Potential mechanism for the effect of shock wave rate in shock wave lithotripsy

Abstract

Artificial stones break significantly better when shock waves (SWs) are delivered at 0.5 Hz than at 2 Hz, and patients treated at slower rates have improved stone-free rates. One possible explanation may be cavitation bubbles that might persist between SWs at high rate and distort subsequent SWs sufficiently to reduce their effectiveness at stone comminution. High-speed photography gives evidence that bubble numbers are greater at higher rates. B-mode ultrasound echo in the free field typically disappears between pulses administered at 0.5 Hz but persists at 2 Hz. Fiberoptic hydrophone measurements at 2 Hz showed, in the free field of an electrohydraulic lithotripter, that SW negative tail was truncated, and proximal to a stone, that SW waveform varied and was distorted such that often the positive pressure amplitude was reduced. Changes in waveform proximal to stone declined as the stone disintegrated and fell away. Thus, data support the persistence of cavitation bubbles at high SW rate, and consequent distortion of waveform. Additionally, debris from a stone appears to accentuate this effect. These measurements may help not only define an effective SW rate and shape, but may further improve our understanding of the comminution process. [Work Supported by NIH-DK43881, DK55674, and ONRIFO-N00014-04-1-4010.]