The chief problem at present with artificial heart valves is their
thrombogenicity, which requires the recipients to undergo
dangerous anticoagulant therapy, which is not always successful.
The present accepted method of testing valve thrombogenicity is
implantation in experimental animals. This technique is unsatisfactory
because of its high cost, the lack of control, the length of the
experiments and because differences between the blood of various
species render the results of dubious value. A more fundamental
approach to the problem based on fluid mechanics'fails because
of the complexity of the situation and because of the opposed effects
of a given fluid mechanical phenomenon. The in vitro use of blood
is not possible due to the need for a single pass system to avoid
recirculating clots, necessitating some thirty gallons of blood for
a run of only one hour.
An analogue method using a fluid which does not have the
disadvantages of the alternatives described above, therefore, seems
worth exploring. Rennetized milk is a possible fluid for such a
purpose and experiments were performed to ascertain whether it
behaves in a manner analogous to blood in respect of its essential
clotting properties. The first indications from the Lee-White
inverted test tube test were very promising with milk behaving in a
manner apparently identical to blood. Further experiments using the
Stagnation Point Flow Chamber showed that there was a striking
similarity in the microscopic appearance of the deposits forming at
and around the stagnation point when a jet of milk/blood impinges
normally onto a glass slide. A subsequent experiment, in which the
fluids are pumped through a mesh, revealed that the sequence of clot
growth as measured by the pressure variation upstream of the mesh is
the same for both fluids. Finally, full scale tests of the clotting
propensity of various heart valves were performed using an artificial
heart system, which showed that the location and appearance of the
clots forming on the valves with rennetized milk were similar to
those found in humans and that the results were reproducible. It
is, therefore, apparent that rennetized milk shows great potential
as an analogue for the flow related clotting of blood and can be used
for testing artificial heart valves provided care is taken in the
choice of materials for construction
