By: Sadwika Salain
Artificial Valves
1 Introduction
Anatomically, the human heart comprises of four chambers—two atria and two ventricles. (Detail of the heart components is shown on figure 1). Physiologically, the heart can be considered as a positive displacement pump that generates blood flow to support systemic and pulmonary circulation. For this purpose, the heart undergoes both contraction and expansion in alternating period. It is estimated that the heart contracts and expands as many as on average 100,000 times everyday, pumping approximately 2,000 gallons of blood (Bender, 1992).
To ensure the blood flows in a forward direction, two valves at each ventricle – one at the inlet – open and shut in a synchronized manner. The four heart valves are:
- Tricuspid valve : located between the right atrium and the right ventricle
- Pulmonary valve : located between the right ventricle and the pulmonary artery
- Mitral valve : between the left atrium and the left ventricle
- Aortic valve : between the left ventricle and the aorta
The blood come from the veins, which has less oxygen, enters the heart via the right atrium. After filling, the right atrium then contracts and pushes the blood to the right ventricle through the tricuspid valve. From this ventricle, blood is sent to the lung through the pulmonary valve. In the lung the blood is oxygenated. Furthermore, the oxygenated blood returns to the left atrium then goes to the left ventricle through the mitral valve. The final stage is, the blood is squeezed out from the left atrium into the aorta and other circulatory system through the aortic valve. Detail of how the heart valve is working will be described in the following section.
2 The Heart Valve in Function
The heart valves function to prevent backward flow of the blood within the heart chamber. As the heart consists of four chambers – two atria and two ventricles – the valve works as a one-way inlet through on one side of ventricle and one-outlet which pass through other side of the ventricles. To maintain the blood flowing into one direction, the valves functions by opening and closing themselves mechanically. These open-and-shut mechanisms are driven by the heart muscle contraction and relaxation, allowing the blood flows into the ventricles and atria chamber at the alternating time.
The following procedure is a description of how the valves function:
- After the left ventricle contracts, the aortic valve closes and the mitral valve opens, to allow blood to flow from the left atrium into the left ventricle.
- As the left atrium contracts, more blood flows into the left ventricle.
- When the left ventricle contracts, the mitral valve closes and the aortic valve opens, so blood flows into the aorta.
In case of disease, the heart valve does not able to shut and open properly and tend to undergone the heart valve failures.
3 Heart Valve Failures
The hearts valves may fail due to generative, rheumatic fever, endocarditis and congenital birth defects (Borrero et al., 2003). These diseases may obstruct the blood flow across the valves. Two possibilities that may occur once the valves damage are:
- Incompetence
This is a condition where the valve could not shut completely, and therefore allowing blood backflows to the origin position. Blood backs up, engorges the veins in the lungs and other parts of the body, and causes a congestion of fluid in body tissues. Fluid may collect in the lungs, obstructing the passage of air and oxygen exchange and interfering with breathing (Nair et al., 2003).
- Stenosis i.e the valve does not sufficiently open. As the result, the amount of blood entering the destination chamber is less than it should be.
- Blood Clothing
Another serious heart valve complication is formation of blood clots. The clots may be formed once the surface of the valve is damage and roughened. This condition may affect the steady flow of blood and then generating swirl flows of the blood, which stimulate the blood clots. Once this coagulated blood is detached and travel through the bloodstream and gets stuck in a small blood vessel, it will disturb the organ function. Once it occurs in the brain, a person will undergo a stroke (Nair et al., 2003).
These circumstances will disturb the balance system of the body. Once the valve severely damage, and the native valve repair is not likely possible, the valves should be replaced with prosthesis or artificial valve to extent the patient lifespan.
4 Artificial Heart Valves
Artificial heart valves are engineered devices used to substitute the natural valves of the heart (Nair et al., 2003). Based on materials of origin, the artificial valve can be categorized in two types; mechanical and biological valves. Some scholars are sub-devided the prosthetic valve – another term of artificial valve – into three groups; mechanical, biological and polymer based valves (Borrero et al., 2003).
