Implications of cavitation phenomena for shot intervals in extracorporeal shock wave lithotripsy.

Objective: To study the influence of shot intervals and the quality of the coupling medium (degassed water) in extracorporeal shock wave lithotripsy (ESWL).

Materials And Methods: Acoustic scattering and damping (and thereby also the focal size) were modelled with respect to the conditions of the coupling medium, using continuous wave high intensity ultrasound. The influence of the shot interval on rise time and attained shock wave pressure were quantified with a modified ESWL apparatus and a specially designed hydrophone.

Results: In tap water, which has a high content of dissolved gases, the focus produced was bulb-shaped, with a diameter of approximately 4 cm, whereas degassed water gave a small focus that produced a water jet only a few millimetres in diameter. The efficacy of the acoustic energy decreased with shot intervals shorter than 1.2s. Longer pauses resulted only in prolonged treatment times.

Conclusion: ESWL becomes inefficient if the intervals between pulses are too short, particularly when ECG R-wave triggered pulses are used on patients with tachycardia. The gas content of the coupling medium is also important for successful ESWL. The efficacy of the system used for degassing the water should be checked at regular intervals.