Introduction
Cardiovascular valve replacement started in the 1960’s and most failures were caused by design and structure related to the valve. Valve replacement has rapidly advanced with new technological developments. Patients live longer, healthier lives today because of valve replacement and the two main replacement valves are mechanical heart valves (MHVs) and Bioprosthetic heart valves (BHVs). There are approximately just over 250,000 replacement valves put into patients worldwide each year, with 55% being MHVs and 45% BHVs. MHVs are non biological material that are made of polymers, metal, and carbon, where the BHVs are composed of human or animal tissues that are mounted onto a stent that has been covered by medical fabric. Stentless valves give advantages over stented valves because they enhance haemodynamics, ventricular remodelling (recovery to the natural state), and a higher patient survival rate. The differences between these two valves lie not only in their composition, but also in their durability, thrombogenicity, and their haemodynamic profile created as blood passes through. Each of the valves is suited to certain age groups depending upon the complications that are encountered.
The mechanical heart valves used require that the patient use a blood thinning pharmaceutical such as Warfarin, which allows easier passage of blood without the risk of tearing the valve or the sutures where it was connected. However, risks are associated with the mechanical valves because they do not have the same durability as the BHV valves and there is always a risk of bleeding when a patient is on a blood thinning pharmaceutical. The BHVs are much broader in their classification and design and are what most surgeons are moving towards in terms of surgery and Siddiqui et al., have developed a great table to show the different valves available on the market.
The BHVs can be derived from human or animal tissue and the human tissues fall into two categories:
· Aortic/pulmonary valve allografts come from a genetically non-identical member of the same species, which could be a graft from someone of your family, which is not very common. These have been shown to have great haemodynamics and low incidences of thromboembolic (blood clotting) complications and the interstitial and endothelial cell layers are not very sustainable.
· Aortic/pulmonary valve autografts come from a different part of the patient’s body and are put into another place, and this is a much more common procedure performed. Usually the pulmonary valve is grafted into the aortic site, and an allograft is put where the pulmonary valve used to be.
· Valves obtained from animals are known as xenografts and these are usually from porcine (member of pig family) and bovine tissue which most closely resemble the human heart. Usually these require stents to be placed around them in order to function as closely to the natural valve as possible. However, when a stent is added this will take up space in the lumen of the artery and can lead to residual stenosis (narrowing) of up to 20%.
Modes of Failure
Patients that receive valve replacement surgeries only have a certain time frame where these valves will improve their quality of life, and then further surgery may be required to put a new valve in or their life may be ended. Siddiqui et al., have found that children and adolescents have the highest rate of tissue failure and occurs in approximately 5 years after the valve replacement surgery was performed. The exact mechanism by which tissue degeneration occurs is not understood, however they hypothesize that host IgM/IgG antibodies deposit macrophages which break down the collagen in the tissues which leads to calcification. Siddiqui et al., has broken down the modes of failure into five main categories.
· Calcification is a major contributor to valve failure and is a result of the interaction of aldehyde groups phospholipids in the tissue and circulating calcium ions. Approximately 75% of porcine valves occur as a result of calcification which causes stiffening of the valve which can be related to calcium metabolism in the body. The zona fibrosa usually the most affected layer of tissue which in histological preparations shows the greatest amount of calcification. As well, cross linking of collagen fibers during the healing process creates crypts where calcification can build up.
· Cusp Tears are a result of the great pressure that results when the valves open and close. The design of the valve has a great deal to do with this, and the replacement mitral valves have shown to be more prone to tearing.
· Pannus occurs when there is excessive growth of the host tissue onto the implanted valve, which is not part of the natural healing process. Pannus consists of myofibroblasts, fibroblasts, and capillary endothelial cells which are initiated by small amounts of thrombus and inflammatory cells as part of injury during the surgery. As healing continues there is more collagen that appear in the pannus, and this causes changes in the structure of the tissue such as the ability of the cusps to open fully. Overgrowth is usually experienced by the stentless valves more than the stented valves.
· Infective Endocarditis usually occurs during the first 2 years of replacement and is more common in patients whose valves were replaced due to previous endocarditis, which is a condition where the heart becomes inflamed usually due to infections from Staphylococcus epidermidis and Staphylococcus aureus. Usually the inflammation occurs around the rings where the valve was sewn into the tissue and this can be seen by an increase in mononuclear cell infiltrate and fibrins in that area.
· Valve Thrombosis is a very rare condition that only occurs in 0.1-0.75% of patients who receive a valve replacement, but is very high during the first three months of recovery from surgery. The main cause of this is the improper use of anticoagulant pharmaceutical therapy. This can be diagnosed by poor peripheral circulation and pulmonary congestion.
What I Think
I thought that the paper was well written and easy enough to read so that someone with a science background would understand everything that the author was describing. Since the journal that the article was published in was a Histopathology journal, I expected it to contain equal amounts of pathology and histology, but I found that there was more pathology than histology. The paper really took an in depth look in the various types of heart valves available for replacement, and the most common modes of failure. The paper did not take the common format of a lab report, but it was more so reviewing the pro’s and con’s of the various valves. I think that this is a very important branch of medicine because not only are these surgeons able to save lives, they are also extending lives and allowing for their quality of life to increase. Heart replacement valves are important for everyone to know about because with such high incidences of heart disease, these procedures are becoming more common and showing people that these valves are available, but only for small time periods may be enough to help scaring them into changing their lifestyles. I have seen these procedures happen to patients in the OR when I was volunteering with a heart surgeon over the past two summers, and to see these procedures happen and how they can make such an improvement in a patients life is quite an amazing feat and with the technology and procedures improving there is only room for improvement.
