The Physics Behind
Artificial Heart Valves 
 

Claire Carson, Julia Richman, and Robin Creel present
Group 30's Web Project for
Mr. Deardorff's Physics 24 lecture
December 4, 2000

   Through this project we hope to shed some light upon the human heart valves, the causes of some irregularities in their function, and the devices man has strived to perfect to repair the heart's proper functioning.  For over half a century, doctors and scientists have been working diligently to create the best funtioning artificial heart valve, and they have nearly completed their task.  In their trials of experimentation, doctors have discovered the immense complexity of the physics within our own heart, and their task of creating the perfect articial heart valve has taught them an even greater appreciation of the intricate inner-workings of the human body.
 

Anatomy:
        The heart consists of four chambers.  The upper two are the right and left atria; the lower two, the right and left ventricles.  In a healthy body, deoxygenated blood is pumped from the body through the right atria and ventricle to the lungs.  From the lungs, oxygenated blood flows through the left atria and ventricle. The blood then leaves the heart via the aorta, and the cycle continues.  Each chamber has a sort of one-way portal, the valve, at its exit that prevents blood from flowing backward.  There are four valves** in the heart;  the tricuspid and bicuspid (mitral) valves separate the right and left ventricles respectively.  The semilunar (also called the pulmonary and aortic) valves precede the entry into the lungs and body (via the aorta) respectively.  If these valves do not function appropriately, many complications can occur.

Physiology:
        Cardiac valves have three functional properties:  (1) preventing regurgitation of blood flow from one chamber to another, (2) permitting rapid flow without imposing resistance on that flow, and (3) withstanding high-pressure loads.  These valves work by a several principles related to physics.  However, this discussion is limited to pressure gradients.  Fluid flows from areas of high pressure to areas of low pressure.  In the heart the valves open and close in response to pressure gradients, i.e., valves open when pressure in the preceding chamber is higher and close when the gradient reverses.  These one-way valves are important in ensuring that the blood flows in the proper direction.  One “heart beat” consists of two phases, the diastolic and the systolic.  During the diastolic phase the heart muscles are “relaxed” and blood flows into the chambers from the body and lungs.  The tricuspid and the bicuspid valves are open at this time, allowing the atria and ventricles to fill with blood (the pulmonary and aortic valves are closed during diastole).  The heart then contracts during the systolic phase and pumps blood into the lungs and body.  At this time the pulmonary and aortic valves open and the tricuspid and bicuspid valves “snap” shut.

Physics:
        An example of how pressure gradients work in the heart is as follows:  at the beginning of systole the goal is to raise the pressure within the ventricle chambers.  As previously described, two things cause the sharp rise in pressure:  the contraction of the ventricle itself, and the fact that for a brief moment all four valves are closed.  Thus, the contracting muscles elevate ventricular pressure without a change in volume.  This moment of contraction is termed isovolumetric, meaning “maintenance of a constant volume.”  At the point that the pressure inside the ventricles exceeds the diastolic pressure within the preceding tract, the aortic and pulmonary valves open and ejection begins.  In this case ejection is the movement of the blood out of the ventricles into the lungs and body.  Initially during ejection the left ventricular pressure rises above that of the aorta and produces a very rapid flow.  Later in ejection, ventricular pressure drops below aortic pressure, resulting in a diminished velocity of outflow.  (Specific values for intracardiac pressures are not discussed here for simplicity’s sake).

(Note: for a review of heart anatomy, go to: www.howstuffworks.com/heart.htm).
**Image from MedFacts.com

            Valvular heart disease (VHD), a non-specific, all-encompassing name for various diseases affecting the heart valves, can be classified into two categories: congenital and acquired.  Congenital valvular heart disease is present from birth, and occurrs in about 0.6% of non-premature live births.  It can be caused by chromosomal abnormalities, such as trisomy 18 or trisomy 21 (known as Down syndrome).  In most cases, however, the causes of congenital valvular disease are unclear.
             Acquired valvular heart disease is much more common than congential VHD.  Acquired VHD is generally caused by a disease or injury to the heart, which affects the individual at some point in their lifetime.  An autoimmune disease related to the streptococcus virus, acute rheumatic fever is a serious illness which can cause forms of VHD such as valvular stenosis (hardening of the valves).  Other cases of VHD include tumors of the heart muscle, injury to the chest, lupus, and many others.
            There are two main types of faulty valves that may or may not require valvular replacement surgery.  There are valves which do not close properly and leak blood into another quadrant of the heart (regurgitation) and valves which are hardened and don’t open properly (stenosis).  Valvular regurgitation causes the heart to work less efficiently because it has to pump some blood twice, and usually results in an enlargement of the heart chambers because there is more blood to pump to compensate for the leaking blood.  However, in severe cases the heart is not strong enough to compensate for the efficiency loss and it results in congestive heart failure.  Valvular stenosis causes higher blood pressure in the heart because blood builds up behind the closed valve and forces the cardiac muscle to work harder to pump the higher-pressure blood through the heart.  The heart usually compensates by growing a thicker layer of muscle.  In extreme VHD cases, valvular replacement surgery is becoming an extremely feasible option.  Both mechanical and biosynthetic valves are used, and prognosis of surgical patients is fairly good (see chart below).

