AUGUST 1992, VOL 56, NO 2

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Extracorporeal Shock Wave Lithotripsy THEELECTROMAGNETIC METHOD Louise Kenney, RN

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he basic concepts surrounding lithotripsy were discovered when aeronautical engineers noticed pitting on the hulls of supersonic jets after flight.' This led to the discovery that high-speed collisions with water droplets in the atmosphere produce shock waves. Researchers investigated the possibility of generating and controlling this energy source under clinical conditions, which led to the birth of extracorporeal shock wave lithotripsy (ESWL).? Shock wave lithotripsy was first used to successfully treat a patient with renal calculi in 1980.3Rapid acceptance and widespread use have made this form of therapy the treatment of choice for more than 80% of patients with renal calculi.4

generation lithotripters. Thus, the electromagnetic method is known as the dry method. As the shock waves strike substances of differing densities of matter or acoustic impedances, the tensile strength of those objects is put under great pressure. Repetitious shock waves sharply focused on the objects (eg, calculi) fragment them into particles that can be passed spontaneously through the urinary system. Two shock wave tubes, or heads, are installed on a specially designed bed (Fig 2) for separate treatment of left and right renal

Electromagnetic Method

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high-energy field of pressure (ie, shock wave) is created in water by a sudden release of energy in a confined space.5 The shock waves are propagated through a small, water-filled, silicone tube, which is held against the patient's flank with constant pressure.6The goal of this procedure is to target the shock waves at the area of the stone only and not to disperse waves to other areas of the urinary system. To limit shock wave effects to surrounding tissue, the water-filled, silicone tube is designed with an acoustical lens to focus the waves (Fig 1). The local coupling of the silicone head during the electromagnetic method eliminates the need for a water tank and consumable electrodes needed with first-

Louise Kenney, R N , MSM, is the director, Wisconsin Laser Center, St Francis Hospital, Milwaukee. At the time this article was written, she was a staff nurse, Universal Medical ScannerslLinc [email protected] Imaging, Farmingmn, Conn. She earned her bachelor of science degree in nursing f r o m Alverno College, Milwaukee, and her master of science degree in management from Cardinal Stritch College, Milwaukee. 251

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hard ceramic suppot? elecm'cal coil insulatingfoil

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metallic membrane acoustical lens waterpath

Fig I . The principle of an electromagnetic shock wave generator: (from left) high voltage capacitator discharges a voltage pulse that acts against a slab coil to produce a current. The current then is focused through an acoustical lens.

Fig 2. Shock wave tubes on the specially designed bed allow separate treatment of left and right renal and/or urethral stones. 252

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and/or urethral stones. Clinical investigations report little evidence of tissue damage during stone fragmentation by electromagnetic ESWL.’ Electromagnetic ESWL units are smaller, require less anesthesia or analgesia for the patient, and produce less-powerful shock waves than first-generation lithotripters.8 The radiologic changes of calculi treated with the electromagnetic unit are subtle, and stone disintegration with minimal tissue damage is optimal when treating stone disease.g There essentially are three types of shock waves: high-pressure waves with large focal areas and high energy results, high-pressure waves with small focal areas and low energy results, and low-pressure waves with medium focal areas and medium energy results. Patients have been treated successfully with low-pressure waves. This is important because highpressure shock waves with large focal areas may result in extensive renal tissue damage if the waves cannot be targeted entirely to the stone. Also, high-pressure waves with small focal areas may not generate adequate energy to break up some stones.I0 One study reveals that more than 100,000 patients have been treated successfully with electromagnetic units.“ Animal studies reveal renal injury secondary to shock wave energy, including description of renal vessels, damage to tubular epithelium, and eventual formation of focal and segmented interstitial fibrosis when higher than clinically recommended levels of energy are used.I2In adults, immediate changes in renal morphology after lithotripsy, such as subcapsular bleeding, peritoneal fluid collections, and loss of corticomedullary differentiation, have been described in as many as 63% of patients.13 These studies identify a correlation between the amount of energy used and the extent of renal tissue damage. The long-term effects of ESWL in children have not been documented. Researchers believe that lower amounts of energy, as used in electromagnetic units, may minimize the degree of renal damage in children, whose body tissue impedance to shock waves is much less than in

adults. Pediatric patients account for 2% to 3% of all renal calculi patients.I4 With electromagnetic ESWL, they can be treated on an outpatient basis, and because this low-energy procedure does not cause severe pain, IV conscious sedation can be used.

Stone Localization

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enal and ureteral stones are located with biplanar x-ray units. An anteroposterior x-ray tube is mounted vertically above the specially designed bed. A second x-ray tube is placed in an oblique, caudocranial, 38-degree angle (Figs 3 and 4). Both x-ray units may be used for fluoroscopy, and plain films for kidney, ureter, and bladder (KUB) x-rays or spot kidney x-rays can be obtained with the anteroposterior x-ray unit. Focusing the shock waves requires precise localization of the target within the specified effective range. The effective shock wave range is approximately 8 cm x 4 mm, and it forms a cigar-shaped area at the center of the radiology monitor. Electronic enhancement imaging helps the urologist localize the stone by stopping the image and allowing the urologist to adjust the brightness and contrast of the monitor. The urologist focuses the waves by placing the center of the effective range and the intersection of the x-ray beams at the same point (Fig 5). The x-ray beam axes are superimposed on the monitors in the form of cross hairs (Fig 6). The patient’s weight and its distribution, congenital anomalies, intestinal gas, bony processes, and the stone composition may affect the urologist’s ability to view the stone. The nurse may need to use various positioning techniques and devices (eg, wedge, oblique pillow, blankets, elbow and knee protectors) to enhance the urologist’s view of the stone on the monitors. The nurse may need to place the patient prone and reverse him or her on the bed. The electromagnetic lithotripter is designed as a flat-top surface that is excellent for radiologic functions. The surface allows urologists to treat patients of all body sizes and shapes as well as patients with physical deformities who normal253

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Fig 4 . Stone targeting and patient positioning. The anteroposterior x-ray tube allows the operator to move the tabletop longitudinally (ie, from head to foot) along the x-axis and to establish a transverse y-axis coordinate (ie, from left to right). The oblique, caudocranial x-ray tube allows the operator to adjust the table transversely and to establish a z-axis (ie, height and head-to foot imaging). (Draning courtesy of Siemen’s Coip, Iseiin, N J )

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Extracorporeal shock wave lithotripsy. The electromagnetic method.

AUGUST 1992, VOL 56, NO 2 AORN JOURNAL Extracorporeal Shock Wave Lithotripsy THEELECTROMAGNETIC METHOD Louise Kenney, RN T he basic concepts surro...
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