Biomedical Engineering Reference
In-Depth Information
12.2.2
Aortic Valve Substitutes
There are several diseases, both congenital and acquired, that render the aortic valve
incompetent. The clinical indicators which determine the decision for replacing a
malfunctioning valve are not discussed here, as they are out of the scope of this
textbook. We should bear in mind, however, that whatever the cause of disease,
besides the valve, other structures may be adversely affected. That is to say that if
the valve leaflets have been calcified probably the aorta is also calcified, meaning
that an accommodation process has been going on for sometime and replacing the
valve will leave the aorta still calcified, therefore stiffer, and that must be taken into
consideration for choosing, for example, the proper valve substitute.
The last statement poses therefore the question: why are there different types of
substitute aortic valves? How it all started? Which criteria are used for selecting one
substitute valve over another? What is the role of the materials and/or the design in
the overall performance of the valve in the recipient?
In the following, we shall attempt to address the above posed questions by
focusing mainly on the biomaterials aspects associated with this particular situation.
Brief History and Development of Substitute Valves
The anatomy of the human heart valves was known at least since the Hellenistic
times [
384
]. Leonardo da Vinci has produced remarkable human anatomical draw-
ings, the valves also included [
10
]. A representative example of these drawings is
depicted in Fig.
12.5
. It was in 1931 that the topographic anatomy and histology
of the valves of the human heart was described in a publication by Gross and
Kugel [
385
]. However, it took another 25 years, i.e., until 1955 for heart valve
replacement to be used clinically. Why this delay?
The reason was that to surgically operate on the heart, the heart itself could not
operate as the pump of the blood circulation while at the same time the lungs could
not serve their purpose for the gas exchanges of the blood elements. Therefore, an
assist system to carry out these functions was necessary so that the surgeon could
operate on the heart. The, so-called, heart-lung machine provided the solution to
this problem. This
machine
receives the venous blood containing CO
2
just before
entering the right heart, via a catheter. It pumps the blood through a
membrane
oxygenator
, where gas exchange takes place, i.e., the CO
2
leaves the blood, while
O
2
is loaded onto the blood by reacting with the protein
hemoglobin
. Then the
oxygenated blood is pumped back to the patient's aorta. Figure
12.6
shows the first
artificial oxygenator developed by the Gibbons.
The cardiac surgeon then, from the mid-1950s, could operate for a limited time
on the heart. The time is limited by blood-material interaction and is discussed
in Sect.
12.4
. He could excise, for example, a malfunctioning valve and suture a
substitute device in its place to perform the excised valve's function. What could
such a device be?