(Data from Fundamentals of Anatomy and Physiology Applications Manual by Frederic H. Martini and
Kathleen Welch, Prentice Hall 1998)

Brief History of Prosthetic Heart Valves:
        In 1952, Charles Hufnagel used the first artificial heart valve, an acrylic ball valve, to correct aortic incompetence.  This procedure was rarely performed until the heart-lung machine was created in 1953, when surgeons were first able to perform open-heart surgery under direct vision.  Several forms of the ball valve were created and used in valvular replacement, but were phased out starting in 1965 when the first disk valve was created by Kay and Donald Shiley.  This valve was thought to encompass traits that the ball valve did not possess, but it too had problems with blood pressure fall (poor hemodynamics) inside the heart.  Doctors then turned to tissue replacement valves, using human pericardium and fascia lata over a synthetic form, but these valves failed after a period of approximately five years because of insufficiency and calcification of the tissue.
        A Japanese surgeon invented a second form of the disk valve, this time with a tilting disk that hindered falls in blood pressure.  This new valve tended to wear out too quickly, and in 1969 a tilting disk valve with a floating disk made of Delrin was created that lasted much longer than its predecessor.  In 1971, the Delrin disk was replaced with a disk of carbon pyrolite when it was discovered that the carbon pyrolite disk would not react with the blood and become swollen and immobile inside the valve the way that the Delrin disk had.  In 1976, the carbon pyrolite disk was given a convexoconcave shape, giving it an aerodynamic form that allowed the valve to open and close quicker and with less space for blood to leak through.  In 1977, the bileaflet valve, termed the "St. Jude Cardiac Valve Prosthesis," was introduced.  This valve consisted of two disks rather than one, and was more capable of controlling blood flow and resisting blood clotting and bacterial infection than the previous designs.  This final design, with minor alterations to its composition, has been used with a high degree of success since its advent.  In addition to the bileaflet valve, porcine and bovine tissue valves (biosynthetic valves) are now used in valvular replacement when the situation calls for a more delicate replacement valve.

Hemodynamics:
        Physicians look for three main criteria when assessing the hemodynamic performance (proper blood flow) of a particular valve design.  The valve in question should (1) function efficiently and present the minimum load to the heart, (2) be durable and maintain its efficiency for the patient's entire lifespan, and (3) not cause damage to molecular or cellular blood components or stimulate thrombus formation (blood clotting).  In addition to these criteria, good hemodynamics ensures that the valve will hold the blood in the heart quadrant between heartbeats, and release the blood upon heartbeat without leaking in either case.  The valve must respond to the appropriate pressure gradients in order to open and release blood at the right moment.  If there is a drop in the body's blood pressure, the valves are responsible for holding blood in the ventricle until the pressure has reached a level great enough to restore appropriate pressure to the vessels.  As you can see, a great deal of skill is involved in prosthetic valve design.

What They are Made of:
    -Mechanical Valves
              Mechanical valves are synthetic valves that come in a variety of materials and forms.  Two of the most popular varieties are the Ball Valve and the Disk Valve.

                    Ball Valves*
                            When the first ball valves were designed, doctors were primarily concerned with securing  the structure in place and preventing thrombosis from occurring.  They knew that all the materials selected would have to be biocompatible--that is, they would not react negatively with the antibodies present in humans.  The materials they selected ranged from Teflon cloth for the sewing ring and silicone rubber for the occluder ball, to stainless steel, solid Teflon, and Lucite for the cage components.
                            The first ball valve implanted in a human had a cage constructed entirely of Lucite, with a ball made of a solid Silastic sphere rather than a stainless steel ball coated with rubber, as was previously done in animal testing. This model had a problem with thrombosis, and the patients died of related complications.  Other models had a stainless steel cage covered with a Teflon cloth and a Silastic sphere for a ball, but thrombosis occured with this model as well.
                            One of the final versions of the ball valve was the Smeloff-Cutter valve (shown), which was contructed of a commercially pure titanium cage in a bare, uncoated nature.  This cage had a nonoverlapping, full-orifice with a ball constructed of silicone rubber.  The design of this valve permitted slight regurgitation of blood that would, in effect, wash the ball and cage, preventing thrombosis.  The Smeloff-Cutter valve enjoyed over a decade of success in valvular replacement with an established record of durability.

