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Introduction

In heart the blood flows between different chambers of heart must flow through a heart valve. The blood which flows out of heart into large arteries must also flow through a heart valve. All heart valves open up enough so that blood can flow through in a specific manner. The heart valves then close to keep blood from flowing backward. There are four valves in the heart: Aortic valve, Mitral valve, Tricuspid valve, Pulmonic valve. Replacement of heart valve is used to repair or replace malfunctioning heart valves. Out of the four heart valves the aortic valve is the most common valve to be replaced, the mitral valve is the most common valve to be repaired. In rare cases the tricuspid valve or the pulmonic valve needed to get repaired or replaced.

Replacement of heart valves has gone through a long journey of test and trials for last many decades. Replacement of heart valves is not any more an operation that is exclusively approached through a simple sternotomy using only sutured prostheses. Now a day’s surgical valve replacement can be performed through many of minimally invasive ways employing conventional mechanical or bioprostheses (tissue valves). In both cases, the direct surgical access allows a clear inspection of the valve, complete excision of the effected leaflets, and debridement of the annulus in a clear and detailed way under a visual system. Which is a better manner employed to treat not only valve pathologies of all natures but also aetiologies. When the process is compared with transcatheter valves in patients for risk it is found that a high or intermediate preoperative predictive risk, conventional surgery has not been shown to be inferior to transcatheter valve implants. To understand the comparison of sutureless valves and their practical applicability with the minimally invasive surgery advances, the trauma of surgery and invasiveness can be reduced and we can get better results.

Replacement valves durability and performance

Before replacement of heart valves an evolution is always done regarding all the possibilities to cure the heart. If there is a possibility the surgeon can repair heart valve, the ring annuloplasty is performed. In this process the surgeon repairs the ring-like part around the faulty valve by sewing a ring of cloth, plastic or tissue around the valve. In the process of repair the surgeon shapes, trims or rebuilds the defected leaflets of the valve. The flaps that open and close the valve are called leaflets.

Replacement is usually needed in case of the aortic valve which is not generally repaired. If the valve is too damaged, the replacement is needed. This process is called valve replacement surgery. The surgeon will remove the damaged valve and replace it with a new one. Mainly there are two types of valves used for replacement. One is Mechanical valve made of man-made materials, such as metal (stainless steel or titanium) or ceramic. These mechanical valves have the longest durability. The person will have to be on medicine support system (blood-thinning medicine) for the rest of the life to get the optimum performance. The second one is Biological or Bioprosthetic- made of human or animal tissue. The bioprosthetic valves has a comparatively less durability. It last 12 to 15 years, but the person may not need to take medicine support (blood thinners) for life. In some cases, surgeons can use patient’s own pulmonic valve to replace the damaged aortic valve. In such cases the pulmonic valve is then replaced with an artificial valve (this is called the Ross Procedure). This procedure may be useful for people who do not want to depend on blood thinners for the rest of life. In this process the new aortic valve does not last very long and may need to be replaced again after a period of time by either a mechanical or a biologic valve.

Replacement valves risks and medication requirement

Replacement of heart valve is a decision taken after having a thorough investigation and consideration of all risk factors. The main question for the patient is whether a mechanical or bioprosthetic valve should be used keeping in view the risk and benefits of the two options mechanical or bioprosthetic valves. In case of mechanical valve because of thrombogenicity of materials used a high shear stress is generated around the hinge points and backflow jets which results in damaging blood and activate clotting-pathways, in such cases patients require lifelong anticoagulation therapy (blood thinning medicines such as warfarin or aspirin) to avoid blood clotting. Even with the use of anticoagulation this risk is apparent like spontaneous bleeding (e.g. gastrointestinal bleeding) or trauma-related bleedings (e.g. subdural hematoma), which cause considerable mortality and morbidity.

