Biomedical Engineering Reference
In-Depth Information
complex but it suffices to say that they are antagonistic to vitamin K which in a
certain form is necessary to activate the coagulation factors II, VII, IX and X.
Heparin is also used in vitro when blood is withdrawn from the body and
collected in plastic bags for transfusion or retrieval of cellular or proteinous com-
ponents by separation techniques, or in tubes for clinical analysis in hematological
labs or to run experiments for blood-material interactions (if blood is placed in
a glass tube it will clot in 4-8 min). Other anticoagulants used in vitro are the
ethylenediamintetraacetic acid (EDTA)
and the
trisodium citrate
which are chelat-
ing agents that bind Ca
2
C
. As it has been mentioned previously Ca
2
C
(along with
phospholipids) accelerate many times the conversion reactions to produce the acti-
vated factors in the coagulation cascade. The difference between EDTA and citrate
is that EDTA causes some injury to platelets so that their aggregation is prevented
and therefore the blood sample drawn into EDTA can be used for platelet counting
(carried usually in an electronic particle counter).
Depending on a particular test that should be carried out sometimes a mixture
of anticoagulants may be used. It is also noted that apart from anticoagulants there
exist
fibrinolytic
and
thrombolytic
(such as
plasminogen
, a
2
-antiplasmin
) drugs, as
well as
antiplatelet
drugs, such as
aspirin
.
In summary it is noted that with regard to testing blood-material interactions,
which are complex and involve many cells and proteins, care should be taken to use
a particular anticoagulant, for a particular time, at a specific concentration.
Heparin is used in another way as well. Alone or in combination with albumin or
urokinase is adsorbed/bound onto the surface of polymeric biomaterials in order to
improve their hemocompatibility [
397
].
12.6
Blood Flow Through the Heart Valves
Certain serious problems and complications associated with the clinical use of
artificial heart valves include thromboembolism, hemolysis, tissue overgrowth and
damage to the endothelial tissue lining in the vicinity of the valves. These prob-
lems are directly related to the hemodynamics (fluid dynamics) in the valvular
region [
398
]. Other associated problems such as tissue overgrowth, infection, tear-
ing of sutures, valve failure due to material fatigue or chemical change are indirectly
related to the fluid mechanics.
Knowledge, therefore, of the blood fluid mechanics in the vicinity of the natural
valves, including the pressure difference across the valve, the velocity profile at the
outflow, the development of disturbed flow regimes (flow separation, vortices, etc.)
provide useful information in understanding the mechanisms of opening and closing
of the valves.
In order to obtain such information in a healthy human subject or in an animal
(in vivo) measuring probes and sensors should be inserted in the blood stream for
accurate monitoring. Such a procedure is certainly impossible for healthy humans
and quite cumbersome accompanied with adverse effects in the animals. Of course