Mechanical heart valve is made from artificial materials of synthetic origin like metals and ceramics. The newest type of the mechanical heart valve is made from polymer to enhance properties of metal and ceramics. The second type the valves is biological valves – named bio-prosthetic. These valves are taken from the heart valve of other animal such pigs. Additionally, this valve is also can be produced from animal’s tissues such as pericardial tissue of the cow, homografts or allograft which is obtained from cryo-preserved – preserved in liquid nitrogen – cadavers.
4.1 Mechanical Heart Valves
4.1.1 Caged Ball Valves
The history of mechanical heart valve was commenced in 1952, when Charles Hufnagel developed an acrylic ball valve to repair malfunction of the aortic valve of a patient. Physically, this valve was formed as a caged ball. The mechanism of this valve mimics that the mitral valve yet with higher durability. This is the major advantage of the caged ball valve. Whilst, it’s major drawback lied on its shape. As it was designed to have a central occlusion, a larger pressure drop across the valve was resulted. This event further initiated a higher turbulent stresses, distal to the valve. The large profile of the valves increased the possibility of interfering the anatomical structures after implantation (Bhuvaneshwar et al., 1991). To answer this issues, between 1965 – 1967 there were a major development of the caged ball valve i.e ‘Cross–Jones’ and ‘Kay–Shiley and Beall caged-disc’ design.
4.1.2 Tilting Disc Valves
In 1965, the major improvement of Kay–Shiley and Beall caged-disc occurred by the invention of tilting disc by Bjork-Shilley (Yoginathan, 1995) . Since then, domination of the caged ball valve was eventually replaced by the disk valve. This first generation of disk valves was not 100% able to overcome the entire problem left by the ball-caged valve. It has a major dearth i.e initiating blood pressure drops, due to poor haemodynamics inside the heart. Additionally, the valve’s wear was much higher than that of the caged ball. This was eventually overcome by invention of tilting disc in 1967, which able to prevent falls in blood pressure and minimizing the flow turbulence (Nair et al., 2003).
4.1.3 Bileaflet Valves
The bileaflet Valve’s principle is a low profile – hinge mechanism. The major advantage of this valve is the high protrude protection. In term of flow profile, this valve offered steadier flow. It was due to its large effective orifice area resulting less obstruction and turbulence of flow (Nair et al., 2003). However, dearth of the previous two valves – caged ball and the tilting disc – was still remained i.e it still has tendency of thromboembolism[1], in both with and without anticoagulation. Therefore, to be clinically used, lifelong anticoagulation was still required (Ribeiro et al., 1986).
4.2 Biological Valves
In 1977 St. Jude Cardiac Valve prosthesis introduced biological valves – which made from biological tissues[2]. This first biological valve was manufactured from porcine or bovine tissue. In term of design, the absence of mechanical occluders became the most obvious distinction between this type of valve and all of mechanical valves. Further development of this first generation lied on the material combinations. The bio-prosthetic valve leaflets were manufactured from combination of xenogenic tissue and synthetic materials.
Compared to the mechanical valves, bio-prosthetic valves have major advantages i.e have better thrombo-resistance and haemodynamic performance as they possess closer trait and functional properties of native valves than do mechanical valve. Yet, limited durability is a major drawback of this type of valve (Borttolotti et al., 1987). Below is the mechanical and biological valves which are available on the market.
