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Artificial Heart Valves
Tuğra Erol
Table of Contents:
1.1 Introduction
1.2 The Requirements Involved in the development of a Prosthetic Heart Valve
1.3 Types of heart valves
1.4 Basic Overview of Tissue Valves (Bioprosthetic)
1.5 Basic Overview of the Evolution of the Mechanical Heart Valves
1.6 The Leading Mechanical Heart Valve Designs and Materials
1.7 Recent Research and Emerging New Technologies for the Prosthetic Heart Valves
1.8 Conclusion
1.1 Introduction
One organ replacement that has become so common today, is the replacement of heart valves. The need for prosthetic heart valves were long recognised, but till the 1950's the mission seemed impossible.
Before going any further lets have a look, what are the reasons that initiated the need for replacement of the natural heart valve tissues.
Blood Circulation in the Heart and the Heart valves:
The blood circulates through out the whole body, and acts as a medium for deposition and feeding. Actually a 70L body contains 5l of blood. For the circulation of this medium (fluid) to be properly carried out, in mechanical terms, a good pump and the appropriate pipes are required. The arteries and the veins being the pipe lines and the pump being the heart, this circulation goes on for the individuals whole lifespan. The heart pumps the blood to the arteries and to the lungs on an average of 60-80 beats for every minute of our life passing. Genetically induced or age related malfunctions (rheumatic illness is one of the main reasons for incompetent heart valves) of this vital organ can cause pain and lead to some detrimental consequences or even worse death.
The human heart consists of four chambers, two atria and two ventricles. Each atrium is connected to its corresponding ventricle by a valve. Ideally these valves close and open fully, but some defects might occur by two ways:
- Stenotic heart valves cannot open completely due to stiffened valve tissue, causing the blood to be pumped through smaller opening which requires more work.
- The second is the incompetent valves, which fail to close completely causing fatal flaw resulting in inefficient blood circulation via allowing blood regurgitation (back-flow).
One remedy for this serious health issue is the use of artificial heart valves to replace the faulty valves. The replacement valves are designed to function like the healthy heart valves in order to sustain the ease and the direction of the blood flow in the human transport system.
1.2 The Requirements Involved in the development of a Prosthetic Heart Valve
As mentioned earlier, the need for the prosthetic heart valves had long been recognised, but in order to be successful in replacing the original tissue by a man made "artificial" one, the union of two criteria were needed. The two factors that were sought after were, the biologically (in our case blood compatible as well) compatible materials and designs which are hemologically tolerant [1].
What would be expected from a biologically blood interfacing material can be stated as:
1. Should not to cause Platelet or Leucocyte activation.
2. Formation of thrombi on the device surface.(pannus)
3. Transport of thrombotic or other material from the surface of the device to another site via the circulatory system.
4. Should not cause injury to circulating blood cells, e.g. ” Hemolysis”
5. Should not cause injury to cells or tissue adjacent to the device.
6. Should not cause overenthusiastic cell proliferation on or adjacent to the device resulting in reduced or turbulent blood flow of blood at the device location.
7. Should not enable adsorption of proteins, lipids, or calcium from the blood onto the surface of the device.
As well as these properties of the materials that constitute heart valves, the design of the valve has to possess certain properties and design considerations, which are related with fluid dynamics and mechanics of materials.
As there was no driven path in front of the surgeons, who wanted to solve these problems with their accumulated experiences, the only way was the trial and error method that lead to an evolution of the valves till this day. Many alternatives appeared from the mist of problems experienced. Most of the time cardiovascular surgeons collaborating with engineers developed prosthetic heart valves and therefore most of the prosthetic heart valves are identified by the name of the surgeon and the engineer who has developed it.
The first implemented alternative was a mechanical valve, followed by many mechanical valve designs in the coming years. The tissue or bio-prosthetic valves were the later additions to this inventory of valves.
