82
Dynamics
of
the Vascular System
addition, it has been suggested that the separation of cardiac mechanical
pump function and contractile state can be obtained. Pump function is
related to loading conditions whereas contractility or inotropic state
depends only on the intrinsic properties of the contracting muscle fibers
or the myocardium.
The importance of P-V loop or pressure-diameter
(P-D)
loops of the
ventricle has been frequently emphasized. This diagram points
to
the
ESV as one of the functional determinants of this representation
of
ventricular pump function. Preload is indicated by the EDV and
SV
EDV
-
ESV
is the stroke volume.
(4.1.5)
4.1.4
Ejection Fraction, Cardiac Performance, Preload and Afterload
The amount of blood ejected out of the ventricle per beat or the stroke
volume is dependent not only on the operating Starling mechanism, or
the preload, it is also dependent on afterload, contractility and heart rate.
Starling's law can be expressed in terms of the ratio of the volume that is
ejected to the total resting volume. This ejection fraction is defined as:
EF
=
SV
I
EDV
(4.1.6)
The normal mammalian heart has an ejection fraction of 0.5 to
0.7.
Table 4.1.1 provides some of the reported values of ejection fraction in
different species of mammals.
Table
4.1.1
:
Ejection fractions in some mammalian species
Man
0.67
Dog
0.65
Cat
0.64
Rabbit
0.61
Together with cardiac output, the product of stroke volume and heart
rate,
