Biomedical Engineering Reference
In-Depth Information
4.6.3 Heart Valve Diseases
As discussed before, the heart valves regulate the flow of blood through the heart and
prevent the backflow of blood into the various heart chambers. A disease of the heart
valve obstructs these functions. There are two common ways for a disease of the heart
valve to develop. The first is associated with rheumatic fever, and the second is associated
with an increased calcium concentration in the body. Rheumatic fever is a disease in
which a bacterium enters the body and produces a variety of proteins. These proteins are
similar to some of the innate proteins within the body, and therefore, antibodies produced
by the host's immune system attack the bacterial proteins and some host proteins. The
heart valve leaflets are likely to be attacked by these antibodies, causing severe damage to
their structure. Normally, the mitral valve and the aortic valve are the first to be attacked
because they are exposed to more flow/mechanical disturbances during normal function-
ing (due to the higher pressure forces acting on these valves). If the disease progresses far
enough, it is common to see lesions within the tricuspid and the pulmonary valves.
There are two stages of valve leaflet destruction that can occur during rheumatic fever.
During the early stages of the disease, the leaflets of adjacent valves can become stuck
together due to the protein destruction and remodeling. In this case, the leaflets can typi-
cally close fully, but they cannot open fully, causing a stenosed valve. Blood is not ejected
as efficiently through a stenosed valve as a normal valve. Clearly, if the valve is stenosed
enough, not enough blood will enter the ventricles and/or the systemic or pulmonary cir-
culation. During later stages of rheumatic fever, scar tissue can form along valve leaflets
due to the extensive destruction of their protein structure. As the reader knows, scar tissue
is mechanically stiffer than the very pliant valve leaflets. This prevents the valve leaflets
from opening and closing fully. A valve that does not close properly causes backflow of
blood into the atria or in the ventricles, and this type of valve is said to experience regurgi-
tation. Regurgitation occurs during the cardiac cycle when the valves should be closed but
they are not fully closed. The end product again is a reduced cardiac output. In most cases
of rheumatic fever, there is some degree of valve stenosis along with regurgitated flow.
Another common heart valve disease occurs when the valve leaflets become calcified. It
is currently not clear what the mechanism behind this phenomenon is. However, what
occurs is the formation of a bone-like matrix (i.e., hydroxyapatite) within the valve leaflets.
Similar to the rheumatic fever disease scenario, calcified valve leaflets are much stiffer
than are normal leaflets. This will induce some combination of valve stenosis and regurgi-
tation flow through the valve and effectively reduce the cardiac output.
END OF CHAPTER SUMMARY
4.1
The human heart consists of two separate pumps that work in synergy to transport deox-
ygenated blood to the pulmonary circulation and oxygenated blood to the systemic circula-
tion. Each pump is composed of a primer pump, the atrium, and an active pump, the
ventricle. The cardiac muscle cells are highly interconnected to allow for the transfer of elec-
trical signals between the muscle cells. Troponin and tropomyosin are present within cardiac
muscle to regulate actin-myosin cross-bridge formation,
in a similar manner to skeletal
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