STONE DISEASE

Kidney damage and renal functional changes are minimized by waveform control that suppresses cavitation in shock wave lithotripsy
Evan AP, Willis LR, McAteer JA, Bailey MR, Connors BA, Shao Y, Lingeman JE, Williams Jr JC, Fineberg NS, Crum LA
Department of Anatomy and Cell Biology, Indiana University School of Medicine, Indianapolis, Indiana, USA
J Urol. 2002; 168:1556-62

• Purpose: In studies to understand better the role of cavitation in kidney trauma associated with shock wave lithotripsy we assessed structural and functional markers of kidney injury when animals were exposed to modified shock waves (pressure release reflector shock pulses) that suppress cavitation. Experiments were also performed in isolated red blood cells, an in vitro test system that is a sensitive indicator of cavitation mediated shock wave damage.
• Materials and Methods: We treated 6-week-old anesthetized pigs with shock wave lithotripsy using an unmodified HM3 lithotriptor (Dornier Medical Systems, Marietta, Georgia) fitted with its standard brass ellipsoidal reflector (rigid reflector) or with a pressure release reflector insert. The pressure release reflector transposes the compressive and tensile phases of the lithotriptor shock pulse without otherwise altering the positive pressure or negative pressure components of the shock wave. Thus, with the pressure release reflector the amplitude of the incident shock wave is not changed but cavitation in the acoustic field is stifled. The lower pole of the right kidney was treated with 2,000 shocks at 24 kV. Glomerular filtration rate, renal plasma flow and tubular extraction of para-aminohippurate were measured in the 2 kidneys 1 hour before and 1 and 4 hours after shock wave lithotripsy, followed by the removal of each kidney for morphological analysis. In vitro studies assessed shock wave induced lysis to red blood cells in response to rigid or pressure release reflector shock pulses.
• Results: Sham shock wave lithotripsy had no significant effect on kidney morphology, renal hemodynamics or para-aminohippurate extraction. Shock waves administered with the standard rigid reflector induced a characteristic morphological lesion and functional changes that included bilateral reduction in renal plasma flow, and unilateral reduction in the glomerular filtration rate and para-aminohippurate extraction. When the pressure release reflector was used, the morphological lesion was limited to hemorrhage of vasa recta vessels near the tips of renal papillae and the only change in kidney function was a decrease in the glomerular filtration rate at the 1 and 4-hour periods in shock wave treated kidneys. Red blood cell lysis in vitro was significantly lower with the pressure release reflector than with the rigid reflector.
• Conclusions: These data demonstrate that shock wave lithotripsy damage to the kidney is reduced when cavitation is suppressed. This finding supports the idea that cavitation has a prominent role in shock wave lithotripsy trauma.
  
  
• Editorial Comment
  The Indiana group continues to demystify the phenomena of shock wave lithotripsy and to systematically elucidate the mechanism of shock wave-induced stone fragmentation and tissue injury. Although cavitation has been suggested as a mechanism of stone comminution in shock wave lithotripsy, it has also been implicated in vascular and tissue damage. By placing a pressure release reflector insert into the standard brass ellipsoid reflector of the unmodified Dornier HM3, these investigators were able to “uncouple” shock waves from their cavitation effect without otherwise altering the positive or negative pressure components of the shock wave. Using a porcine model, they compared shock wave treatment using a standard rigid reflector with shock wave treatment using a pressure release reflector or sham control lithotripsy. They found that shock waves associated with the pressure release reflector resulted in only minor hemorrhage near the renal papillae and a mild decrease in GFR post-treatment in contrast to shock waves administered using the standard rigid reflector which demonstrated characteristic intraparenchymal hemorrhage and subcapsular hematomas as well as more pronounced decreases in renal plasma flow and GFR. Not only do these findings suggest a role for cavitation in shock wave-induced renal tissue injury, but they also imply that alteration of the shock waveform could potentially be used to maximize stone comminution effects or minimize tissue trauma.

Dr. Margaret S. Pearle
Associate Professor of Urology
University of Texas Southwestern Med Ctr
Dallas, Texas, USA