Heart valve prosthesis

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

• Minimal regurgitation - This means that the amount of blood lost upstream as the valve closes is small. For example, closure regurgitation through the mitral valve would result in some blood loss from the left ventricle to the left atrium as 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 physiological heart function (i.e. complete functional envelope of cardiac output vs. heart rate).
• Minimal transvalvular pressure gradient - Whenever a fluid flows through a restriction, such as a valve, a pressure gradient arises 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 endothelium continuous with the endothelium lining the heart chambers they are are not normally thrombogenic. This is important as should thrombus form on the heart valve leaflets and become seeded with bacteria, so called "bacterial vegetations" will form. Such vegetations are difficult for the body to deal with as the normal physiological defense mechanisms are not present within the valve leaflets because they are avascular and 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 bacteria into the arterial tree which may lead to bacteremia or 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 arteries leading to myocardial infarction, cerebral arteries 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 presense of regenerative cells (e.g. fibroblasts) in the connective tissue from 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 cell technology may eventually offer such capabilities.

• Biological heart valves
  • Allograft/autograft/isograft
  • Xenograft
• Artificial heart valves
  • Percutaneous implantation
    • Stent framed
    • Not framed
  • Sternotomy/Thoracotomy implantation
    • Tilting disk
    • Bi-leaflet
    • Tri-leaflet

• Thrombogenesis
  • Mechanisms:
    • Forward and bckward flow shear
    • Static leakage shear
    • Presense of foreign material (i.e. intrinsic coagulation cascade)
    • Cellular masceration

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

• Page describing types of heart valve replacements