1.3 Types of heart valves
The types of prosthetic heart valves used today in modern science can be classified as:
Mechanical Valves
- Ball Valves
- Disk Valves
- Single Leaflet Valves
- Bileaflet Valves
Tissue Valves
- Animal Tissue Valves ( Xenografts, heterografts)
- Human Tissue Valves ( Homografts, Autografts, Ross Procedure)
The reason for the variety of prosthetic heart valves lies in the different circumstances that a patients situation can create and lead to different kind of needs of application, materials, treatment and etc. Different types of valves possess various advantages over one and other.
As the main concern of this paper will be to assess two of the leading mechanical heart valves and their application and design criteria, just an overview of the other types of prosthetic heart valves and their materials will be provided in this section.
1.4 Basic Overview of Tissue Valves (Bioprosthetic)
Considering tissue valves, they are generally used for patients, which are pregnant or too old for anti-coagulation therapy. People, who have tendency to have a haemorrhage are also included to this list. Thus the main advantages and disadvantages in using these types of valves can be stated as:
· Advantages:
- Reduced rate of thromboembolism
- Freedom from anticoagulant related haemorrhage
- No immunosuppresive therapy needed
· Disadvantages:
- Disappointing long term performance
- Degradation caused by calcification and tearing of leaflets.
- Stress concentration leading to degeneration
Types of Tissue Valves
The type of tissue used classifies the tissue valves and they can be either an animal tissue or a human tissue. The type of tissue can be a valve tissue or it may not be like the pericardial tissues used.
Animal tissue valves: are called xenografts from the Latin prefix "Xeno-" for foreign or heterografts.Xenografts can be of valve tissue, typically porcine or pig valve tissue; or they can be of non-valve tissue, for example bovine or cow pericardium[2].
To examine the structure of a Xenograft, Carpentier-Edwards (CE) Porcine valve would be a good example, which is made from a pig valve. The valve tissue is sewn to a metal wire stent, which is bent to form 3 "U" shaped prongs. A cloth (Dacron) sewing skirt is attached to the base of the wire stent and the stents are also covered with cloth. The valve has good durability data and good hemodynamics. Some surgeons prefer the CE Porcine and pericardial valves because of the material and design of the sewing ring [2].
Materials: Porcine valve tissue fixed in glutaraldehyde, stents made of wire, Elgiloy, which is a cobalt-nickel alloy; sewing ring-knitted Teflon
The pericardial valves have somewhat different structures compared tot the pig valves. The application is the same as the pig valves but the production of these valves are more complex compared to the porcine valves. The increase in complexity leads to problems and limiting the use of the type of valve. Examining again the pericardial valves of the same manufacturer, the main factor limiting the use of the CE Pericardial valve was the failure because the tissues could not be secured on to the stent properly, and the installation giving in to stress accumulation after 6 to 8 years post implantation. However, it appears that analyzing the causes of failure of the prior pericardial valves and devising an ingenious method of securing the pericardial tissue to the stent posts and avoiding the high stress regions that led to tears have improved the durability of the valve considerably [2].
Human tissue transplants from another person are called Homografts and are similar to a valve transplant. An autograft is a valve moved from one position to another within the same patient or a valve self-transplant. The most common autograft procedure is moving the pulmonic valve to the aortic position, also called the Ross Procedure [2].
Homograft valves are donated either by patients or by their families. The valves are then preserved in liquid nitrogen (cryopreserved) until needed. The valve must be thawed overnight before it is used. This means that a surgeon must know in advance what size and type of valve he is going to use.
The main advantage of the Ross procedure is that the patient receives a living valve in the aortic position. The hope is that in children, the valve will continue to grow as the child grows older. Other potential benefits are better hemodynamics (there is essentially no pressure drop across the valve) and better durability. However it remains unclear whether the durability of the Ross is better than standard porcine or pericardial valves.
There are many potential complications in less skilled hands after the Ross procedure is carried out perhaps the commonest of which is leakage of the valve (aortic regurgitation)[2].
1.5 Overview of the Evolution of the Mechanical Heart Valves
As the supreme dominator of the heart valve market and the best-proven designs belong to the mechanical heart valves, our main focal point will be the evolution of the design and materials used for these types of valves.