                    -Disk Valves
                               The first disk valve used in human valvular replacement occurred in 1965, but it was not until the advent of the tilting disk valve with a floating disk that any major success occurred, putting the disk valve ahead of the ball valve in terms of the longevity of the patients and lack of complications with the valves.
                               -Single Leaflet Disk Valves*
                                        The large size of the ball valve was thought to be associated with certain deaths--deaths that were the result of irritation of the ventricular cavity at the hands of the ball valve's large cage.  This lead scientists to develop a valve with a smaller cage, which would thusly require a smaller ball, or rather a disk.  The first disk valves resembled a shallow ball valve with a disk in place of the ball (shown), where the disk simply moved up and down with the heartbeat to open or occlude the valve.  Like the ball valves, these original disk valves lacked optimal hemodynamics, the ability to properly control blood flow.  Other designs followed, having disks oriented on an angle to optimize hemodynamics, and having a floating disk that prevented wear along the axis of attachment.  The many versions of the tilting disk valve have been popular for decades because of their ability to prevent periprosthetic leak, endocarditis, and thrombosis, among other complications.
                            -Bileaflet Disk Valves*
                                        The bileaflet design first introduced in 1977 has been heralded for its success.  The design (using two disks, a hinge mechanism, and a low profile) was found to be very durable when constructed of an apporpriate material (usually carbon pyrolite) and has enjoyed low thrombogenicity and superior hemodynamics.  Of the half-dozen or so varieties of the bileaflet valve, the St. Jude prosthesis (shown) has been used most often in mitral and aortic valve replacements.  Of the three varieties of the St. Jude prostheses, all are constructed of carbon pyrolite, and have recently been fitted with a circular metallic ring below the polyester sewing ring for aid in x-ray visibility.

   -Tissue Valves (Biosynthetic Valves)
          Tissue valves are classified in two varieties based on what type of tissue is used.  They can either consist of human tissue or animal tissue

                        -Human Tissue Valves (Homographs, Autographs)
                                Cognizant of the problems with the ball valves, doctors in the 1960s turned to the idea of homographs (or allographs) in which the patient receives a replacement heart valve from a deceased donor.  The donated valve is frozen in liquid nitrogen until time for surgery, when the valve must be thawed for 24 hours before transplantation.  Because the delicate nature of the valve, surgeons must be careful to ensure the valve is the proper size and to take the utmost care during surgery, since it is harder to install than all other forms of tissue valves.
                                In an autograph tissue valve, the tissue is from the body of the patient receiving the valve replacement.  The surgeon can take tissue from a variety of regions, including, but not limited to, the dura mater, pericardium, fascia lata, peritoneum, and vena cava.  The tissue is then attached to a synthetic frame, usually of stainless steel, which gives the structure a stronger form and is therefore easier to transplant than a homograph.
                        -Animal Tissue Valves (Xenographs, Heterographs)
                                Unlike homographs and autographs, the terms xenograph and heterograph are used interchangeably.  Both refer to animal tissue implants by coining them either "foreign" or "different."  The animal tissue chosen for human valvular prosthesis is porcine or bovine.  In certainporcine implants*, the pig valve is attached to a steel frame (called a stent), giving the tissue shape, and the stent is then covered with a Dacron cloth.  Other porcine valves are transplanted without a stent, with the theory that the stent and the sewing cloth decrease the hemodynamics of the valve.
                                Another xenographic tissue implant comes from bovine pericardium, where the tissue is attached to a Dacron covered titanium stent. Bovine pericardium valves* exhibit non-thrombogenic properties and excellent hemodynamic traits, and have begun to replace mechanical valves and porcine valves used for the same purpose.
*link to Cedars Sinai Medical Center

        Science has come very close to designing artificial valves that function like healthy cardiac valves, however the ideal heart valve still does not exist.
 
 
 
 

Links to Other Sites on Valvular Prosthesis:
        Medical Carbon Research Institute, LLC
        St. Jude Medical
        Sulzer Carbomedics, Inc.
        Vi Vitro Systems

Some Interesting Links (used as references):
        http://www.heartpoint.com/valvularheartdxmore.html
        http://www.allina.com/Allina_Journal/Fall1996/rose.html
        www.prenhall.com/martini/fap