The effect of thromboresistent surfaces on the development of blood emboli and hemolysis.

Heparinized dog blood was sheared in conicylindrical test cells which had been coated with thrombo-resistant materials. Hemolysis and blood emboli formation in the coated cells were as great or greater than in uncoated polycarbonate cells. These results suggest that materials which resist the buildup of thrombi may still be stimulating emboli formation and red cell damage.

The effect of shear, specific surface, and air interface on the development of blood emboli and hemolysis.

Heparinized dog blood was exposed to shear and foreign surfaces in conicylindrical test cells. The cells were injection molded from polycarbonate and were filled using a technique that avoided contact of the blood with air. Particulate-matter formation was measured and was found to be dominated by the surface-to-blood-volume ratio and to be independent of shear rate. Hemolysis was also measured and was found to vary linearly with shear rate and to increase with increasing surface-to-blood volume ratio. Thus, at low shear rates and high specific surface conditions, the degree of hemolysis was found to be minimal while particulate-matter formation was high. The results suggest that the safety of extracoporeal perfusion procedures cannot be inferred from hemolysis measurements alone. In one series of tests, a gas-blood interface was generated at a rate equivalent to the rate of surface renewal in conventional disc oxygenators. The gas-blood interface failed to contribute significantly to the damage indices, which suggests that the apparent superiority of membrane oxygenators may be a result of factors other than the absence of a blood-gas interface.

The thrombogenicity of radiation grafted polymers as measured by the vena cava ring test.

Radiation grafted hydrogels on silicone rubber with water contents ranging from 10--90% have been evaluated by the vena cava ring test. All hydrogel grafted materials were found to accumulate less thrombus than ungrafted silicone rubber, and graft level was found to have little effect on thrombus accumulation. Acrylamide grafts demonstrated superior thromboresistance by this test. Poly (2-hydroxyethyl methacrylate-ethyl methacrylate (HEMA/EMA) copolymers were examined along with pure HEMA and EMA grafts. At two weeks implantation, intermediate HEMA/EMA copolymers were found to have the least thrombus accumulation of any of the polymers in this series. EMA grafted rings were highly thrombogenic. The results from these studies indicate that the sensitivity of the vena cava ring test for detecting certain differences in polymer structure is poor. Also, the significance of the vena cava ring test for evaluating materials for use in contact with flowing human blood is questionable, based upon comparisons of results for similar materials tested in other blood compatibility evaluation systems.

Plasma glucose concentrations and the development of blood emboli and hemolysis.

Plasma glucose concentrations were measured in heparinized dog blood as the blood was sheared in polycarbonate coni-cylindrical test cells. The glucose depletion rate during the 4 hr tests increased with increasing shear rate (480 to 2100 sec-1) but was independent of the surface/volume ratio (7.0 and 12.7 cm-1). These results suggest that the glucose metabolism in red cells (the dominant consumer of blood glucose) increases more from shear-induced stretching or tearing of red cells that adhere to the wall than from cell-boundary collisions. (Changes in platelet and leukocyte glucose metabolism would not have been detected in our measurements of whole blood glucose concentrations.) The rate of glucose depletion was also independent of the extent and rate of hemolysis and the extent of microemboli formation (as measured by SFP). When insulin-induced hypoglycemic blood was used to fill the test cells, the resulting hemolysis and SFPs were not significantly different from hemolysis and SFPs produced in normoglycemic blood from the same donors. Thus red cells (which have negligible glycogen stores) do not depend significantly on plasma glucose to resist hemolysis, and platelets may rely on their substantial glycogen storage to form microemboli during hypoglycemia.