Lithotripter

Extracorporeal shock wave lithotripsy
Intervention
Some of the passed fragments of a 1-cm calcium oxalate stone that was smashed using lithotripsy.
ICD-9-CM	98.5
MeSH	D008096
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Extracorporeal shock wave lithotripsy (ESWL) is a non-invasive treatment of kidney stones (urinary calculosis) and biliary calculi (stones in the gallbladder or in the liver) using an acoustic pulse. It is also reported to be used for salivary stones and pancreatic stones.

Beginning in 1969 and funded by the German Ministry of Defense, Dornier began a study of the effects of shock waves on tissue.In 1972, on the basis of preliminary studies performed by Dornier Medical Systems, an agreement was reached with Egbert Schmiedt, director of the urologic clinic at the University of Munich. The development of the Dornier lithotripter progressed through several prototypes, ultimately culminating in February 1980 with the first treatment of a human by SWL.The production and distribution of the Dornier HM3 lithotripter began in late 1983, and SWL was approved by the U.S. Food and Drug Administration in 1984.

The lithotriptor attempts to break up the stone with minimal collateral damage by using an externally applied, focused, high-intensity acoustic pulse. The patient is usually sedated or anesthetized for the procedure in order to help them remain still and reduce possible discomfort. The patient lies down in the apparatus' bed, with the back supported by a water-filled coupling device placed at the level of kidneys. A fluoroscopic x-ray imaging system or an ultrasound imaging system is used to locate the stone and aim the treatment. The first generation lithotriptor known as the Dornier HM3 (Human Model 3), has a half ellipsoid-shaped piece that opens toward the patient. The acoustic pulse is generated at the ellipsoidal focal point that is furthest from the patient and the stone positioned at the opposite focal point receives the focused shock wave. The treatment usually starts at the equipment's lowest power level, with a long gap between pulses, in order to accustom the patient to the sensation. The length of gap between pulses is also controlled to allow cavitation bubbles to disperse minimizing tissue damage. Second and later generation machines use an acoustic lens to focus the shock wave. This functions much like an optical lens, focusing the shock wave at the desired loci. The frequency of pulses are currently left at a slow rate for more effective comminution of the stone and to minimize morbidity while the power levels are then gradually increased, so as to break up the stone. The final power level usually depends on the patient's pain threshold and the observed success of stone breakage. If the stone is positioned near a bone (usually a rib in the case of kidney stones), this treatment may be more uncomfortable because the shock waves can cause a mild resonance in the bone which can be felt by the patient. The sensation of the treatment is likened to an elastic band twanging off the skin. Alternatively, the patient may be sedated during the procedure. This allows the power levels to be brought up more quickly and a much higher pulse frequency, often up to 120 shocks per minute (although this rate of treatment has been largely abandoned due to an association with ancillary morbidity).