Bioprosthetic valves (tissue valves) are made of either porcine aortic valves or bovine pericardium, in some cases it may also be produced from equine or porcine pericardium. The main benefit of bioprosthetic valves is that they do not require support of medicine for life-long. But as we use tissue it does introduce the possibility of degeneration of the valve, which is virtually non-existent in mechanical valves. The main risk with bioprosthetic valves is reoperation for structural valve deterioration (SVD) due to the limited durability of bioprosthetic valves. The average lifespan of a bioprosthetic valve is estimated at 15 years in elderly patients, but this risk is higher in younger patients in whom SVD is accelerated due to a more pronounced immunologic response to the valve and enhanced calcification of the valve In addition, elderly patients generally have a shorter life expectancy during which they are at risk for requiring replacement of a deteriorated bioprosthetic valve and thus less often require reoperation. The most common reason for reoperation is SVD. Risk of non-SVD is considered to be equally low to that of mechanical valves. Particularly important is the risk of prosthetic valve endocarditis that has a similar incidence in mechanical and bioprosthetic valves and is a devastating diagnosis that often requires the need for reoperation.

Types of Replacement Heart valves

Replacement of heart valve is an effective way to treat problem valves. This procedure has been performed for last many decades. Generally it is safe and effective procedure that can help many patients to live healthy and active life. There are mainly two types of Heart valves used in replacement procedure. In case of patients with significant heart valve disease, when valve replacement is performed, the valve is chosen by considering many parameters of the patient’s condition, medical history, age etc. The selection of the type of valve is important to ensure the safety, durability and better health after the surgery for as long as it could be. Both the type of valve has different characteristics and own advantages and disadvantages.