To reason this success over other types of valves currently available on the market, a closer look to the advantages and disadvantages of mechanical valves would be appropriate:
· Advantages:
- Evolution of designs and materials, by trial and error method, since 1952
- Highly durable, reliable
· Disadvantages:
- Continuous anti-coagulation therapy (associated with risks of bleeding and thromboembolic complications)
- Inability to grow, repair and remodel (younger patients,children)
Investigating the evolution and the history of mechanical heart valves, the first replacement heart valve was designed and constructed Charles Hufnagel, a surgeon, and implanted in 1952[1].
The first mechanical valve was a ball valve implanted in the descending thoracic aorta. The design incorporated the material methacrylate and nylon rings containing teeth for securing the valve on the aorta.
Figure 1. Figure 2.
Figure 1. The complete valve with an outer methyl methacrylate casing and ball.
Figure 2. The device which is a plastic ring with multiple teeth projecting from its inner surface.
(http://members.aol.com/amaccvpe/history/hufn.htm)
After the first valve till the 1960's, innovator's efforts were directed toward developing flexible leaflet, silicone and urethane-coated fabric prosthetic valves. But they were ultimately unsatisfactory [1].
Following these attempts, new materials and designs kept on emerging incorporated to form the ultimate mechanical heart valve. To have a glimpse of the just what kinds of materials were used and what kinds of problems led to their failure here are some briefly stated examples:
- Knit Teflon aortic cusps (flexible fabric)/ after 24 months in vivo stiffening, tearing because of fibrin deposition and ingrowth of tissue.
- Nylon fabric impregnated with silicone/ absorption of lipids
- Caged ball valve silicone ball/ wear on the ball caused by the cage
- Non- tilting disc valves/ developed notching from rubbing against the struts
- Fabric covered orifice, teflon poppet/ Poppet notched the paralel struts
- Flexible fabric valves after some months of operation, smooth endothelial covering.
- Plastic Delrin disc/ swollen in a fluid medium and lock up.
In about 1973 a definite pattern emerged with the ball and disc valves. It was observed that a harder titanium strut would erode a soft poppet (silicone rubber), whereas a harder poppet (pyrolyte) will erode a softer (titanium) strut or knitted Teflon or Dacron velour [1]. Another very important observation during the trials was that the ball valves functioned well in some patients, but the large valve profile made their use difficult in patients with small ventricles and narrow aortic roots [4]. The hemodynamic performance of the ball valves in the small valve sizes was less than optimal.
After all the trial and error experience, today, two of the mechanical tilting disc valves, the St. Jude and Medtronic hall valves seem to be the ones which have been the most established both in design and materials. Although these valves were designed in the year of 1976, the accomplishments of the designs still serve the humanity.
1.6 The Leading Mechanical Heart Valve Designs and Materials
Having a close look at the technology and design of well-established mechanical valves, essential requirements for success are:
1. Increasing the flow volume through the small orifice.
2. Decreasing the pressure drop across the valve in order to reduce platelet aggregation relevant to turbulent flow formation (hemodynamics).
3. Using a disc to allow for maximum flow area within the valve housing.
4. Using titanium and pyrolytic carbon materials, which had proven to be extremely durable and which made it possible to construct the valve housing out of titanium or carbon, and avoid welds.
5. Using a PTFE sewing ring to reduce fibrous overgrowth or pannus formation.
6. Reducing valve sound.
The first three of the listed criteria's are related with the study of biomechanics and fluid dynamics. Basically, the important factors and variables in the design of a heart valve related with the discipline of fluid dynamics (hemodynamics), are the velocity and turbulence shear stress peak values (TSSmax). TSSmax is a relevant parameter to the assessment of the risk of hemolysis and platelet activation associated with it[ewtyer]. The problem caused by the turbulence in the flow of blood is that it leads to thrombi formation. Another problem related with the turbulence associated Reynold`s shear stress which can lyse red blood cells and if not sublethal damage occurs at lower levels of shear stress impairing the normal function of them[5]. Thus it is essential to keep the levels of turbulence as low as possible if an artificial heart valve design is going to be successful.
