Stone fragmentation during shock wave lithotripsy is improved by slowing the shock wave rate: studies with a new animal model

Abstract

Purpose: The current trend toward ungated shock wave lithotripsy means that more patients are being treated with shock waves delivered at a rapid rate (120 shock waves per minute or greater). However, no benefit of an increased shock wave rate has been shown and in vitro studies indicate that slowing the shock wave rate actually improves stone fragmentation. We tested the effect of the shock wave rate on stone comminution in a new animal model.

Materials and methods: Gypsum model stones were inserted via upper pole percutaneous access into the lower pole calix of the kidneys of female pigs weighing approximately 100 pounds. Shock wave lithotripsy was performed (400 shock waves uninterrupted at 20 kV. and 30 or 120 shock waves per minute) 2 hours later using an unmodified HM3 lithotriptor (Dornier Medical Systems, Marietta, Georgia). After en bloc excision of the urinary tract stone fragments were collected and sieved through 2 mm. mesh. The particles were weighed and surface area was determined.

Results: Stones treated at 30 shock waves per minute broke more completely than stones treated at 120 shock waves per minute. The percent of fragments greater than 2 mm. was significantly higher for stones treated at the fast rate of 120 versus the slow rate of 30 shock waves per minute (mean +/- SEM 81% +/- 14% versus 45% +/- 12%, p <0.005). When stone fragmentation was expressed as the percent increase in fragment surface area, significantly greater fragmentation occurred at the slower than at the more rapid rate (327% +/- 63% versus 135% +/- 136%, p <0.02).

Conclusions: Slowing the rate of shock wave administration during shock wave lithotripsy significantly improves the efficiency of stone fragmentation in vivo.