  1. Mechanical valve– Mechanical valves are made up of man-made materials. Mechanical valve is made up of metal (stainless steel or titanium) or ceramic. They usually consist of one or two leaflets and a metal ring surrounded by a ring of knitted fabric, which is used to sew onto the heart in place of the original faulty valve. Mechanical valves bring revolution in the heart valve replacement for the person with faulty valves. In due course of time the professionals faces many challenges in respect of proper functioning of mechanical valves and along with other new health related issues caused by the valves. Many studies were done to overcome the challenges and after many intensive researches many types of mechanical valves were invented. Each type of mechanical valve was even better and improved than the previous one. The revolution in this field is still going on to make it the best. There are mainly three types of mechanical valves  used till date.
  2. Caged ball Valve – Caged ball valve is the very first mechanical valve successfully implanted. Caged ball valve consists of a silastic ball with a circular sewing ring and a cage formed by three metal arches. These are no longer used for replacement of heart valves. There are still several thousands of patients have caged ball valves implanted, and these patients require a regular follow-up. The first implantation of an aortic valve substitute was performed in 1960 with a caged-ball valve. It consists of a silastic ball encaged by Stellite-21 (alloy of cobalt, chromium, molybdenum and nickel) arches and a Teflon sewing ring. Some studies report > 40 years of proper functioning of this type of valve in some patients. In this type of design, the blood flow is partially obstructed by the ball and has to flow around it. Due to the obstruction in the flow of blood generates a large energy loss. The ball also causes blood cell damage and blood stasis due to flow separation, which further causes clotting of blood known as thrombosis. Even though it has a  good durability but the caged-ball valve provides a poor hemodynamic performance with a much high risk of thrombosis. Due to so many complications caged ball valve’s use is discontinued after the discovery of tilting disc valve.
  3. Tilting disc valve – The tilting disc valve was made keeping view of the complications in the performance of caged ball valve. It is made in such a way to overcome all the drawbacks. In tilting disc valve the ball is replaced by a circular carbon-coated or pyrolytic carbon disk (mono-leaflet) fixed to a rigid ring by metal struts and rotating with an opening angle ranging from 60 to 80 degrees. In tilting disc valve the blood flows through two orifices of different sizes when the valve is open, the blood flows in such a manner creating a semi-circular flow jet through two orifices the large one and another through the smaller one. In this type of valve due to the angular opening the damage to the blood cells is reduced but still causes slight energy loss and turbulence, especially through the larger orifice. The thrombosis is still a big challenge in tilting disc valve also and the patient has to use blood thinner medicine for the rest of the life. For the safety side highly durable materials for the disk and the rigid ring are used. Even with all improvement in some cases of struts disruption due to fatigue and fracture have been reported due to fabrication defect of the outlet strut causing the malfunctioning of the valve. It is again a search of a new type of valve has begin resulting even a better option bileaflet valve.
  4. Bileaflet valve– Bileaflet valve is the new discovery in the process of search for a better mechanical valve after tilting disk valve. Bileaflet valve is made of two semilunar disks attached to a rigid valve ring by small hinges. In  bileaflet valve the opening angle of the leaflets relative to the annulus plane ranges from 75° to 90°. In bileaflet valve the open valve consists of 3 orifices: a small slit-like central orifice between the 2 open leaflets and 2 larger semicircular orifices laterally. When the valve is open, the resulting transvalvular flow is thus symmetrical and makes bileaflet valves less obstructive mechanical valves (with the largest effective orifice areas). The material used for the leaflets also makes it a much better choice. The material used to make bileaflet valve are – solid pyrolytic carbon or graphite coated with pyrolytic carbon. It provides adequate biocompatibility, strength and durability. It also provide better thrombo resistance as well than other mechanical valves. However, hinges can be a source of blood stagnation and subsequent thrombus formation. Keeping in view of such problems, all mechanical valves have a built-in normal small backflow allowing blood wash-out. Bileaflet prostheses have the highest regurgitant fraction (10%). The professionals are making constant efforts to enhance this washing-effect by modifying the hinges’ design (butterfly geometry, convex open pivots). All these improvements makes it a better choice but still have many drawbacks like high leakage flow velocities, turbulent flow, vortex structures and viscous shearing. Such drawbacks may in turn cause damage to blood cells. Bileaflet valve is more widely used mechanical valve all over the world. The major issue with bileaflet valve is the inherent thrombogenicity (resulting from potential blood stasis) and hemolytic risk (due to viscous shearing). Patients with mechanical valves require a life-long anticoagulation therapy which  increases the risk of bleedings. These valves should be avoided in women of child-bearing age, or pregnant women as warfarin is not safe for use in pregnancy, and those with a high risk of falls or bleeding. In the process of improvements many measures taken to reduce the thrombosis issue of bileaflet mechanical valves, like new surface coating with hydrophilic polymers and improved designs to get closer to the native valve and provide a better physiological flow. Even with all improvements bileaflet valve is the better choice and used widely but certain drawbacks are still there. Keeping in view one more new valve is in the process of development the triflo valve, a trileaflet valve with pyrolytic carbon leaflets has been developed (not yet in clinical use) and it provides a large effective orifice area with soft closure, reduced flow separation and regurgitation (when the valve doesn’t close all the way and blood flows backward into the heart). There are many improvements made like the ability of the valve to rotate within the sewing ring to improve to the flow or supra-annular designs to provide larger valve orifice areas. These improvements resulted in good hemodynamics and excellent durability. It could be proved a better choice for future.

Advantage of Mechanical Valve

The mechanical valve has one very important advantage which is their durability. In case of an elderly person or a fully grown patient the mechanical valve have been known to work more than 40 years. Mechanical valve in general less likely to have a reoperation or resurgery. Mechanical Valve is safe for the person who have overactive parathyroid gland (affecting blood calcium levels).

Disadvantage of Mechanical valve

The mechanical valve has many disadvantages like causing blood damage, life time blood thinning medicine to be consumed rest of the life, cavitation, noise issue, fluid mechanics etc. We will discuss few of them in detail.

Blood Damage– The blood flow and its specific constituents are the very important for normal functioning of our body. Our heart controls the regular blood flow in our body. The smooth blood flow has specific role in maintaining basic constituents of the blood like blood platelets. The complex blood flow phenomena caused by the mechanical prosthesis are responsible to induce serious thromboembolic complications. Many studies shows the models realistic sized suspended platelets for assessing potential blood damage in flow through Mechanical valve due to the nature of the materials and the design hemodynamics. The patients implanted mechanical valve are at a higher risk for clotting and therefore cause embolism. Even though many improvements made in the mechanical valves but high leakage flow velocities, turbulent flow, vortex structures and viscous shearing are the few side effects may be expected. All such effects may in turn cause damage to blood cells. The only possible way to overcome it up to certain levels is taking appropriate medicines throughout life. The medication improves some complications but equally creates the new one like excessive bleeding issues etc. 

