86
Dynamics
of
the
Vascular System
ventricle no longer has the ability to develop pressure. Neglect
V,
results
in
a
simplified relation
Ees
=
Pes
Nes
(4.1.19)
Despite its obvious limitations, the simple ratio
of
E,/Ees
has been
used extensively to characterize the interaction
of
the heart and the
arterial system (e.g. Kelly et al.,
1992;
Starling,
1993).
Fig.
4.1.5
illustrates the steady state coupling
of
the ventricle and the arterial
system using the ratio
of
E,/E,,
(Sunagawa et al.,
1983).
ves
Ved
Volume
(ml)
Fig.
4.1.5:
Diagram defining the effective arterial elastance
E,,
showing its relation to the
ventricular elastance,
Ees.
It has been shown by that the use
of
peripheral vasoactive agents such
as. the potent vasoconstrictor methoxamine and vasodilator nitroprusside
(Li and Zhu,
1994),
do not alter the slope
of
the end-systolic pressure-
volume line, or
E,,
(Starling,
1993).
But the slope
of
E,
is
increased with
methoxamine and decreased with nitroprusside, as expected.
As
a result,
the ratio
of
E&,,
is altered, indicates changes in ventricle-arterial system
coupling. With dobutamine, on the other hand,
E,
stays unaltered, while
