Hemodynamic Measurements
and
Dynamics
of
Assisted
Circulation
227
Fig.
8.1.9:
Illustration
of
the electromagnetic blood
flow
measurement principles. The
moving conductor (blood with velocity, v) in a magnetic field (B) induces
an
electric
motive force and the potential
(E)
is picked up by the electrodes.
Wyatt (1 984) has recently reviewed
blood flow and
velocity
measurement by electromagnetic induction. The cannular flowmeters
with uniform field and point electrodes have the defect of
a
high degree
of dependence on velocity distribution when they are not symmetric.
The perivascular flowmeters have
two
defects: in addition to their
sensitivity to velocity distribution, they are also sensitive to wall
conductivity effects. The former can be reduced by using insulated- or
multiple-electrodes. which improve the signal-to-noise ratio.
Mills'
(1966) catheter-tip flowmeter has the advantage that it is unaffected by
the vessel wall. Boccalon et al. (1978) has devised
a
noninvasive
electromagnetic flowmeter which provides useful clinical applications.
8.1.3.2 Ultrasound Doppler Velocimeters
Ultrasonic methods of measuring blood flow velocity are based on either
the transmission or the reflection of ultrasound. Ultrasound propagation
velocity through biological tissue
is
about 1560 m/s.
Its associated
wavelength can be easily calculated from
C
A=-
f
(8.1.14)