Noise– Some people hear a ticking sound. It’s the valve leaflets opening and closing sound. But it is not necessarily shown in all people.

Cavitation– Cavitation is known to be the rapid formation of vaporous microbubbles in a fluid due to a local drop of pressure below the vaporization pressure at a given temperature. Cavitation in the blood can be a major cause to mechanical heart valve failure, so cavitation testing is consider as an essential part of the valve design verification process. In the mid 1980s after a series of valve failures observed and as a results it is correlated with a direct association of cavitation. The damages when tested were found to be due to cavitation. Many studies showed the bubble formation and collapse of cavitation at mechanical heart valves. It was concluded that the cause of damage to the formed elements of blood could be cavitation and as a result enhance the risk of thromboembolic complications seen in mechanical heart valve patients.

The mechanical heart valve’s quality is quite high, and implantation has become a routine clinical procedure with a low operative mortality (< 5%). Most of the patients still face the risks of blood cell damage, thromboembolic complications and material failure of the prosthetic device. It is found that cavitation has been responsible by visualization causing of high frequency pressure fluctuations (HFPF).The tilting disc valves are found to be with higher cavitations risk than bileaflet valves because of higher closing velocities. But the thromboembolic potential seems to be the same in both valves. More studies are therefore needed to investigate the cavitation potential of bileaflet valves in vivo. The post processing of HFPF have shown difficulties when seen on bileaflet valves because of asynchronous closure of the two leaflets.

Fluid mechanics– The blood circulation system in the body works on fluid mechanics. Human heart performs a series of task to maintain the blood flow in the whole body. Heart valves have a major role in maintaining the proper flow of blood that to at a certain pressure. Heart valve help to maintain unidirectional seamless blood flow through the heart and to open and close the valves like door at the right time and close tightly to avoide backward flow. Mechanical valve has the issue of blood flow disturbances. It is found that many of the complications of mechanical heart valves can be explained through fluid mechanics. In studies we came to know blood clot formation is a side effect of high shear stresses created by the design of the valves. If we look the pressure phenomenon used by an ideal heart valve would produce minimal pressure drops, have small regurgitation volumes, minimize turbulence, reduce prevalence of high stresses, and never create flow separations in the safety of the valve. Even though many improvements makes in the mechanical valves but high leakage flow velocities, turbulent flow, vortex structures and viscous shearing may in turn cause damage to basic structure of blood cells.

2.    Bioprosthetic valve- Bioprosthetic valve is a step towards the natural solution of the heart valve problem. Bioprosthetic valves (tissue valves) are made up of either natural tissues porcine aortic valves or bovine pericardium, in some cases it may also be produced from equine or porcine pericardium. Bioprosthetic valve is in general made in two types – stented or stentless bioprotheses. Stented Bioprostheses –The stented bioprostheses is made in such a way to mimic the native aortic valve. In porcine bioprosthetic valves 3 porcine aortic valve leaflets cross-linked and mounted on a metallic or polymer supporting stent hence it is called stented. Pericardial valves are made from sheets of bovine pericardium and mounted inside or outside a supporting stent. Stentless Bioprostheses – It a more improved version with better hemodynamics and durability of the valve. Many versions of stentless bioprosthetic valves have been developed. Stentless bioprostheses are created from whole porcine aortic valves or fabricated from bovine pericardium. Percutaneous Bioprostheses – There are patients with symptomatic aortic stenosis (when the opening of the valve gets too narrow and not enough blood flows out) considered to be at high or prohibitive operative risk and in such cases Percutaneous aortic valve implantation is usinging as an substitute to standard aortic valve replacement (AVR). Usually a percutaneous transfemoral approach is used to implant the valve. To reduce the issues of vascular access and associated complications, a transapical approach through a small thoracotomy may also be used sometime. The procedure appears promising at present. The percutaneous bioprostheses remains experimental and is currently undergoing further investigation.

