Extracorporeal shock wave lithotripsy

Published on 07/02/2015 by admin

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Last modified 07/02/2015

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Extracorporeal shock wave lithotripsy

Jonathan A. Faust, MD

Urolithiasis is a common condition with a lifetime prevalence of 12% in the United States. It is more common in men than women and most often presents in the third to fourth decade of life. Most urinary stones can be passed spontaneously; however, 10% to 30% require urologic intervention. Since the introduction of the first lithotripter in 1980, extracorporeal shock wave lithotripsy (ESWL) (Figure 167-1) has gradually replaced open and percutaneous surgical approaches as the treatment of choice for most urinary stones requiring intervention in the kidney or upper ureter.

Figure 167-1 Extracorporeal shock wave lithotripsy. (Netter illustration from www.netterimages.com. © Elsevier Inc. All rights reserved.)

Technical aspects

All lithotripters consist of (1) an energy source that creates a shock wave, (2) a system to focus the energy of the shock wave, (3) a coupling medium that facilitates transfer of the shock-wave energy to the patient, and (4) an imaging system to provide localization of the stone and to guide energy delivery to the stone. First-generation lithotripters, such as the Dornier HM-3, require patients to be immersed in a water bath as the coupling medium and use a sparkplug to generate an 18-kV to 24-kV discharge. This spark causes water vaporization and a cavitation bubble that rapidly expands and then contracts, leading to the creation of a shock wave. The origin of the wave is termed the F₁ focal point. A semiellipsoid reflector focuses the energy wave to converge at the stone (located at the F₂ focal point) under the guidance of fluoroscopy. The shock wave travels through the water bath and the patient with little attenuation because of the similar acoustic impedance of water and body tissues. The urinary stone presents a change in impedance, resulting in the release of compressive energy and a mechanical stress on the stone. Repeated shocks (1000 or more) lead to disintegration of the stone, and stone fragments are excreted in the urine.

Some newer lithotripters use piezoelectric crystals or electromagnetic shock generators. These devices are more durable and require less frequent maintenance. Piezoelectric lithotripters have the advantage of having a wider aperture, resulting in lower energy density at the skin and, therefore, less patient discomfort. Various methods of focusing the shock wave are used. Most no longer require patient immersion in a water bath because the shock is generated within a water-filled compartment and transferred through a membrane to the patient using a coupling gel. In addition to fluoroscopy, ultrasound is used in some newer lithotripters for stone localization. Some lithotripters can synchronize shock delivery with respiration or the cardiac cycle, although this may limit the maximal rate of shock delivery.

Physiology of water immersion

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