226
Dynamics
of
the
Vascular
System
Tonometer
-*
"\-
Radial
artery
Fig.
8.1.8:
Illustration
of
the volume pulse method (top) and the pressure pulse method
(bottom). Notice the difference in the sensor placement.
8.1.3
Blood
Flow
Measurement
8.1.3.1 Electromagnetic Flowmeter
In-vivo accurate measurement
of
blood flow lagged behind that
of
pressure for decades. The electromagnetic flowmeter
is
based on
Faraday's law
of
induction:
e
=
v
x
B
-
J
I
oc
(8.1.13)
where the induced electric field is
E,
in a conductor moving with a
velocity v in a magnetic field intensity
B.
J
is the current density and
oc
is the conductivity
(of
blood).
Figure 8.1.9 illustrates the principle.
Bevir's (1971) virtual current theory, however, presents a more practical
analysis.
