Hemodynamics
of
Large Arteries
83
CO
=
SV
x
HR
(4.1.7)
ejection fraction has been widely used in the clinical setting as an index
of
LV
performance.
It
is clear that the pressure-volume relationship
provides
a
gross assessment of the global mechanical performance of the
heart.
Another important aspect of the pressure-volume curve is the area
under the loop which represents the mechanical work performed by the
ventricle to overcome
its
vascular load. For this reason, it is often
termed the external work of the heart and the area under the
P-V
loop is
also known as the "work loop". This loop area
is
approximately the
product of mean aortic pressure and the stroke volume, viz.
EWEPXSV
(4.1.8)
As a general definition, afterload can be considered as the forces that
resist ejection of the ventricular outflow. The dynamics
of
the vascular
system thus plays a vital role in determining this outflow.
4.1.5
Coupling of the Ventricle and the Arterial System
In assessing the coupling and the interaction of the heart and the arterial
system several methods have been proposed.
To
simplify the analysis,
approaches have mostly been based on models with lumped parameters.
One such method the arterial system
is
represented by an effective
arterial elastance
(E,),
although
E,
does not directly reflect the physical
elastic properties of the arteries.
Elastance defined here, reflects only a
system property, i.e.
it
does not equal the elastic properties of arteries. It
is derived from the three-element windkessel model and based on the
assumption that the arterial system behaves linearly. As such, it is
a
steady-state parameter that incorporates peripheral resistance, arterial
compliance and characteristic impedance
of
the aorta, and systolic and
diastolic intervals,
Rs
[t,
+
z(l
-
e-q]
E,
=
(4.1.9)
