Hemolytic potential of hydrodynamic cavitation.

The purpose of this study was to determine the hemolytic potentials of discrete bubble cavitation and attached cavitation. To generate controlled cavitation events, a venturigeometry hydrodynamic device, called a Cavitation Susceptibility Meter (CSM), was constructed. A comparison between the hemolytic potential of discrete bubble cavitation and attached cavitation was investigated with a single-pass flow apparatus and a recirculating flow apparatus, both utilizing the CSM. An analytical model, based on spherical bubble dynamics, was developed for predicting the hemolysis caused by discrete bubble cavitation. Experimentally, discrete bubble cavitation did not correlate with a measurable increase in plasma-free hemoglobin (PFHb), as predicted by the analytical model. However, attached cavitation did result in significant PFHb generation. The rate of PFHb generation scaled inversely with the Cavitation number at a constant flow rate, suggesting that the size of the attached cavity was the dominant hemolytic factor.

Determination of the in vivo cavitation nuclei characteristics of blood.

Cavitation has been documented in the in vitro testing of blood-handling devices. To predict whether cavitation will occur clinically, the nuclei content of blood and the threshold pressure for activation of the in situ nuclei must be characterized. A single-pass flow apparatus is described for determining the nuclei characteristics of blood. The flow apparatus consists of a syringe pump and a venturi-geometry hydrodynamic device, called a cavitation susceptibility meter (CSM). Blood is accelerated through the throat of the CSM, thus exposing the nuclei in the blood to a well-defined pressure profile. The apparatus was used in an ex vivo sheep model for the determination of the in vivo nuclei characteristics of blood. The active nuclei concentration of in vivo blood was measured to be at most 2.7 nuclei per liter of plasma at a minimum throat pressure of -1610 mm Hg gauge (i.e., tension of 900 mm Hg). At this pressure, bubble stability theory predicts the active nuclei to have a radius on the order of 0.3 microm. Based on these results, in vitro studies to determine the cavitation potential of blood-handling devices must utilize test fluids that contain a minimum nuclei size distribution and concentration. It cannot be assumed that in vivo blood is nuclei rich, such that it will cavitate at or near vapor pressure.

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 (deltaP) 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 deltaP 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 deltaP 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.