The last three criteria are concerned mainly with the structural organisation of the elements and the materials used in the construction of the artificial heart valve. The reason for the type of material selected by the industry is that pyrolytic carbon (PyC) and titanium have proved out to be appropriately strong and biocompatible throughout the history of trials.
Jack Bokros invented the pyrolytic carbon , that can be acknowledged as a major contribution to the mechanical heart valve material technology. Since 1969 the pyrolyte has been used as components for more than 2 million mechanical heart valves. The initial quest that lead to the invention of such a material was quite different. In 1958 John Bokros was charged with the task of developing a seal for pellets of uranium thoride, a nuclear fuel [1].
Briefly focusing on the material properties, pyrolyte belongs to the turbostratic carbon family. It possesses similar structure to graphite but unlike graphite's covalently bonded carbon atoms in planar hexagonal arrays stacked in layers with weak intermolecular forces between layers, the turbostratic carbon show distorted stacking sequence and distortions exist within each layer. This structural distortion in return provides the ductility and the durability of the pyrolyte (PyC) [6].
For the artificial heart valves, the material is manufactured by co-depositing carbon and silicon carbide on the graphite substrate by chemical vapor deposition, which by this procedure the PyC is alloyed with silicone [6]. By this method the material gains its proper hardness.
Also for added biocompatibility, the PyC can be coated with titanium oxide films by ion-beam enhanced deposition procedure[7]. The formation of thrombus on an artificial biomaterial due to rejection of the human body was found to be correlated with the charge transfer from fibrinogen, to the surface of the implanted materials. The coating of the titanium oxide films inhibit the electron transfer from the fibrinogen to the materials surface by forming Ti+2 and Ti+3 ions [7].
The PTFE sewing ring is the basis of securely mounting the artificial heart valve to the aortic root. As durability is the main concern, the surgically implanted valve has to remain intact and not fail due to repeated loading, the sewing ring is made up of high tensile stress bearing material. PTFE has proven to be one of the most durable materials till now under the circumstances that it has been exposed to.
Leading Valve Designs:
The St. Jude Valve (1976):
The basic design of the St. Jude Medical® mechanical heart valve has remained virtually unchanged in the 23+ years since its first use.
The origins of this valve belong to Chris Posis, an industrial engineer who became interested in prothetic heart valves. Mr Posis together with Dr Nicoloff worked on the new idea of a floating hinge valve with the pivots near the periphery of the retaining annulas and with a central opening. But through the evaluation by Mr Villafana and his engineers, it was found out that this was not a workable design [1]. The evaluation process lead to a new design concept though, having the floating hinges of the leaflets located near the center axis of the rigid housing and opening the outer edge of each leaflet, leaving a small central opening. The name St. Jude has a different story then other valves, as it does no possess the name of the surgeon that created or inventor of the valve. The valve was going to be named Nicoloff valve but the doctor declined and wanted it to he called St. Jude after the St. Jude Thaddeus who is the patron of difficult cases, as taught in church liturgy [1].
Designed and manufactured of pyrolytic carbon, currently the valve has been tungsten impregnated for easy visualization following implantation. The upstream, pivot-based bileaflet design enhances valve responsiveness to changing heart conditions and decreases the volume of retrograde flow during closure[8]. The St. Jude Medical® mechanical heart valve provides optimal thromboresistance and superior hemodynamics, as well as ease of implantability and longevity.
Figures: Analysis and simulation of the hemodyamics of St. Jude valve by ASD.
(http://www.asd-online.com/eng/eng_jsmindex.htm)
The standard valve is available in 19 to 31 mm aortic sizes and in 19 to 33 mm mitral sizes. Various sewing cuffs are available such as double velour polyester or PTFE and expanded versions offer 10 to 15% more sewing flange for the mitral valve and 25% more sewing flange for the aortic valve [8].
Figures: The structure of the SJM valve. With the exception of the sewing ring, it is totally constructed of pyrolytic carbon.
