Engineering Ethics

Chapter 8

1994: Bjork-Shiley Heart Valve

Defect

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THE REPORTED STORY

The New York Times Abstract:

A unit of Pfizer Inc. has agreed to pay $10.75 million to settle Justice

Department claims that the company lied to get Federal approval for a

mechanical heart valve that has fractured, killing hundreds of patients

worldwide. (Meier, 1994)

THE BACK STORY

HEART VALVES

During the cardiac cycle, blood is transported from the atria to the ventri-

cles, to the systemic and pulmonary circulation, and back again. Within the

heart, blood flow is regulated by the four valves: the aortic, mitral, pulmonary,

and tricuspid valves. Each valve is made up of a few thin folds of tissue, called

leaflets or cusps, and keeps blood from flowing backward, or regurgitating,

when closed.

A diseased valve may not open or close completely to regulate blood

flow. Valvular disease may be congenital (present at birth) or may be caused

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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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