Effect of macroscopic air bubbles on cell lysis by shock wave lithotripsy in vitro

In studies of cells or stones in vitro, the material to be exposed to shock waves (SWs) is commonly contained in plastic vials. It is difficult to remove all air bubbles from such vials. Because SWs reflect at an air-fluid interface, and because existing gas bubbles can serve as nuclei for cavitation events, we sought to determine in our system whether the inclusion of small, visible bubbles in the specimen vial has an effect on SW-induced cell lysis. We found that even small bubbles led to increased lysis of red blood cells (1- to 3-mm diameter bubbles, 9.8+/-0.5% lysis, n = 7; no bubbles, 4.4+/-0.8%, n = 4), and that the degree of lysis increased with bubble size. Damage could not be reduced by centrifuging the cells to the opposite end of the vial, away from the bubble. B-scan ultrasound imaging of blood in polypropylene pipette bulbs showed that, with each SW, bubbles were recruited from the air interface, mixing throughout the fluid volume, and these appeared to serve as nuclei for increased echogenicity during impact by subsequent SWs; thus, bubble effects in vials could involve the proliferation of cavitation nuclei from existing bubbles. Whereas injury to red blood cells was greatly increased by the presence of bubbles in vials, lytic injury to cultured epithelial cells (LLC-PK1, which have a more complex cytoarchitecture than red blood cells) was not increased by the presence of small air bubbles. This suggests different susceptibility to SW damage for different types of cells. Thus, the presence of even a small air bubble can increase SW-induced cell damage, perhaps by increasing the number of cavitation nuclei throughout the vial, but this effect is variable with cell type.