BT5011 : Biomaterials Engineering

MECHANICAL HEART VALVES

By

Yanamala Vijay Raj

Praveen Krishna V.

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Heart/Heart Valves

A heart valve normally

allows blood to flow in only one

direction through the heart. The

four valves commonly represented

in a mammalian heart determine

the pathway of blood flow through

the heart. A heart valve opens or

closes incumbent upon

differential blood pressure on each

side

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When Heart Valves Stop Working

ARTIFICIAL VALVES:

An artificial heart valve is

a device implanted in the

heart of a patient with

valvular heart disease.

When one of the four

heart valves malfunctions,

the medical choice may

be to replace the natural

valve with an artificial

valve. This requires open-

heart surgery.

Need and function-when does natural valveneed be replaced

Heart Valve diseases fall into two categories:

Stenosis- hardening of the valve andincompetence- permittance of backflow

3 causes of Heart Disease:

Rheumatic Fever: stiffens valve tissue, causingstenosis

Congenitally defective valves: do not formproperly as the heart develops, but often gounnoticed until childhood

Bacterial infection: causes inflammation ofvalves, tissue scarring, and permanentdegradation

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Evolution of Prosthetic Heart Valves

The development of the originalball-and-cage valve design canbe attributed to the bottlestopper in 1858

In the early 1950’s, it led to theidea of a prosthetic heart valveconsisting of a cage with amobile spherical poppet

On July 22, 1955, at the City GeneralHospital in Sheffield, England, JudsonChesterman implanted the firstsuccessful heart valve

The patient lived 14 hours after thevalve was placed, but died when thepoppet twisted out of position

Valve was made of Perspex, an outercage, a poppet, and 2 buttons to fastenthe valve to the outside of the heart

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•Starr-Edwards valve was first successful long-termvalve created•It was implanted in its first 8 patients in 1961 (6 of8 survived•Ball-and-Cage design•Devised important “Nine Commandments” indeveloping a prosthetic heart valve.

Since this time, over 30 mechanical heart designshave been marketed in the U.S. and abroadThese valves have progressed from the simple cagedball valves, to strut-and-leaflet valves and themodern bileaflet valves, to human and animal tissue

So, To Summarize

•1952 – Dr. Charles Hufnagel

designed and implanted a mechanical

heart valve into a thirty year old

female.

•1960 – The Starr-Edwards ball valve

was created. It was based off of Dr.

Charles Hufnagel’s design.

•1969- The Bjork-Shiley valve started

being used and it was based on a

tilting disc design.

•1979 – Bileaflet valves start being

used and up to this day, their designs

are still being improved upon.

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Types of valves - advantages and disadvantages

•Mechanical- There are three types. The caged ball, tilting disk, andbileaflet. Lasts for over 20 years•Tissue(biological)- valves that are used from animals to implant themback into humans. Typically lasts between 10-15 years

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Characteristics of ideal heart valve“Nine Commandments”:

oEmbolism PreventionoDurabilityoEase and Security of AttachmentoPreservation of Surrounding Tissue FunctionoReduction of TurbulenceoReduction of Blood TraumaoReduction of NoiseoUse of Materials Compatible with BloodoDevelopment of Methods of Storage and Sterilization

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Unidirectional flow Durable : 40million cycles/year Blood compatible: no thrombus, embolus Central flow: Laminar not turbulent Closing not damaging blood cells Last but not the least important – It should be quiet

“An ideal Prosthetic valve”

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Starr-Edwards Ball-in-Cage Valve

The ball valve was the first mechanical heartvalve used and designed by Charles Hufnagel.

The Starr-Edwards ball valve was first usedclinically as a mitral valve replacement in1960.

After the Starr-Edwards valve was established,several other design variations were createdsuch as Magovern–Cromie, DeBakey–Surgitool, and Smeloff–Cutter ball valves.

Ball valves operate on the simple principle thatthe ball will be forced to one side of the valve orthe other depending on which way blood isflowing.

They were modeled after ball valves used inindustrial applications to allow the flow of fluidson only one direction.

When the pressure exerted by the heart onto theblood (and the ball) exceeds the pressure in theaorta, the ball is pushed away from the heart.

This is the open position of the valve and bloodcan flow out of the heart into the aorta. After theheart ejects blood, the pressure inside the heart isgreatly reduced so blood will try to flow backinside the heart.

The negative pressure sucks the ball valvebackwards. It fits over the opening of the heartand prevents backflow of blood

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Björk–Shiley valveConstruction• The Björk–Shiley valve consists of

a single carbon-coated disc in ametal housing.

• The disc is held in place by twometal struts, an inflow and anoutflow strut.

• The housing is made from thealloy Haynes 25, which iscomposed of 51% cobalt,20% chromium, 15% tungsten,and 10% nickel.

• The Björk–Shiley valve wasconsidered very durable and waswidely used in the 1970

The Björk–Shiley valve is a mechanical prosthetic heart valve. The valve was co-invented by American engineer Donald Shiley and Swedish heart surgeon Viking Björk.

Beginning in 1971, it has been used to replace aortic valves and mitral valves. It was the first successful tilting-disc valve

Tilting disc valves can open at an angle of60° and at a rate of 70 beats per minute.

The angular opening of this valve reducesdamage to blood cells.

These are major improvements over the balldesign but the struts of the tilting disc valvestend to fatigue and fracture over longperiods of time.

The convexo concave shape of disc wasreplaced by flat discs starting 1986following lawsuits due to failure of 619valves out of 80,000 implanted.

