Extreme negative pressure does not cause erythrocyte damage in flowing blood

In extracorporeal circulation, negative pressure is thought to be a source of hemolysis. This study was designed to investigate the effects of extreme negative pressure on flowing blood. The study model was pipe flow. The hemolysis generated by negative pressure driven flow was compared with that generated by positive pressure driven flow of equal magnitude to control for the hemolytic effect of shear stress. A series of pressures (720, 600, 500, -500, -600, and -720 mm Hg; n = 8) was tested for pipe diameters of 0.04 and 0.16 cm, with a length-to-diameter ratio of 500. The pressure difference across the pipe (Delta P) was equal to the magnitude of the applied pressure. The hemolysis was quantified by the modified index of hemolysis (MIH). For both pipe diameters, MIH was found to not depend on the Delta P or the blood collection day (multiple regression analysis, p = 0.50 and p 0.63, respectively). There was no statistically significant difference between the MIH for equal Delta P generated by positive or negative pressure (p = 0.50) for both pipe diameters tested. MIH did depend upon the pipe diameter, with 0.04 cm having higher MIH at all pressures (p = 0.0003). Thus, negative pressure is not a significant hemolytic factor in flowing blood.