The sewing cuff are marked for easier operation and suturing, providing reference points for the orientation of the valve in the optimum flow position (Figure 3). A St. Jude Medical® mechanical heart valve Hemodynamic Plus (HP) Series cuff is optional in 19 to 25 mm aortic and mitral sizes, which offers increased geometric flow areas by up to 26% over standard cuffs. Also available is a rotatable version of the standard valve, the SJM® Masters Series valve, which is designed for greater ease of implantation (Figure 4.)[8].
Figure 3. Figure 4.
Features such as a controlled torque rotation mechanism allow for easy intraoperative adjustment. In the aortic position, the pivot guards can be rotated toward subannular obstructions to push them away from the leaflets. In the mitral position, leaflets can be rotated away from obstructions
Medtronic Hall - Kaster Tilting Disc Valve (1977) (1977):
A cardiac surgeon, Dr Hall, having used the available valve types until the mid 1970's, was the idea bearer that improvement could be made on the tilting disc concept [ghsg]. Dr. Karl-Victor Hall of Norway evaluated 450 patients who randomly received Lillehei-Kaster or Bjork-Shiley valves. After following up these patients and publishing the results,he felt that the tilting disc design had potential, but that it could be improved. In cooperation with Dr. Arne Woien of Oslo, Norway, and Bob Kaster of Minneapolis, Minnesota, he planned to improve the tilting disc valves by [9]:
1. Increasing the flow volume through the small orifice by moving the pivot axis centrally.
2. Introducing a sliding motion of the disc to allow disc excursion out of the housing during full opening. The pivoting motion of the doughnut-shaped disc allows washing of all struts and both sides of the disc during the entire opening and closing cycle, thus washing away potential platelet aggregates. The open-ended structural members decrease the possibility of attachment of fibrous material with resultant valvular thrombosis.
3. Using titanium and pyrolytic carbon materials, which had proven to be extremely durable and which made it possible to construct the valve housing from a single piece of titanium, thus avoiding welds.
4. Using a PTFE sewing ring to reduce fibrous overgrowth or pannus formation.
5. Using a non-occlusive, non-overlapping disc to allow for maximum flow area within the valve housing.
The stated objectives were achieved by constructing the Medtronic Hall valve whose disc had a central perforation that allowed moving the pivoting action of the disc more centrally and guiding the opening of the disc by inserting a guiding rod through the opening in the middle of the disc [1,9].
Housing Construction:
The Medtronic Hall valve housing is machined from a single piece of titanium with no welds or introduced bends in valve members to weaken or otherwise compromise valve durability. Readily machinable, titanium polishes well, is electronegative in blood, is non-magnetic, and is both bio- and hemo-compatible. Titanium is a proven heart valve material [9].
Central Guide Strut Construction:
As part of the valve design, a curved, central guide strut provides two benefits:
· it allows assembly of the valve and disc without imparting stress on the valve housing, and
·
allows the disc to move out of the annular plane, the tightest constricture of the resultant outflow tract.
Rotatability:
The Medtronic Hall valve is designed to rotate within its sewing ring to allow the surgeon to align the leading edge of the disc parallel to blood flow and to avoid interference with surrounding anatomy(figure 5). Parallel blood flow enables maximum hemodynamic efficiency and reduces the amount of turbulence and stasis.
Figure 5.
(http://www.medtronic.com/cardiac/heartvalves/medhall/mh_implant_orientation.htm)
Recent aortic valve studies have shown that the Medtronic Hall valve produces lower levels of turbulence than the leading bileaflet valve. The authors determined that, "With optimum orientation ... the Medtronic Hall valve matches the aortic flow pattern to near normal physiology." Comparatively, "the St. Jude Medical valve revealed significant turbulent flow at any orientation."[10].
(http://www.csmc.edu/cvs/md/valve/medhall.htm)
Materials: Cage: titanium, single block, computer machined; Disk: Pyrolytic carbon, sewing ring: knitted Teflon
X-Ray: Radiopaque ring with large attached central curved strut and two smaller side struts. Center strut has bulge in middle, resembles goose-neck. Occluder is faintly radiopaque. Opening angle: 75 degrees for aortic, 70 degrees for mitral. Closing angle: 0 degrees.