5 Comparison among the Heart Valves Prosthesis
In addition to the material of origin distinction as shown above, there is several differences between biological and mechanical valve prosthesis and is shown in a form of ‘advantages and disadvantages table’ as shown below.
| Prosthesis | Mechanical | Biological | ||||
| Homograft (Allografts) | Xenograft | Bio-prosthetic | ||||
| Example | Caged Ball | |||||
Tilting Disc
BileafletCadaver valvesPorcine or bovine tissue valvesTitanium stents + pericardiumAdvantages
- High durability
- Easy to be manufactured
- Structurally sound – built in redundancy in the strut design
- Low levels of regurgitation in the closed phase
- Good hemodynamic characteristics
- Lower valve height and therefore suitable to be place at any anatomical locations
- Uniform flow profiles
- Low tendency of causing structural complications
- Less thromboembolic
- Low tendency of causing immune response, infection or disease transmission
- Less thromboembolic
- Less immuogenicity after Glutaraldehyde treatment
- Lower risk of infection
- Greater supply
- Less thromboembolic
- High durability
Disadvantages
- Lifelong anticoagulant therapy
- Risk of infection
- Noisy operation
- Turbulent flow
- Catastrophic failure
- Relatively large valve height
- Flow separation downstream of the valve, which might lead to thrombus formation
- Lower levels of redundancy in the cage strut structure
- Lower minor orifice flow can lead to tissueover growth and thrombosis
- Strut fracture and related complicationshave occurred in certain models
- Hinge design prone to thrombus formation and valve failure
- Leaflet escapement reported in certainmodels
- Limited supply
- Early failure in young people
- Tend to be degenerated & calcified (requires changes in 5-15 years)
- Progressive degeneration & limited durability
- Early failure in young people
- Possible disease
- Catastrophic failure
- Risk of infection, foreign body response
- Calcification
6 Failure of the Heart Valves Prosthesis
Since the invention of the heart valves, several clinical failures have been encountered such as (Damen, 2003):
- Degradation of valve components
- Structural failure
- Clinical complications associated with the valve.
Furthermore, the above failure caused clinical event which end up of requiring reoperation or even causing death (Bhuvaneshwar et al., 1991). i.e.
- Anticoagulant-related hemorrhage (ACH),
- Prosthetic valve occlusion (thrombosis or tissue growth),
- Thromboembolism
- Prosthetic valve endocarditis (PVE),
- Hemodynamic prosthetic dysfunction, including structural failure of prosthetic components (strut failure, poppet escape, ball variance),
- Reoperation for any other reason (e.g.; hemolysis, noise, incidental) etc.
These failures have driven the researchers to actively search the ideal or nearly ideal heart valves prosthesis. Prior the envisioned prosthesis is manufactured, it is important to describe the functional requirements of the ideal heart valve prosthesis should be.
7 Functional requirements of heart valve prosthesis
Functional requirement of the hearts valve prosthesis are (Damen, 2003):
- The prosthesis should be able to function efficiently without introduction of additional load to the heart,
- It has a good reliability and durability and able to keep good performance in sufficiently long period of time for lifespan of the patient.
- It must be biocompatible – do notintroduce any damage to any other organ and do not stimulate blood clotting.
Other requirements stated by Bhuvaneshwar et.al (1991) are:
- Cause minimal trauma to blood elements and the endothelial tissue of the cardiovascular structure surrounding the valve,
- Show good resistance to mechanical and structural wear.
- Minimize chances for platelet and thrombus deposition.
- Be non-degradable in the physiological environment.
- Neither absorbs blood constituents nor release foreign substances into the blood.
- Have good processibility (especially suitable for sterilization of the device by appropriate means) and take good surface finish.
The ideal design of the heart valve prosthesis is presented on the table below.
Basic design goals |
Other desirable characteristics |
|
Table 2. Ideal Criteria for the heart valve prosthesis
Source: (Damen, 2003)
8 Design challenges of heart valve prostheses
Reviewing the criteria presented in Table 2, several design improvement can be clarified, which focus on:
- Discovering substitutes that able to self-repair and grow
It is known that all current biological heart valve tissue has no ability to grow, repair, or remodel within the recipient body (Michell et al., 1998). A potential solution that satisfies thorough requirement stated on the table 2 is tissue engineering that able to develop an identical copy of a healthy valve of the recipient.
