Hemodynamic Measurements
and
Dynamics
of
Assisted Circulation
233
Tb
-
Td
&-T
F,=
(8.1.2 1)
where
Tid
is the temperature
of
the injectate through the catheter at the
delivery site. F, has been reported to be between
0.8
and
0.9.
8.1.4
Measurement of Vascular Dimensions
Measurements of geometric dimensions of blood vessels, such as length,
diameter
and
wall
thickness,
are
of
considerable
importance in
quantifying dynamic behavior. Strain gages are popular for length
measurements.
Mercury-in-silastic rubber,
constantan,
silicon, and
germanium transducers
are examples.
They are based either on
dimensional change or resistivity change. Change in resistance
(AR)
is
derived from:
R=--
Prl
A
(8.1.22)
where
A
is the cross-sectional area and
1
is the length of the strain gage
wire. The fractional change in resistance is given by:
A1
APr
-(1+
20)-+-
R
1
Pr
AR
--
(8.1.23)
where
G
is the Poisson ratio (ratio of radial strain to longitudinal strain).
The first term
on
the right-hand side is due to dimensional effect, the
second term to piezoresistive effect. Strain gage transducers can be
applied to measure length as well as pressure. In both cases, the resultant
change in resistance is detected by a Wheatstone bridge circuitry.
Superior resolution with high gage factors can be obtained with
semiconductors.
High-resolution dimension measurement can also be obtained with
ultrasonic dimension gages. The disadvantage is more complex circuitry.
The method requires a pair of piezoelectric transducers
(1-15
MHz)
either sutured or glued on to the opposite sides of a vessel for pulsatile
diameter measurement or for wall thickness measurement. It is operated
