84
Dynamics
of
the Vascular System
where t, and td are systolic and diastolic periods, respectively.
The
diastolic pressure decay time constant is shown as before,
z=R,C
(4.1
.lo)
E,
has been approximated by the ratio of end-systolic pressure to stroke
volume
(SV
or
V,),
or
E,
=
P,,
I
SV
(4.1.11)
When the mean arterial pressure is used to approximate
P,,,
then
E,
can
be easily estimated from
E,
=PI
V,
(4.1.12)
Approximation
of
mean pressure to
P,,
is good under
normal
physiological conditions, but poor during strong vasoactive conditions.
Cardiac output is the product of stroke volume and heart rate,
as
defined before,
(4.1.13)
co
=
v,
*
f,
where
1
fh
=r
(4.1.14)
T is cardiac period.
The effective arterial elastance can be rewritten as
R
T
E,
MS
(4.1.15)
since
R,
is simply the ratio
of
mean arterial pressure divided by cardiac
output.
Alternatively, when the diastolic aortic pressure decay time constant
is long compared with the diastolic period, or
z
>>
td,
the denominator of
equation (4.1.9) reduces to
