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Common Urological Problems

Urinary Stone Disease Intervention

  • - General Urology - Common Urological Problems - Urinary Stone Disease
  • Jul 25, 2010
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Tags: | citric acid | computed tomography | dissolution agents | fluoroscopic imaging |

D. EXTRACORPOREAL SHOCK WAVE LITHOTRIPSY
Extracorporeal shock wave lithotripsy has revolutionized the treatment of urinary stones.  The concept of using shock waves to fragment stones was noted in the 1950s in Russia.  However,  it was during the investigation of pitting on supersonic aircraft that Dornier, a German aircraft corporation, rediscovered that shock waves originating from passing debris in the atmosphere can crack something that is hard. It was the ingenious application of a model developed in hopes of understanding such shock waves that ESWL emerged.

The first clinical application with successful fragmentation of renal calculi was in 1980.  The HM–1 (Human Model–1)  lithotriptor underwent modifications in 1982 leading to the HM–2 and, finally, to the widespread application of the HM–3 in 1983 (Figure 16–16). Since then, thousands of lithotriptors have been put into use around the world, with millions of patients successfully treated.

All require an energy source to create the shock wave, a coupling mechanism to transfer the energy from outside to inside the body,  and either fluoroscopic or ultrasonic modes,  or both,  to identify and position the calculi at a focus of converging shock waves. They differ in generated pain and anesthetic or anesthesiologist requirements, consumable components, size, mobility, cost, and durability.

Urinary Stone Disease

Urinary Stone Disease

Focal peak pressures (400–1500 bar), focal dimensions (6 x 28 mm to 50 x 15 mm), modular design, utilization to help increase mobility of frozen joints,  varied distances (12–17 cm) between focus 1 (the shock wave source) and focus 2 (the target),  and purchase price differentiate the various machines available today.

1. Shock wave physics-
In contrast to the familiar ultrasonic wave with sinusoidal characteristics and longitudinal mechanical properties,  acoustic shock waves are unharmonic and have nonlinear pressure characteristics.

There is a steep rise in pressure amplitude that results in compressive forces (Figure 16–17). There are 2 basic types of shock wave sources:  supersonic and finite amplitude emitters.

Supersonic emitters release energy in a confined space, thereby producing an expanding plasma and an acoustic shock wave. Such shock waves occur in nature-the familiar thunderstorm with lightning (an electrical discharge) followed by thunder (an acoustic sonic boom) is an analogous situation.  Under controlled conditions,  such an acoustic shock wave can successfully fragment calculi. The initial compression wave travels faster than the speed of sound in water and rapidly slows down to that speed. The traveling pressure wave is reduced in a nonlinear fashion.

Medical applications have focused such waves to concentrate energy on a calculus (Figure 16–18).

Finite amplitude emitters, in contrast to point source energy systems, create pulsed acoustic shock waves by displacing a surface activated by electrical discharge. There are 2 major types of finite amplitude emitters:  piezoceramic and electromagnetic. The piezoceramic variety results in a shock wave after an electrical discharge causes the ceramic component to elongate in such a manner that the surface is displaced and an acoustic pulse is generated. Thousands of such components placed on the concave side of a spheric surface directed toward a focus result in high stress, strain, and cavitation pressures (Figure 16–19). Electromagnetic systems are similar in concept to a stereo speaker system.

An electrical discharge to a slab, adjacent to an insulating foil, creates an electric current that repulses a metal membrane, displacing it and generating an acoustic pulse into an adjacent medium.  These waves need to be focused toward the offending stone.

All shock waves, despite their source, are capable of fragmenting stones when focused.  Fragmentation is achieved by erosion and shattering (Figure 16–20).  Cavitational forces result in erosion at the entry and exit sites of the shock wave. Shattering results from energy absorption with stress, strain, and shear forces. Surrounding biologic tissues are resilient because they are not brittle nor are the shock waves focused on them.

image Figure 16–16.  Diagrammatic representation of a Dornier HM–3 lithotriptor.

2. Preoperative evaluation-
Physical examination should be as thorough as in preparation for any other surgical procedure.  Vital signs including blood pressure should be noted. Body habitus including any gross skeletal abnormalities, contractures, or excessive weight (>300 lb) may severely limit or preclude ESWL.

image Figure 16–17.  Shock wave. Vertical axis represents pressure and horizontal axis represents time.

Borderline individuals require simulation before treatment. Pregnant women and patients with large abdominal aortic aneurysms or uncorrectable bleeding disorders should not be treated with ESWL. Individuals with cardiac pacemakers should be thoroughly evaluated by a cardiologist. If ESWL is contemplated,  a cardiologist with thorough knowledge and with the ability to override the pacemaker should be present in the lithotripsy suite.

image Figure 16–18.  A: Supersonic shock wave emission from a spark gap electrode. B: Reflecting the shock wave from focus 1 to focus 2 allows for stone fragmentation.

image Figure 16–19. Piezoceramic finite amplitude emitter. Ceramic components are placed on the concave surface of a sphere and each component is directed to an identified focus.

image Figure 16–20.  Incoming shock waves result in fragmentation from erosion and shattering.




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