In the choice of a cardiac prosthesis it is advisable that the
prosthesis is not thrombogenic. The first prostheses were the Starr type,
consisting of a cage containing a ball of various material, which was silastic
(a particular alloy), while the ball inside the cage moved and allowed to block
the flow of blood.
The ball moved inside the cage, under the pressure of the
turbulent flow, during systole or diastole, depending on its position in the
aorta or in the mitral site.
These valves were the most implanted in the world and allowed to carry out
research studies, allowing to save human lives of heart patients. They had the
advantage of being easy to implant from a technical point of view, and of being
user-friendly valves, or very easy to be manipulated.
Cardiac valvular prostheses are divided into two types:
- Mechanical valves: those with the ball that moves inside the cage.
- Biological valves or bioprostheses that are produced with tissues of animal
derivation, for example of equine pericardium or bovine pericardium, or with the
trunk of the aorta of the pig.
The mechanical valves, as I said, are constituted by a movable element, generally carbon, and a structural element which retains the movable element, which is constituted by special alloys or also made of carbon. Depending on the design there are different types of biological or mechanical. From the practical point of view, physicians are concerned that these mechanical valves have a greater tendency to the formation of the thrombi for which it is necessary to prescribe the therapy with anticoagulant drugs in the patient with a mechanical valve.
On the contrary, biological valves do not require anticoagulant treatment, except for the first three months after implantation. Unfortunately, the biological valves have a shorter life span, so a patient, at some point, complains to his doctor of symptoms such as breathlessness, dyspnoea, or a big murmur into the chest. These symptoms may be related to the aging of the biological valve or its degeneration, with the need for this valve to be replaced.
The valves with the moving ball are those of the Starr-Edwards type, they are simply implanted like all the others with stitches, they have a fixing ring and
around this ring it is possible to apply the sutures that are inserted on the native ring or the mitral or aortic valve. Other types of valves, those now in
disuse, had an automatic coupling system so they could be anchored automatically when the surgeon rotated the cage, causing the activation of the coupling
system. The hooks come out and penetrate the area of the valve ring.
After some time the valves of the Sorin type, built by FIAT, were designed, characterized by a mobile element, a fixing ring with small hooks (struts) that
contained the mobile element.
This part of the valve was movable, because it opened as a door, not completely, but at a certain angle, to allow
the passage of blood flow in a less turbulent manner and subsequently the
retrograde flow of blood made it close . The flow through this type of valve, as
you can see, could not be completely a laminar flow because the central portion
did not have a laminar flow and therefore this condition could create turbulence.
Then interest in these mechanical valves, not one element, but from two moving elements, and the valves of the St. Jude type, which in fact today are the most
implanted in the world, have grown. They are built with two moving elements, n. 2 hemidisks, which obviously offer certain advantages, but also disadvantages.
The blood flow in this sketch is better than that type of valves with a single element and this is an element that recommends its installation in the field of
mechanical valves, precisely because they are characterized by a bidisk. The flow through these mechanical valves is very turbulent in the ball valves, in
the monodisk a little less, while in the valves with double disc the flow is closer to the laminar flow. We know that flow turbulence is a risk factor for
thrombogenicity, if a flow is more turbulent, then the flow is at greater risk of thrombogenicity.
Patients with highly thrombotic mechanical valves, such as
the ball valve, must have INR values of anticoagulation a little higher than those that have a valve with two moving elements. Even in some cases of children
who have been implanted this valve with two mobile elements it was possible to stop the anticoagulant therapy, because in children the flow is much higher than
adults, so the valve is much more washed in a child ( in relation to the body surface) compared to the flow of an adult.
There are various types: autologous ones, heterologous ones. The autologous are those taken from the same patient, and therefore the autologous are represented
by the pulmonary valve of the same subject taken and implanted in the aortic site. This is the intervention of Ross, old intervention, which has recently had
a revival phase: some players have been subjected to interventions of this type.
This intervention with autologous valves is used a lot in children because the pulmonary valve has a growth therefore it grows with the child, so this type of
procedure is favored.
In other cases, homologous biological valves are used, which were taken from cadavers. Some of the hearts can not be used for the explant for which valves
are used (aortic) and these valves have an optimal performance, because they are human valves, obviously they have some drawback that is not related to the risk
of rejection, because they do not have a rejection because the histocompatibility antigens on collagen are not expressed, because these valves
have all been disendothelized by a process based on glutaraldehyde. The problem is that these types of valves are scarcely available, and there are few, the
same banks and transplant centers have small banks with these homograft stored or cryopreserved at low temperature, so there are very limited availability.
They are mainly used in case of infection, because the resistance to infections of these autologous valves is much higher than those of a mechanical valve, to a
biological valve on which an infection remains localized and it is extremely difficult to root it. Then there are the biological ones that, as I said, were
produced by industry and are of two types fundamentally. We have the "stented" valves, that is, supported, which last less, and the "stentless" valves, that is
without support, that last longer. The support coupled to the valves is represented by a fabric structure that runs around the valve. They are made of
biological material, from bovine and equine pericardium and there are various types; durability is good, but generally does not exceed 10-15 years, so it must
be established if an elderly person has to receive a valve, mainly a biological valve will be implanted. If the patient is 75 years old it is good practice to
insert a biological valve because the durability of these valves is practically parallel to his life expectancy and, moreover, does not require anticoagulant
therapy, except for the first three months. In elderly patients, especially after age 70, in anticoagulant therapy, there is a very high risk of
subarachnoid cerebral hemorrhage and for this reason biological valves must be used. Thus, in the young patient it is preferable to implant a mechanical valve.
Today, however, mechanical valves consist of two cusps like real valves, have a long life and are not noisy. It has been documented and described in the literature rather than patients who carried two ball valves (one in the mitral and one in the aortic area) because of the noises generated by the first generation mechanical valves have undergone a state of neurosis and depressive syndrome , so that suicides have been described. Biological valves do not require anticoagulation, the duration is 10-12 years and they are silent. These are the advantages of this type of valves.
The surgical approach for the replacement of the heart valve can be of two types:
through the classic surgery with sternotomy (opening of the sternum to access
the heart) or through TAVI (Transcatheter Aortic Valve Implantation). This last
modern approach allows to place the valve in place through a catheter, inserted
at the femoral or trans-apical level (through the apex of the heart), which on
the tip has a balloon with the valve provided with a closed stent.
This type of approach allows avoiding surgery in patients who could not cope (elderly
people in suboptimal conditions, critically ill patients, difficulties in the
management of surgery) with a result very similar to that of classical surgery.
For those wishing to deepen this discussion, I refer to an interesting work on
Medscape which concerns the TAVI technique and the Edwards SAPIEN valves.
The traditional technique requires a longitudinal (vertical) opening of the
anterior wall of the thorax through the sternum which is cut into two parts.
This incision is called 10 vertical median sternotomy. Through this opening the
surgeon can see the whole heart and the ascending aorta. The intervention
requires that the patient be connected to the heart-lung machine. To do this two
cannulae are inserted, one in the upper part of the ascending aorta and one in
the right atrium. They bring blood from the patient to the machine, where it is
enriched with oxygen, and vice versa. Once the extra body circulation has
started, the heart can be stopped with a special mixture of chemicals called
cardioplegia. At this point the aorta is opened, the diseased valve removed and
a prosthesis (mechanical or biological) is inserted in its place. Then the aorta
is closed again. As soon as it receives the blood again, the heart spontaneously
begins to contract. The patient can thus be disconnected from the machine.