Advantage of Bioprosthetic Valve

The advantage of Bioprosthetic valve (tissue valves) is that the patients usually do not require long-term blood thinner therapy (e.g. warfarin) for the rest of their life. These medications has reduce the blood clotting in the system which can cause stroke or embolism. In the long term these medicine creates an increase in risk of bleeding. However, approximately 1/3 of patients with a tissue valve need a blood thinner requirement for other heart or vascular conditions. Bioprosthetic valve is used in case of pregnant women or planning to have children as warfarin cannot be used in pregnancy due to the risk of causing fetal malformations. In increasing age the bioprosthetic valves are suggested keeping in view of higher risk of bleedinmg related issues.

Disadvantage of Bioprosthetic valve

The main disadvantage of tissue valves is their less durability. The bioprosthetic valve is made up of tissues which gets damaged with time and conditions. As the tissue get damaged and require replacement, especially in patients ≤ 65 years old.The durability of tissue valves depends on the patient’s age at the time of the valve replacement surgery takes place. In case of patients less than 65 years old, some aortic tissue valves begin to fail after only 5 years and some mitral tissue valves begin to fail as early as 4 years after the surgery. In some cases tissue valves fail after 7 to 10 years. The risk factors that may increase the possibilities of failure include poor life style of the patient, several conditions affecting calcium metabolism or when calcium containing chronic drug therapies are used, including children, adolescents, young adults and patients on maintenance hemodialysis. The patients with bioprosthetic valves are more likely to get reoperation or repeat replacement surgery. Bioprosthetic valves tend to deteriorate more quickly in younger patients.

Tissue engineered valves

Tissue engineered heart valves (TEHV) is step towards a new revolution in the replacement of heart valves. It is uniquely designed valve which proposed treatment by creating a living heart valve for people who are in need. In present scenario a huge number of prosthetic hearts are implanted in a year all around the world which is expected to be triple or more in the coming next fifty years. 

Our today’s treatment system offers only two provisions such as mechanical valves or biological valves. It’s a hard fact that both are not properly supporting to one’s health, both have limitations like in mechanical valves the lifelong use of anticoagulants is must causing several health issues while biological valves are having durability issue like structural degradation and leads to reoperation. Heart valve disease is major cause of high cardiovascular morbidity and mortality rate in the world. Thus, there is a great need of Tissue engineered heart valve.

Tissue engineering of heart valves is a novel approach that explores the use creating a living heart valve composed of the host’s own cells that is capable of growing, adapting, and interacting within the human body’s natural system. In these models natural and synthetic polymers mimic the extra cellular matrix. Tissue engineering heart valve is a method providing new horizon of hope for constructing both valve replacements and valvular disease models. Tissue-engineered heart valve replacements help us to go beyond the limitations of mechanical and bioprosthetic valves, as they are made up of living tissues capable of active remodeling and self-repair.

Many tissue-engineered valve replacements have got promising results in large-animal trials. Tissue-engineered disease models also provide a scalable way for investigating the pathobiology of valvular diseases like aortic stenosis. These various reviews provide an overview of recent developments, current challenges, and future directions in the field of heart valve tissue engineering.

Research has not yet reached the stage of clinical trials.

Conclusion

Human heart is a very complicated as well as very sensitive organ. Human heart provides nutrition and other support system to all body through the blood flowing through it. Heart has four valves which maintain the regular flow of blood towards the body from the heart and vice-versa. Taking care of is Human heart is very challenging not only today but always. The medical researchers accept the challenges and invent the best possible solution to various problems. Replacement of heart valve was one of the most challenging tasks for the medical professionals.

The journey of replacement of heart valves from early time to modern world shows many ups and downs. Every time from the very first replacement of mechanical heart valve till today the long journey was full of hurdles but every time a new mile stone is created. The development of mechanical valves from caged ball valve to bileaflet valve shows the serious concern and commitment of the medical community. Mechanical valve and bioprosthetic valve prove to be of very useful step in treatment of Heart valve replacement but due to the limitation of both the options a challenge remains always. Heart valve disease is major cause of high cardiovascular morbidity and mortality rate in the world. . Thus, there is a great need of Tissue engineered heart valve.

