Download mechanical valves

Bs Ngô Thị Cẩm Hoa
Khoa Phẫu Thuật Tim- BV 115
CONTENTS
1. Types of heart valve prostheses
2. Mechanical valves
2.1 Types of mechanical heart valves
2.2 Durability
2.3 Blood damage
3. Biological valves
4. Selection of an artificial valve.
5. Antithrombotic therapy in MHV
Types of heart valve prostheses
There are two main types of artificial heart
valves:
- The mechanical valves.
- The biological valves.
Types of heart valve prostheses
- Mechanical heart valves
Ball and cage
Tilting disk
Bi-leaflet
Tri-leaflet
- Biological heart valves
Allograft/ autograft
Xenograft
Mechanical valves
- The
first mechanical prosthetic heart valve was implanted in 1952. Over the
years, 30 different mechanical designs have originated worldwide.
- These valves have progressed from simple caged ball valves, to modern
bileaflet valves.
- Heart valves are designed to fit the peculiar requirements of blood flow
through the specific chambers of the heart, with emphasis on producing
more central flow and reducing blood clots.
Mechanical valves
Caged ball design
Tilting-disc valves
Bileaflet valves
The caged ball valve
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It is one of the early mechanical heart valves.
It uses a silicone ball that is held in place by a
welded metal cage.
When blood pressure in the chamber of the heart
exceeds that of the pressure on the outside of the
chamber the ball is pushed against the cage and
allows blood to flow. At the completion of the
heart's contraction, the pressure inside the chamber
drops and is lower than beyond the valve, so the ball
moves back against the base of the valve forming a
seal.
The caged ball valve
Disadvantages
Caged-ball valves completely block central flow.
Therefore the blood requires more energy to flow
around the central ball.
The ball is notorious for causing damage to blood
cells due to collisions. They have a high tendency to
forming blood clots, so the patient must have a high
degree of anti-coagulation, usually with a target
INR of 2.5-3.5.
Edwards Lifesciences discontinued production of
the Starr-Edwards valve in 2007.
The tilting-disc valves
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In the mid-1960s, a new class of prosthetic valves were
designed that used a tilting disc to better mimic the natural
patterns of blood flow.
The tilting-disc valves have a polymer disc (pyrolytic
carbon disc) held in place by two welded struts (metal
struts).
The disc floats between the two struts in such a way, as to
close when the blood begins to travel backward and then
reopens when blood begins to travel forward again. The
titling-disc valves open at an angle of 60° and close shut
completely at a rate of 70 times/minute.
The tilting-disc valves
Advantages
- This tilting pattern provides improved central flow
while still preventing backflow.
- The tilting-disc valves reduce mechanical damage
to blood cells.
Disadvantages
- The only problem with this design is its tendency
for the outlet struts to fracture as a result of fatigue
from the repeated ramming of the struts by the
disc.
The Medtronic-Hall; Bjork-Shiley; Harken valve;…
Bileaflet valves
In 1979, a new mechanical heart valve were
known as bileaflet valves and consisted of
two semicircular leaflets that pivot on hinges.
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- The carbon leaflets exhibit high strength
and excellent biocompatibility. The leaflets
swing open completely, parallel to the
direction of the blood flow. They do not close
completely, which allows some backflow.
Bileaflet valves
Advantages
-These valves are distinguished mainly for providing the
closest approximation to central flow achieved in a natural
heart valve.
-These bileaflet valves have the advantage that they have
a greater effective opening area (2.4-3.2 square cm c.f.
1.5-2.1 for the single-leaflet valves). Also, they are the
least thrombogenic of the artificial valves.
Disadvantages
- They do not close completely, which allows some
backflow. Since backflow is one of the properties of
defective valves, the bileaflet valves are still not ideal
valves.
Durability
- Mechanical heart valves are today very
reliable and allow the patient to live a
normal life.
- Most mechanical valves last for at least 20 to
30 years
Blood damage
Clots formed by red blood cell (RBC) and
platelet damage can block up blood vessels
and occurs in one of three basic pathways:
- Tissue factor exposure.
- Platelet activation.
- Contact activation by foreign materials.
Blood damage
All MHV models are vulnerable to thrombus
formation due to:
- High shear stress.
- Stagnation.
- Flow separation.
Blood damage
The caged-ball designs:
Experience high stresses at the walls that can damage cells.
Flow separation due to high-velocity reverse flow surrounded
by stagnant flow.
Tilting-disc valves:
Flow separation behind the valve struts and disc as a result of a
combination of high velocity and stagnant flows.
The bileaflet models:
- High stresses during forward and leakage flows as well as
adjacent stagnant flow in the hinge area. The hinge area is the
most critical part of bileaflets and is where the thrombus
formation is usually prevalent.
Biological valves
There are two groups:
- Human tissue valves (Homografts, Autografts)
- Animal tissue valves (xenografts, pericardial sac):
Biological valves
Human tissue valves:
Homografts: within 24h of donor death, sterilized with
antibiotics and preserved at -196C. They are inserted
directly, usually in the aortic position.
Autografts: ROSS Procedure.
Animal tissue valves:
Porcine heterografts ( the Hancock valve, The CarpentierEdwards, The Medtronic Intact valve).
Stentless Porcine Xenografts: St. Jude Medical valve, The
Edwards stentless valve, the Medtronic Freestyle valve).
Pericardial aortic valves: Bovine pericardial valves.
Biological valves
- They do not require long-term anticoagulats.
- They have better hemodynamics.
- They do not cause damage to blood cells.
- They do not suffer from many of the structural
problems experienced by the mechanical heart
valves.
Biological valves
Selection of an artificial valve
The following groups of patients should receive bioprosthetic valves :
- Patients with coexisting disease who are prone to hemorrhage and who
therefore tolerate antocoagulants poorly: bleeding disorders, instertinal
polyposis, angiodysplasia
- Noncompliant with permanent anticoagulants treatment, who are unwilling to
take anticoagulants on a regular basis or who live in developing nations and
cannot be monitored
- Patients older than 65 years old in whom bioprosthetic valves deteriorate
slowly and because of their age may also be at greater risk hemorrhage
while taking anticoagulants
- Patients with a small annulus in whom an unstented bioprosthetic graft may
provide superior hemodynamics
- Woman patients (< 40 years of age)
Conclusion
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There are two main types of artificial heart valves:the mechanical
valves,the biological valves.
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The most used heart valves in the US and EU are those utilizing tissue
leaflets. Mechanical valves are more commonly used in Asia and Latin
America.
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The following companies manufacture tissue heart valves: Edwards
Lifesciences, Medtronic, St. Jude, Sorin, ATS Medical, 3F, and CryoLife.
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InThe Future, A new generation of mechanical valves made of materials with
improved blood contact properties that do not cause clotting in the blood stream.
better wear characteristics and resistance to infection are under development.