Mechanical Blood Trauma in Circulatory-Assist Devices
5. Damage to leukocytes
In addition to changes in RBC deformability, aggregation and fragility, functional alterations to other blood cells caused by mechanical trauma may be important in cardiovascular assist device applications. Kusserow et al. were among the first to report that the mechanical stress during flow in extracorporeal circuits in vitro resulted in significant damageto leukocytes. In human blood in vitro, Dewitz et al. showed that shear stresses of 0 to 2,000 Pa for 2 to 10 min at 37°C significantly reduced leukocyte count, altered morphology and adhesiveness, and disrupted functionality. Subsequent experiments demonstrated that the exposure of human leukocytes to shear stress (0 to 80 Pa) in a rotational viscometer led to a reduction of functionality; particularly chemotactic ability, which was significantly reduced at stress levels above 15 Pa and had over a threefold reduction at 45 Pa. Further, hexose monophosphate (HMP) shunt activity following particle ingestion was also significantly lower after exposure to shear stresses of 15 Pa or more for ten minutes. Polymorphonuclear (PMN) chemiluminescence was also reduced following whole blood stress trauma of 45 Pa, and after 15 Pa in leukocyte suspensions. Since the changing of the surface/volume ratio in the viscometer by more than a factor of three did not change the damage caused at the same shear stress levels, the authors concluded that the leukocyte trauma was due to shear stress alone and not due to surface interaction effects. Interestingly, the shear stress levels required for leukocyte damage were quite similar to those required for platelet functional alteration, and much lower than those required for RBC hemolysis for the same exposure times.