Hemodynamics
of
Large Arteries
99
Pace
(1
975)
at aortic arch junction suffice to illustrate these (Fig.
4.2.5).
It is clear from these impedance spectra that ascending aorta has the
lowest impedance modulus and the impedance modulus is higher through
descending thoracic aorta, the left subclavian, the brachiocephalic and
the carotid arteries. The characteristic impedances are also higher in
smaller arteries due to their increased stiffness and reduced lumen
diameter.
Resistance of vascular beds that are perfused by smaller
arteries is also higher.
Ventricular afterload is defined
as
all external factors that oppose
ventricular ejection. For this reason arterial input impedance has been
suggested as being afterload. It is important to note that both the ability
of the left ventricle to do work (myocardial performance) and the
properties of the arterial system are important in determining the power
generated by the ventricle.
In general, input impedance as predicted by the three-element
windkessel model gives a reasonable overall estimate of experimentally
measured input impedance. This is more
so
in moduli than in phase.
With vasoconstriction, the impedance modulus is increased and its first
minimum is shifted to a higher frequency.
With vasodilation, the
impedance modulus decreases and its first minimum is shifted to a lower
frequency. The corresponding zero-phase crossing also applies. This
indicates that wave reflections arrive earlier due to a closer effective
reflection site in the case of vasoconstriction.
4.3 Wave Propagation Phenomena
4.3.1
The
Propagation Constant
For a pressure pulse wave propagating along a uniform artery without the
influence of wave reflections, the pressures measured simultaneously at
any
two
sites along the vessel are related by:
P2
=
PFF
(4.3.1)
