###### Abstract  
The use of intra-operative blood salvage, dialysis, and artificial organs are associated with the application of non-physiological mechanical stress on red blood cells (RBCs). To explore the effect of these procedures on red cell deformability, we determined it before and after the mechanical stress application both in an in vitro system and following a blood-saving procedure. RBC from eight healthy donors and fifteen packed RBC units were subjected to mechanical stress. RBCs from five patients undergoing orthopedic surgery were also collected. We measured the percent of undeformable cells (%UDFC) in the red cell samples using our cell flow properties image analyzer, which provides the distribution of RBC deformability in a large cell population. Mechanical stress systematically reduced the cell deformability and increased the %UDFC, while simultaneously causing hemolysis of rigid, undeformable RBCs. Ultimately, the overall result depended on the initial level of the undeformable cells; the stress-induced change in the proportion of rigid cells (\(\Delta\)%UDFC) increased (\(\Delta\)%UDFC > 0) when its initial value was low, and decreased (\(\Delta\)%UDFC < 0) when its initial value was high. This suggests that the final impact of mechanical stress on the percent of rigid cells in the RBC population is primarily determined by their initial concentration in the sample. red blood cells; deformability; shear stress; mechanical stress; blood salvage +
Footnote †: journal: Medical Stress  
of patients undergoing perioperative auto-transfusion [18; 25; 26; 27]. However, since during the RBC collection and purification, they are exposed to mechanical stress, blood salvage is associated with a reduction in the lifespan of erythrocytes [28; 29; 30]. Although the effect of mechanical action is beyond doubt, the detailed mechanism of the influence of blood salvage procedure on the functionality of RBCs remains under discussion [29]. Depending on the level of shear stress and its duration, the exposure can lead to cell hemolysis. [31; 32; 33; 34; 35; 36; 37]. However, even before cell destruction, there is a change in the properties of the cytosol, cytoskeleton, and cell membrane [12]. The RBC undergoes structural changes in response to the supraphysiological mechanical stress [38; 36; 39; 40; 16]. Prolonged exposure to supraphysiological shear stress may promote the spread of abnormal erythrocyte morphology [22], phosphatidylinositolserine externalization [41], and increased vesiculation [41; 42]. Ultimately, these changes lead to the destruction of RBCs [31; 32; 33; 34; 35; 36; 37]. The effect of mechanical stress on erythrocytes was analyzed using a numerical approach [35], in vitro and ex vivo experiments [32; 33; 36; 37; 43; 44]. [PERSON] et al. [41] demonstrated that the flow of stored RBCs through a heart-lung machine provoked alterations in RBC structure and functionality. These include elevation in osmotic fragility, alteration in cell aggregability and deformability, and acceleration of microvesicle formation. The degree and kinetics of these changes depend on the cell storage duration in the blood bank. The article's authors [41] conclude that their data will help develop, improve, and control the quality of the extracorporeal circuit approach. [PERSON] et al. [14] used a shear (counter-rotation) system coupled to a microscope to directly monitor RBC fragmentation and hemolysis, which were recognized after exposure to shear stress of 288 Pa for 40 s. Later, the same research group [12] visualized the presence of abnormal RBCs (with damaged morphology) under prolonged (more than 100 s) exposure to shear stress of 60 Pa that induced asymmetric cell elongation. In regards to physiological conditions, [PERSON] et al. [45] demonstrated (using model simulations of the flow of erythrocytes through the human spleen's venous fissures) that erythrocytes' deformation can contribute to cell membrane vesiculation and subsequent decrease in its surface-area-to-volume ratio.