Mechanical Blood Traumatization by Tubing and Throttles in In Vitro Pump Tests: Experimental Results and Implications for Hemolysis Theory

Abstract: Blood has become essential as a test fluid to evaluate hemolysis and biocompatibility of blood pumps in vitro. The blood is usually pumped from a blood bag into a circuit against elevated pressure. A throttle or a length of tubing is used to produce the pressure head. Blood damage caused by the shear stress in these pressure-reducing devices should be minimal. It is not known whether the high but short-lasting shear stress in a throttle is more or less damaging to the blood than the low but long-lasting stress in tubing. In this study, throttles (width 11 mm, minimal height 0.9 mm, length 30 mm; shear stress = 136 N/m² lasting for 3.23 ms); and tubing (inner diameter 9.5 mm, length 4.5 m, shear stress = 4.5 N/m² lasting for 3.5 s) were compared at a flow of 5 L/min and a pressure drop of 150 mm Hg. Experiments (n = 10) with bovine blood were performed in two parallel setups using Bio-Medicus pumps BP80. Free hemoglobin in plasma (fHb) and thromboxane B₂ (TXB₂) were measured. After 6 h, the fHb increase was 31.9 ± 19.1 mg% for the throttle setup and 32.3 ± 16.2 for the tubing setup. The TXB₂ release was 296 ± 70 and 305 ± 54 pg/0.1 ml respectively after 4 h. In summary, no significant differences between the two setups for either fHb or TXB₂ could be detected. So the use of a throttle, which requires far less priming volume and a smaller blood-contacting surface while also offering a wider range of adjustment, seems preferable. Furthermore, in contrast to some other in vitro studies, these results strongly indicate that blood is quite insensitive to high shear stress exposure in the millisecond range even if repetitively applied.