Heart valve prosthesis

Functional requirements of heart valve prostheses

The functioning of natural heart valves is characterised by many advantages:

• Minimal regurgitation- This means that the amount of bloodlost upstream as the valve closes is small. For example, closure regurgitation through the mitral valvewould result in some blood loss from the left ventricleto the left atriumas the mitral valve closes. Some degree of valvular regurgitation is inevitable and natural (Fixme: Give indicative value). However, several heart valve pathologies (e.g. rheumatic endocarditis) may lead to clinically significant valvular regurgitation. A desirable characteristic of heart valve prostheses is that regurgitation is minimal over the full range of physiologicalheart function (i.e. complete functional envelope of cardiac outputvs. heart rate).
• Minimal transvalvular pressure gradient - Whenever a fluidflows through a restriction, such as a valve, a pressuregradientarises over the restriction. This pressure gradient is a result of the increased resistance to flow through the restriction. Natural heart valves have a low transvalvular pressure gradient as they present little obstruction to the flow through themselves (Fixme: Give indicative value). A desirable characteristic of heart valve prostheses is that their transvalvular pressure gradient is as small as possible.
• Non-thrombogenic- As natural heart valves are lined with an endotheliumcontinuous with the endothelium lining the heart chambers they are not normally thrombogenic. This is important as should thrombusform on the heart valve leafletsand become seeded with bacteria, so called "bacterial vegetations" will form. Such vegetations are difficult for the body to deal with as the normal physiologicaldefense mechanisms are not present within the valve leaflets because they are avascularand largely composed of connective tissue(Fixme: Create article discussing the pathgonesis of leaflet bacterial vegetations.). Should bacterial vegetations form on the valve leafets they may continually seed bacteriainto the arterial treewhich may lead to bacteremiaor septicaemia. Portions of the vegetation may also break off forming septic emboli. Septic emboli can lodge anywhere in the arterial tree(e.g. brain, bowel, lungs) causing local infectious foci. Even dislodged fragments from non-infectious vegetations (Fixme: Is this the correct terminology?) can be hazardous as they can lodge in, and block, downstream arteries(e.g. coronary arteriesleading to myocardial infarction, cerebralarteries leading to stroke). A desirable characteristic of heart valve prostheses is that they are non or minimally thrombogenic.
• Self-repairing - Although of limited extent compared to well vascularised tissue (e.g. muscle), the valve leaflets do retain some capacity for repair due to the presence of regenerative cells(e.g. fibroblasts) in the connective tissuefrom which the leaflets are composed. As the human heart beats approximately 3.4x10¹² times during a typical human lifespan this limited but nevertheless present repair capacity is critically important. No heart valve prostheses can currently self-repair but replacement tissues grown using stem celltechnology may eventually offer such capabilities. (State that they wear).
• Rapid dynamic response - STD

Types of heart valve prostheses

• Biological heart valves
  • Allograft/autograft/isograft
  • Xenograft
• Artificial heart valves
  • Percutaneousimplantation
    • Stentframed
    • Not framed
  • Sternotomy/Thoracotomyimplantation
    • Ball and cage
    • Tilting disk
    • Bi-leaflet
    • Tri-leaflet

Design challenges of heart valve prostheses

• Thrombogenesis / haemocompatibility
  • Mechanisms:
    • Forward and backward flow shear
    • Static leakage shear
    • Presence of foreign material (i.e. intrinsic coagulation cascade)
    • Cellular masceration
• Valve-tissue interaction
• Wear
• Blockage
• Getting stuck
• Dynamic responsiveness
• Failure safety
• Valve orifice to anatomical orifice ratio
• Trans-valvular pressure gradient
• Minimal leakage

Typical configuration of a heart valve prosthesis

• Anchor
• Leaflets
• ...

(Editing note: Will be spending an hour or so a day to build up this article.)

External link

• Page describing types of heart valve replacements

This article is licensed under the GNU Free Documentation License.
It uses material from the http://en.wikipedia.org/wiki/Heart+valve+prosthesis Wikipedia article Heart valve prosthesis.