Subspecialty Management |
|Chapter 53:||Anesthesia and the Renal and Genitourinary Systems|
Extracorporeal Shock Wave Lithotripsy
Extracorporeal shock wave lithotripsy (ESWL) was first conducted in Munich, Germany in 1980, and in 1984 the lithotripter was introduced into the United States. Since then, ESWL has become the treatment of choice for disintegration of urinary stones in the kidney and upper ureter. The first clinical model of the lithotripter introduced into common practice (Dornier HM-3) uses a water bath in a steel tub and a metal gantry chair to support the patient suspended in a sitting position. This, the first-generation lithotripter, is still commonly used and presents complex challenges of immersion physiology and monitoring difficulties. Second- and third-generation lithotripters (Siemens, Lithostar, Wolf Piezolith, Dornier HM-4, MFL 5000, etc.) have since been developed and introduced into clinical practice. They have evolved mainly in the direction of eliminating the water bath and producing a pain-free lithotripter. However, all lithotripters share similar technologic principles in having three main components: (1) an energy source, most commonly a spark plug (alternatively an electromagnetic membrane or piezoelectric elements may be used in some machines); (2) a system to focus the shock wave, such as an ellipsoid or reflecting mirrors; and (3) a system to visualize and localize the stone in focus, namely fluoroscopy or ultrasound. 146
The Dornier HM-3 lithotripter, the gold standard, uses a water bath for treatment and x-ray equipment to localize the stone. These are contained in the all too familiar steel tub. The patient is strapped in a gantry chain in a semisitting position, with support under the shoulders and hips and with the flank exposed. The chair is hoisted to the ceiling, travels on ceiling rails, and is then slowly lowered into the tub. The patient is immersed in the water up to the clavicles. An electrode (or spark plug) is placed in the water at the base of the tub in an ellipse and is connected to a generator that supplies 16 to 24 kV of electricity. The electrical energy creates a spark across the gap in the electrode with generation of a loud noise, intense heat, and explosive vaporization of water. The sudden expansion of air bubbles thus created sets up a pressure wave (shock wave) with distinct mechanical and acoustic properties. The shock wave is focused by the ellipse to a focal point called the Fz focus (the tip of electrode is the first focus). The shock wave travels through water and body tissues without significant localized dissipation of energy because the acoustic impedance of the two media is similar. However, when the shock wave arrives at the entry surface of the stone, it encounters a sudden change in impedance, causing it to release compressive energy of the magnitude of several atmospheres. Similarly, when the wave exits on the opposite surface of the stone, an interface is again encountered, and shock wave energy is released in a blast. Repeat applications cause the stone to disintegrate. Shock wave energy is most concentrated in the Fz focal zone and rapidly decreases beyond it. 147, 148, 149 These physical properties of the shock wave must be understood to prevent injury to tissue or to any prostheses and to ensure that the shock wave passes unimpeded to its focal point for most effective lithotripsy results. The following discussion elucidates this further.
Biomechanical Effects of Shock Wave
For shock waves to be most effective, the stone should remain in the Fz focus during treatment. 149 Pressure energy measurements show an exponential decrease beyond this small focal zone. The kidneys and hence the kidney stone follow the up-and-down movements of the diaphragm during respiration. It is likely, therefore, that the stone will move in and out of focus during respiration. This issue has been studied, and innovative ventilatory techniques have been tried to minimize stone movement during ventilation. 150, 151, 152, 153, 154, 155 High-frequency jet ventilation has been shown to decrease stone movement and has been claimed to increase the efficacy of the treatment. 151 High-frequency conventional ventilation using fast respiratory rates and small tidal volumes has also been effective in decreasing stone excursions during respiration. 155 However, other data 156 and wide clinical experience with success rates of lithotripsy do not support or justify the routine use of these techniques, which add their own complexity and complications to the procedure. 153 Furthermore, regional anesthesia and analgesia-sedation are frequently used for lithotripsy, in which case spontaneous ventilation is the only option. Studies in sedated patients with intercostal block and local infiltration anesthesia have documented that the stone movement with spontaneous respiration is mainly restricted to the Fz focal zone during ESWL. 157 Therefore, conventional ventilation during general anesthesia or spontaneous respiration during regional anesthesia is acceptable for lithotripsy. Occasionally in an awake patient, one may have to carefully titrate sedation or, if the patient is under general anesthesia, may need to manipulate respiratory parameters to decrease abnormally large stone movement.
For effective stone disintegration, shock waves should reach the stone with very little loss of energy. Therefore, the flank area should be kept free of any medium that would provide an interface for dissipation of shock wave energy. For example, nephrostomy dressings should be removed and the nephrostomy catheter should be taped clear of the blast path. If epidural anesthesia is used, great care should be taken that the catheter and the gauze are taped well clear of the blast path in the flank on the side being treated. Pandit et al 158, 159 noticed an unusually high rate of failure of lithotripsy in their patients receiving epidural fentanyl analgesia. They discovered that the foam tape placed to secure the epidural catheter was frequently in the blast path of the shock waves and could absorb up to 80 percent of the shock wave energy.
