Hemodynamics
of
Large Arteries
97
The characteristic maxima
and minima associated
with the input
impedance spectrum is closely related to reflections in the arterial
system.
The initial large decrease in impedance modulus accompanying a
negative phase indicates that the load facing the left ventricle at low
frequencies is capacitive in nature, while at high frequencies it is
inductive. The viscous losses in the proximal aorta are small, but are
more appreciable elsewhere in the arterial system (Li, 2000).
Hence,
approximating characteristic impedance by the high frequency values of
the input impedance is more accurate in the proximal aorta than in other
vessels.
(4.2.9)
Values of characteristic impedance of the aorta estimated from high
frequency average of the input impedance spectrum have been variable,
mostly due to the frequency range used for the estimation and the body
size. In the dog,
Z,
has been found to be
200-300
dyn.s.cm5 when a
typical high frequency range used for taking the average is
3-1
0
Hz.
In the time domain, a simple method to approximate characteristic
impedance
of
the aorta
is
to utilize the fact that in early ejection,
reflected waves cannot reach the proximate aorta. In this case,
2,
can be
obtained, simply from the ratio of instantaneous aortic pressure and flow
above their end-diastolic levels (Li,
1986):
(4.2.10)
This method is valid for the first
60-80
ms
of
ejection.
influence the measurement, is the classic water-hammer formula:
Another method, also with the assumption that reflected waves do not
pc
zo
=-
m2
(4.2.1
1)