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Medtronic-Hall Valve Tilting Disk

The Medtronic-Hall valve was developed to improve on existing tilting disc valves by

reducing the risk of valvular thrombosis.

This was to be accomplished by improving the hemodynamics and by allowing the

disc to move

downstream away from the orifice during opening.

The valve was also designed for maximal structural durability to minimize the risk of

mechanical breakage.

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Two semicircular leaflets that rotate about struts attached to the valve housing

Good hemodynamic performance - improved flow characteristics, lowertransvalvular pressure gradients, less blood flow turbulence, improvedhemodynamics at a given annular diameter, a larger orifice area and low bulkand flat profile

the least thrombogenic of the artificial valves

most commonly implanted mechanical valve

Bileaflet valves

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Hemodynamics of blood flowHowever, in spite of improved design and hemodynamics – still haunted by numerous

complications and the most dreaded one of valve thrombosis

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Biomaterials used•Many different materials are used in the creation of artificial heart valves.

• Metal alloys consisting of stainless steel or titanium are often used to give mechanical strength and for their corrosion resistance properties.

• The struts on some leaflet valves and the cage on caged-ball models are commonly made of metal alloys due to their strength and durability requirements

Pyrolytic carbon is another valuable material for its strength and its ability to prevent clotting.

This material has a similar structure to graphite and was originally developed forapplications in the nuclear fuel industry as a coating for nuclear fuel particles. However, it wassoon realized that pyrolytic carbon had biomedical applications.

It is biocompatible, thromboresistant, resistant to wear, and has high strength and durability.

It is able to stand up to the repeated opening and closing cycles it must endure when used inmechanical heart valve.

It is commonly used for the inner orifice and the leaflets of bileaflet valves. The ATS Bi-leafletvalve shown here has leaflets made of pyrolytic carbon

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oA material often used for the suture ring (which is used to attach the valve to the body) isDacron.

o Dacron is a long chain polyester made from ethylene glycol and terephthalic acid. It is asynthetic fiber that has many uses in industry, including thermal insulation and sails forboats.

o In biomedical applications this material is also commonly used for vascular grafts. It isrelatively inert and its porosity allows tissue in growth.

oAnother material that is commonly used for the suture ring is Teflon

oTeflon is used in many medical applications because of its signature low coefficient offriction

o Teflon is relatively inert and highly biocompatible. As with Dacron it is often used forvascular grafts. 16

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Failure modes in mechanical valve

Problems that interfere with the successful performance of valves can be grouped as below.

•Degradation of valve components•Structural failure•Clinical complications associated with the valve.

Clinically, valve failure has been considered to be present if any of the followingevents require reoperation and/or cause death:

•Anticoagulant-related hemorrhage (ACH),•Prosthetic valve occlusion (thrombosis or tissue growth),•Thromboembolism•Prosthetic valve endocarditis (PVE),•Hemodynamic prosthetic dysfunction, including structural failure of prostheticcomponents (strut failure, poppet escape, ball variance),•Reoperation (valve replacement) for any other reason (e.g.; hemolysis, noise,incidental) etc

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Evaluation of Prosthetic Valves20

Patient’s height, weight, and BSA should be recorded to assess whether prosthesis-patient mismatch (PPM) is present.

Valves should be imaged from multiple views, with attention to:

Opening & closing motion of the moving parts (leaflets for bioprosthesis and occluders for mechanical ones)

Presence of leaflet calcification or abnormal echo density attached to the sewing ring, occluder, leaflets, stents, or cage

Appearance of the sewing ring, including careful inspection for regions of separation from native annulus & for abnormal rocking motion during the cardiac cycle

Echo Imaging of Prosthetic Valves

Regurgitation occurs at the disc margins The regurgitant jets converge toward the center of the valve

PARAMENTERS

CLINICAL INFORMATION Date of valve replacement

Type and size of the prosthetic valve

Height, weight and body surface area

Symptoms and related clinical findings

BP and Heart Rate

IMAGING OF THE VALVES Motion of leaflets or occluder

Presence of calcification on the leaflets or

abnormal densities on the various

components of the prosthesis

Valve sewing ring integrity and motion

PARAMENTERS

DOPPLER ECHOCARDIOGRAPHY OF

THE VALVE

Contour of jet velocity signal

Peak velocity and gradient

Mean pressure gradient

VTI of the jet

DVI

Pressure half time in MV and TV

EOA

Presence, location and severity of

regurgitation

Caged-Ball Valve

Single-Leaflet Valve

Bileaflet Valve

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Improvements that researchers will attempt to make in the future include:

•Longer lifespan of the heart valves.•Lower rejection rate.•Less risk of blood clots in mechanical heart valves.•Remove the need for blood thinning medication.•Implant the artificial heart valves through a less intrusive method (currently open-heart surgery).•Polymeric Heart Valves - Scientists are looking more into polymer materials for heartvalves because it’s easy to fabricate, has a large range of polymer properties, anddurability.•Tissue engineered heart valves- Obtaining the number of types of cells for tissuevalves, lack of scaffold material

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References

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Peck, Peggy. "Replacement Heart Valves Built to Last, and Even Grow." WebMD. WebMD, n.d. Web. 16 Sept. 2012. <http://www.webmd.com/heart-

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"Pericardial Heart Valves." Wikipedia. Wikimedia Foundation, 04 Jan. 2012. Web. 19 Sept. 2012. <http://en.wikipedia.org/wiki/Pericardial_heart_valves>.

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Pick, Adam. "Porcine Valves – What Is A Porcine Heart Valve Replacement?" Porcine Valves – What Is A Porcine Heart Valve Replacement? N.p.,

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