1.7 Recent Research and Emerging New Technologies for the Prosthetic Heart Valves
The tissue engineering is anew approach whereby techniques are being developed to transplant autologous cells on to biodegradable scaffolds to ultimately form new functional tissue in vivo and in vitro. New research focuses on the tissue engineering of heart valves, fabrication of trileaflet heart valve scaffold from a biodegradable polymer, a polyhydroxyalkanoate.
The biodegradable and biocompatible trileaflet scaffold was formed from a porous polyhydroxyalkanoate (Metabolix Inc., Cambridge, MA). The porous heart valve scaffold (pore size 100 to 240 mm) was seeded with vascular cells grown and expanded from an ovine carotid artery.
It was concluded that polyhydroxalkanoates could be used to fabricate a porous, biodegradable heart valve scaffold. The cells appeared to be viable and extracellular matrix formation was induced after the pulsatile flow exposure [11]. 1.8 Conclusion
The ultimate goal of the artificial organs was always to imitate and create the closest conditions that the natural human body had created for itself through evolution. Till now the humans mastered materials technology which the raw minerals were derived from the earth and processed to form the desired shape and property. Also some replacements were obtained from our related mammal species. The deficiencies of these prosthetic heart valves were compensated as much as the conditions allowed it to be.
The development in the field of tissue engineering, which will master the raw materials and processing of the human body, will lead to the ultimate goal of having spare part of our body constructed totally of our own genetic material. These developments inevitably will take over the prosthetic heart valves that were constructed from the materials, which are extracted from another source. But until that moment arrives, current technology provides hope and supplies the needs of the patients.
References:
[1] Richard A. DeWall , MD, Naureen Quasim, MD, and Liz Carr, BFA. Evolution of Mechanical Heart Valves. Ann Thorac Surg 2000;69: 1612-21.
[2] Steven Khan, MD: Cedars- Sinai Medical Center Prosthetic Heart Valve Information page [Online]. Available: http://www.csmc.edu/cvs/md/valve/default.htm
[3] Mauro Grigioni, Carla Daniele, Giuseppe D`Avenio, Vincenzo Barbaro. The Influence of Leaflets` Curvature on the flow field in Bileaflet Prosthetic Heart Valves. Journal of Biomechanics 2001;34: 613-621.
[4] Herr R, Starr A, McCord CW, et al. Special Problems Following Valve Replacement: Embolus, Leak, Infection, and Red Cell Damage, Ann Thorac Surg1965;1:403.
[5] W.L. Lim, Y.T.Chew, T.C. Chew, H.T. Low. Steady Flow
Dynamics of Prosthetic Heart Valves: A Comparative Evaluation by PIV Techniques. Journal of Biomechanics 31 (1998) 411-421.
[6] Materials for Medicine [Online] Available: www. Britannica. Com.
[7] James Lankford, Ph.D. "Assuring Heart Valve Reliability" [Online] Available: http:// www.swri.edu.
[8] St Jude Medical Inc. [online] Available: http://www.stjudemedical.com/
[9] Medtronic Inc. [Online] Available: http://www.medtronic.com/cardiac/heartvalves/medhall/
[10] Kleine P, Perthel M, Nygaard J, et al. Medtronic Hall versus St. Jude Medical Mechanical Aortic Valve: Downstream turbulences with Respect to Rotation in Pigs. J Heart Valve Dis 1998;7:548-555.
[11] Ralf Sodian, MD, Simon P. Hoerstrup, MD, Jason s. Sperling, MD, Sabine H. Daebritz, MD. David P. Martin, Ph. D, Frederick J. Schoen, MD, Ph D, Joseph P. Vacanti, MD, and John E. Mayer, Jr, MD. Tissue Engineering of Heart Valves : In Vitro Experiences. Ann Thorac Surg 2000;70: 140-4.