- While tissue engineering is seem still to far to be implemented, improving the mechanical design as well as material of the mechanical valves is still relevant. For this purposes, polymeric materials show great potential in overcoming problems of material fatigue, while at the same time maintaining natural hemodynamics and functional characteristics. The ability of these polymeric materials to maintain or closely simulate natural body hemodynamics is greatly based upon the fact that they have a soft texture which simulates the lubricity exhibited by the natural heart valves, and allows them to contract and expand freely and body like as the blood flows through them(Nair et al., 2003).
9 Conclusion
The heart can be considered as a positive displacement pump, function to flow the blood to the aortic and pulmonary system. The heart posses four heart valves that functions to maintain the blood goes into forward direction, by opening and closing mechanism in alternating manner. Heart Valve failures are signed by the valves does not able to open and close completely. In severe damage, the heart valve must be replaced by the heart valve prosthesis; mechanical or biological valves. The mechanical valves are made from artificial-manufactured materials such as metal, ceramics and polymer. Whist, the biological valves are made from biological tissues of either animal or human. Allograft, Xenograft or homograft are the terms to describe the origin of transplanted valve tissue. The major advantage of the mechanical valve is high durability, yet has issues of biocompatibility such as introducing blood clothing and vice versa of the biological valve. The challenges of the heart valve development in the future are to produce the heart valve that able to self-grow, repair. Tissue engineering seems to be the answer of the challenges. However, it is still too far to be applied recently. The improvement of the existing mechanical valves is still required. Polymer apparently improves the dearth of the existing mechanical valves properties.
10 References
– (2009) Wikipedia.
BENDER, J. R. (1992) Hear Valve Disease. Major Cardiovascular Disorder. Yale.
BHUVANESHWAR, G. S., MURALEEDHARAN, C. V., RAMANI, A. V. & VALIATHAN, M. S. (1991) Evaluation of materials for artificial heart valves. . Bull. Mater. Sci. , 14, 1361-1374.
BORRERO, J. R., CURE, J., FABRE, N. J. & ROSADO, E. (2003) Mechanics of Prosthetic Heart Valves. Applications of Engineering Mechanics in Medicine. Mayaguez, University of Puerto Rico.
BORTTOLOTTI, U., MILANO, A. & THIENE, G. (1987) Mechanical failures of the Hancock pericardial xenograft. Thoratic Cardiovascular Surg., 94, 200-2007.
DAMEN, B. S. (2003) Design, Development and Optimisation of a Tissue Culture Vessel System for Tissue Engineering Applications. Groningen, Swinburne University of Technology.
MICHELL, R., JONAS, R. & SCHOEN, F. (1998) Pathalogy of explanted cyropreserved allograft heart valves: comparison with aortic valves from orthotopic heart transplants. Journal of Thorac Cardiovascular Surgery, 115, 118-27.
NAIR, K., MURALEDHARAN, C. V. & BHUVANESHWAR, G. S. (2003) Developments in mechanical heart valve prosthesis. Sadhana 28, 575-587.
RIBEIRO, P. A., ZAIBAG, M. A. A., IDGIS, M., KASAB, S. A., DAVIES, G., MASHAT, E., WAREHAM, E. & FAGIH, M. A. (1986) Antiplatelet drugs and the incidence of thromboembolic complications of the St Jude Medical Aortic prosthesis in patients with RHD J. Thorac. Cardiovascular Surgery, 91, 92-98.
YOGINATHAN, A. P. (1995) Cardiac valve prosthesis., NewYork, CRC Press.
http://www.pages.drexel.edu/ ~nag38/Types/bil.html
http://www.heart-valve-surgery.com/mechanical-prosthetic-heart-valve.php
[1] Thromboembolism is the formation of a blood clot (thrombus) inside a blood vessel, obstructing the flow of blood through the circulatory system – (2009) Wikipedia.
[2] Autograft, xenograft and homograft are the terms that describe the origin of the host tissue – can be retrieved in detail on the appendix.
Leave a Reply