Hemodynamics
of
Large
Arteries
87
E,,
is
increased, indicating changing cardiac contractility (Fig. 4.1.6).
Thus, the coupling ratio
E,/E,,
can be altered by either change in cardiac
contractility or arterial load.
Volume
Imll
0
80
120
150
2
Vdurne
Irnl)
Fig.
4.1.6:
Pressure-volume diagrams of the ventricleillustrating the steady state coupling
of
the ventricle and the arterial system using the ratio of
E,/E,,.
With methoxamine and
nitroprusside infusions, the slope
of
Ees is unchanged, while E,
is
altered.
With
dobutmine infusion, Ees is increased, while
E,
is unchanged. From Starling
(1993).
The effects of alterations of windkessel model parameters on the
ventricular pressure-volume relation under steady state conditions have
been investigated (Maughan
et
al.,
1984).
The changes in arterial
compliance and peripheral resistance have little effects on the slope
of
the end-systolic pressure-volume relation,
However, the shape and the
trajectory
of
left ventricular elastance are significantly altered.
It should be noted here that there are other indices to cardiac
performance, such as the pump function curve proposed by Van den
Horn et al.
(1
984), the maximum velocity of cardiac muscle shortening
concept suggested by Brutsaert (1
974)
based on force-velocity-length
relations.
4.1.6
Dynamic Heart-Arterial System Interactions
Blood
pressure varies during the cardiac
cycle. Thus, vascular
compliance
is
also expected to vary continuously. This dynamic time-
varying, pressure-dependent compliance property is incorporated in a
