Biomedical Engineering Reference
In-Depth Information
Since the percutaneous valve is not sutured in place, it can migrate with time.
Also the need to collapse the entire valve structure, both the stent and the tissue,
into a very small cross-sectional area may damage the tissue, resulting in
reduced durability.
As discussed previously on valve design, many percutaneous valves are fixed
rigidly to the aortic wall, unlike a surgical valve where the stent is suspended
somewhat in the central orifice. While the stent on a surgical valve can flex and
provide some compliance to the tissue when it is under heavy loads, a
percutaneous valve cannot. Therefore, the commissures of the leaflets on a
percutaneous valve remain in a fixed position during periods of heavy loads,
which greatly increases the forces on the tissue. If the forces exceed the
mechanical properties of the tissue, the leaflet can tear prematurely, resulting in
reduced durability.
The limitations of current percutaneous valve technology stand in stark
contrast to the relative success of traditional surgical valves, but proponents
argue that it is a risk±benefit discussion (Chiam and Ruiz, 2008). Percutaneous
valves are intended for use in high risk patients who are too sick to survive open
heart surgery (Webb et al., 2009). As such, these patients have a greatly reduced
life expectancy compared with the traditional surgical patient, and any
theoretical reduction in long-term durability is sufficiently balanced by the
immediate benefits to the patient of improved cardiac output and extension of
life. There also remains the interesting possibility that these terminally ill
patients may recover additional cardiac function once the defective valve is
replaced. Such recovery is seen in terminal patients awaiting a heart transplant
who have been implanted with a ventricular assist device (Klotz et al., 2008).
An additional difficulty in the development of percutaneous valves is that it
greatly disrupts the current practice of medicine, where the cardiovascular
surgeon is no longer the implanting clinician. Because percutaneous valves are
usually implanted through the femoral artery, the skills of interventional
cardiologists and radiologists are more appropriate. This has added a con-
siderable amount of debate to the issue, although the groups seem to be realizing
and appreciating the skills of the other, and in Europe, a team-based approach is
emerging, which will undoubtedly provide the best level of care for the patient.
A final dynamic that is certainly important to investors and others who look
at the financial aspects of new technology development is that the manufacturer
can obtain much higher payments for a percutaneous valve than for a traditional
surgical valve. True, a percutaneous valve requires the development of an entire
system of products, not just the valve, but also the catheter, the specialized
stent, and development of the implantation procedure. Still, those features were
also required when surgical valves were originally developed, although today's
surgical valves benefit from the investments in the tools and techniques made
years ago. The upside is substantial, and while Edwards Lifesciences reports
average selling prices of $4500±5000 for a traditional surgical valve, with
