratio of pressure to flow or the impedance modulus for that
harmonic component obtained can be erroneous.
Input impedance of the systemic arterial tree have been obtained in
the systemic pulmonary and coronary arterial circulations.
Vascular
impedances have been measured in man, dog and other mammalian
species. Fig.
4.2.4
gives an example of the modulus and phase of the
vascular impedance measured at the ascending aorta or the input
impedance of the systemic arterial tree in normal adults.
0.
Fig.
4.2.4:
Modulus and phase
of
the impedance measured at the ascending aorta or input
impedance
of
the systemic arterial tree in a normal adult
(A)
and the average obtained
in
five adults
(B).
Z,
indicates characteristic impedance
of
the proximal aorta. From
Nichols and O’Rourke
(1998).
Input impedance shows
a
large decrease in magnitude at very
low
frequencies
(<2
Hz), then oscillates, exhibiting maxima and minima, and
eventually reaches
a
somewhat constant level-low compared to
its
zero
frequency
(DC)
value, at higher frequencies
(>5
Hz).
The input
impedance
(Z)
approaches the characteristic impedance
(Z,) of
the
proximal aorta at these high frequencies. The phase of the impedance is
initially negative, becoming progressively more positive and crossing
zero at about
3-5
Hz, and remains positive but close to zero thereafter.
