234
Dynamics
of
the Vascular System
in the PW mode at
f
=I KHz.
Figure
4.1.1
gave an example
of
dimensional measurement (cardiac muscle segment length) with the
ultrasonic dimension gages. The small size of piezoelectric crystal
ultrasonic dimension transducers allows their implantation for chronic
and conscious animal studies. Dynamic measurements of large vessel
diameter and wall thickness can be simultaneously recorded with
ultrasound operating in M-mode.
However, its limitation lies in
boundary identification and resolution. Similar problem is encountered
with angiographic recording. Magnetic resonance imaging affords high
resolution, but the disadvantage of the inability to provide real time
recording. Recent advance in intravascular ultrasound
(NUS)
provides
structural detail. as well as dimension measurements.
8.2
The Assisted Circulation and the Intra-Aortic Balloon Pump
8.2.1
Mechanical Assist Devices and the Assisted Circulation
The heart is generally viewed as an energetic mechanical pump. As
such, a means to assist its function under adverse conditions need
to
be
mechanical in nature. With limited donors for heart transplants and the
limited success with long term use of artificial hearts (although the most
recent patient
survived about
4
months), other avenues of cardiac
assistance for a failing heart take center-stage. Mechanical assistance and
replacements for damaged hearts have grown in application in recent
years and is expected to grow in the near future. While many problems
have developed during clinical studies of total artificial hearts, artificial
heart assist devices appear to operate without such extensive difficulties.
The versatility of such devices are in their varied designs to be either
temporary or permanent, internal or external and can operate in
a
number
of modes relative to the natural heart. The most common mode for
implantable heart assist devices is to operate in parallel with the left
ventricle with blood pumped from the
left
atrium to the ascending aorta.
In
this mode (parallel to the left ventricle) the assist device can take over
varying percentages of the pumping work and blood flow. The most
common mode of operation for temporary assist devices, for example,
the intra aortic balloon pump (IABP),
is
in-series with the left ventricle.
The IABP is surgically inserted into the descending aorta below the
