Hemodynamic Measurements and Dynamics
of
Assisted Circulation
2
17
where
P,
=
measured pressure for the nth harmonic and
Po,
=
actual
pressure of the nth harmonic component.
For
a
distortion-free blood
pressure measurement system, or one with
a
flat frequency response, it is
necessary that the amplitude ratio
P,/Po,
=1
.O,
or there is no difference
between the measured pressure and the actual pressure. Under this
condition, the phase angle
$,
=
0, i.e., there is no phase shift between the
two.
For the pressure measurement system to record the arterial blood
pressure waveform faithfully, it must have sufficient dynamic frequency
response (Li et al.,
1976).
This often results in changing the needle size
or length of the needle, especially when an additional pressure transducer
of different compliance specification is unavailable.
8.1.1.2
The Catheter-Pressure Transducer Systems
For a catheter-pressure transducer system, frequently an underdamped
system, compliance as well as geometric factors are important. Figure
8.1.1
provides
a
lumped approximation of the system. The above second-
order representation can be applied to evaluate dynamic frequency
response of the system.
Fig.
8.1.1
:
Lumped model representation of the catheter-manometer system.
R=
Poiseuille resistance
of
the fluid in the catheter.
C
=
compliance combination of the
catheter and the manometer (C=C,+C,;
C,=compliance
of
the catheter and C,=
compliance
of
the transducer). L
=
inertia of fluid.
Either a sinusoidal pressure generator or a step-response "pop-test"
are common methods to evaluate dynamic frequency response of the
catheter system.
.
Commonly, a step increase in pressure is applied
against the catheter-transducer
system and the balloon
which is
connected to the same chamber as the catheter is inflated. The balloon
