100 Engineering Ethics: An Industrial Perspective
by an infection that invades the tissue. It may also be caused by rheumatic
fever, heart attack, stroke, or aging. Symptoms include dizziness, shortness of
breath, fatigue, irregular heart rhythms, and fluid retention. The disease may
be managed by drug therapy or, when severe, by surgical replacement.
A prosthetic replacement is necessary when stenosis or insufficiency
occurs. During stenosis, the opening of the valve has decreased, causing the
heart to work much harder to transport blood. During insufficiency, the valve
is leaky, causing blood to flow backward. A prosthetic replacement may
either be created from artificial materials or from animal or human cadaver
tissue. Mechanical heart valves, made of artificial materials, last for life but
may increase the risk of blood clots, causing a patient to take blood thinners
for life. Biologic heart valves need to be replaced every 10 to 15 years.
THE BJORK-SHILEY HEART VALVE
Dr. Viking Bjork, a cardiac surgeon at the Karolinska Hospital in
Stockholm, Sweden, first began to replace native valves in his patients with
mechanical heart valves during the 1960s. After experience with the Bahnson
Teflon cusp, Starr valve, and Kay-Shiley valve, he designed his own valve with
American engineer Donald Shiley. They based their design on the tilting disc
valve because this valve design decreased the pressure gradient across the
valve compared with the gradient across the Kay-Shiley valve. Additionally,
their free-floating disc valve, which was first composed of Delrin polyacetal
plastic, opened in the aortic position to 60 degrees. The original flat disc was
held in place by an inflow strut (shown in Figure 8.1 at top) and outflow strut,
which were welded to the valve ring.
A carbon-coated double-flange universal sewing ring surrounded the
valve. This first version of the Bjork-Shiley valve was first implanted in one
of Bjork’s patients in 1969. Later, in 1971, the flat disc material was
changed from Delrin to Pyrolyte carbon.
In order to minimize blood clot formation after implantation, Bjork and
Shiley modified the original disc shape to be convexo-concave in 1976.
Additionally, the disc pivot point was moved 2.5 mm downstream, allowing
the disc to pivot away slightly from the orifice ring in the open position.
The inflow strut was made an integral part of the valve ring without any
welds; the outflow strut was welded. The result was additional clearance
between the disc and the flange, which diminished the low-flow area behind
the disc by 50% and provided for a “washing effect” of the disc. It was claimed
that this new shape not only decreased blood clot complications by 50% but
kept the valve completely open with half the flow required with a flat disc.
It also had a much more rapid reaction on closure, resulting in reduced
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