Tissue engineering of heart valves is a research that explores the creation and use of a living heart valve composed of the host’s own cells which are capable of growing, adapting, and interacting within the human body’s natural system. Tissue enginnered heart valve is the most revolutionized step towards the future of replacement of heart valve. Tissue engineered heart valve will surely overcome all the limitation of both mechanical and bioprosthetic valves. Tissue engineered heart valve is the future of Replacement heart valve. Many tissue-engineered valve replacements have got promising results in large-animal trials.

Research has not yet reached the stage of clinical trials. But Hoping for a better future of tissue engineered valve.

FAQ

Q1. What is a heart valve?

Ans. Human heart has four valves: the mitral valve and the aortic valve on the left side of the heart and the tricuspid valve and the pulmonic valve on the right side of the heart. In order for blood to move properly through the heart, each of these valves must open and close properly as the heart beats. The valves are usually referred to as leaflets or cusps. This leaflets comes together to close the valve, preventing blood from improperly mixing in the heart’s four chambers (right and left atrium and right and left ventricle)

Q2. When do the heart valve open and close?

Ans. It could be notice that the beating of the heart makes a “lubb-dubb, lubb-dubb” sound. This sound corresponds to the opening and closing of the valves in the heart. The first “lubb” sound is softer than the second; this is the sound of the mitral and tricuspid valves closing after the ventricles have filled with blood. As the mitral and tricuspid valves close, the aortic and pulmonic valves open to allow blood to flow from the contracting ventricles. The blood collected in the left ventricle is pumped through the aortic valve to the rest of the body, while blood cloocted in the right ventricle goes through the pulmonic valve on to the lungs. The second “dubb,” which is much louder, is the sound of the aortic and pulmonic valves closing after releasing the blood.

Q3. How often do the heart valves open and close ?

Ans. The average human heart beats 100,000 times per day. In the whole life span an average person 70-year-old, that means over 2.5 billion heart beats/life.

Q4. How big are the heart ?

Ans. The heart is about the size of the two hands clasped together.

Q5. How can the heart valve be replaced?

Ans. If the heart valve cannot be repaired, the consultant may decide to replace the native valve with a prosthetic valve. The prosthetic valves are usually of two types: a mechanical valve or a tissue valve. A mechanical valve is made from synthetic (manmade) materials and a tissue valve is usually made from the animal/human tissues. Valve replacement can be performed with a full open-chest or through less invasive or minimal incision approaches as well.

Q6. Are there any complications or other risks with heart valve surgery that should be known?

Ans. Serious complications, sometimes leading to re-operation or death, may be associated with heart valve surgery. It is important to discuss the particular situation with the consultant to understand the possible risks, benefits, and complications associated with the surgery.

Q7. How do valve should be taken care of ?

Ans. Be sure the dentist and general consultant know that heart valve surgery is done. Ask the dentist and general consultant about taking antibiotics before dental or surgical procedures or endoscopy to help prevent valve infection. Always follow The consutant’s instructions carefully.

Q8. How long after heart valve repair or replacement surgery “normal” levels of activity could be resumed?

Ans. After a valve replaced or repaired, the normal recovery period is four to eight weeks. The ability to return to the normal daily activities depends on several factors, including the type of valve repair/replacement one had, how well the incision is healing, and the advice of the consultant. A supervised cardiac rehabilitation program under the guidance of the consultant is always helpful to regain energy and ensure overall good health.

Q9. What are the type of mechanical valves?

Ans. There are mainly three types of mechanical valves

  1. Caged ball valve
  2. Tilting disc valve
  3. Bieleaflet valve

Q10. What is tissue engineered valve?

Ans. Tissue engineered heart valves (TEHV) is step towards a new revolution in the replacement of heart valves. It is uniquely designed valve which proposed treatment by creating a living heart valve for people who are in need. Tissue engineering of heart valves is a novel approach that explores the use creating a living heart valve composed of the host’s own cells that is capable of growing, adapting, and interacting within the human body’s natural system. In these models natural and synthetic polymers mimic the extra cellular matrix.

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