Except when piezoelectric lithotripters or lithotripters with very low shock-wave energy are used, the procedure is painful and requires some form of anesthesia. Shock waves produce sharp stinging pain at the entry site in the flank combined with a sensation of deep visceral pressure discomfort.
Although shock waves pass through most tissues relatively unimpeded, they do cause tissue injury, the extent of which depends on the tissue exposed and the shock wave energy at the tissue level. For example, skin bruising and flank ecchymoses are not uncommon at the entry site. Painful hematoma in the flank muscles may occur. Hematuria is almost always present at the end of the procedure and results from shock wave-induced endothelial injury to the kidney and ureter. 147, 148 Adequate hydration is necessary to prevent clot retention. A decrease in postoperative hematocrit should arouse suspicion of a large perinephric hematoma. Punctate hemorrhages have been observed in the stomach and bowel; and this injury might be responsible for abdominal distention, nausea, and vomiting in some patients.
Lung tissue is especially susceptible to injury by shock waves. Air trapped in alveoli presents the classic water (tissue)air interface to the shock wave, causing dissipation of energy with alveolar rupture and hemoptysis. Massive hemoptysis and death from pulmonary damage have been reported in laboratory animals after a single exposure of the thorax to shock wave. 160 Shock waveinduced hemoptysis in a child and a pulmonary contusion with life-threatening hypoxemia in an adult have been reported. 161, 162 Children are more likely to suffer pulmonary damage from shock waves because of the shorter distance of the lung bases from kidneys as compared with adults. It is recommended that a Styrofoam sheet or Styrofoam board be placed under the back in children to shield the lung bases from shock waves during ESWL. 163
Shock waveinduced cardiac arrhythmias occur in 10 to 14 percent of patients undergoing lithotripsy despite the fact that shock waves are synchronized with the patients ECG and are delivered in the refractory period of the cardiac cycle. 164 These arrhythmias are believed to be due to mechanical stresses on the conduction system exerted by the shock waves. Even though the shock waves are produced by electrical energy, the intricate grounding system of the lithotripter is such that the current density reaching the myocardium is minuscule compared with other sources (e.g., cardiac pacemakers). Hence, current-induced arrhythmias are unlikely. As mentioned previously, arrhythmias do occur frequently and are due to mechanical effects of shock waves. Atrial and ventricular premature complexes, atrial fibrillation, and supraventricular and ventricular tachycardia have been reported. 164, 165, 166 ECG artifacts are also common. Artifacts and arrhythmias usually disappear once the lithotripsy is stopped. Occasionally, however, arrhythmias may persist, requiring treatment.
Physiologic Changes During Immersion Lithotripsy
Water immersion with the Dornier HM-3 lithotripter produces significant changes in the cardiovascular and respiratory systems (Table 53–9). Cardiovascular changes include an increase in central blood volume, with an increase in central venous and pulmonary artery pressures. 167, 168, 169, 170, 171, 172 Weber et al 167 observed that increases in central venous pressure and pulmonary artery pressure were directly correlated with the depth of immersion. On the other hand, the sitting position, together with general or epidural anesthesia, would tend to cause peripheral pooling and decreased venous return. A 1987 study noted a decrease in cardiac output and an increase in systemic vascular resistance during ESWL under general anesthesia. 172 Respiratory changes with immersion up to the clavicles are significant: functional residual capacity and vital capacity are reduced by 20 to 30 percent; pulmonary blood flow has been shown to increase 170, 171, 172, 173, 174 ; and tight abdominal straps and the hydrostatic pressure of water on the thorax impart a characteristic of shallow, rapid breathing pattern. 174 Ventilation/perfusion mismatch and hypoxemia are thus more likely. The renal effects of immersion include diuresis, natriuresis, and kaliuresis. A decrease in antidiuretic hormone and renal prostaglandins occurs. The temperature of the bath water can cause profound changes in the patients temperature. This heat transfer is further augmented by vasodilation produced by general or epidural anesthesia. Both hypothermia and hyperthermia have been reported. 175, 176
TABLE 53–9. Changes on Immersion During Lithotripsy
Anesthetic Choices for Lithotripsy
Anesthetic regimens successfully used for lithotripsy include general anesthesia, epidural anesthesia, spinal anesthesia, flank infiltration with or without intercostal blocks, and analgesia-sedation. 177, 178, 179, 180, 181, 182, 183, 184 General anesthesia offers the advantages of rapid onset and control of patient movement. Ventilatory parameters can be controlled to decrease stone movement with respiration. Extra-long circuit tubing and monitoring cables are required. Disadvantages include a likelihood of positional injury and the possible need to transport an anesthetized patient to other locations if adjunctive procedures become necessary.
Epidural anesthesia offers the advantage that the patient is awake and can help with transfer in and out of the gantry chair, thus reducing the likelihood of injury. With epidural anesthesia and use of loss of resistance to air for identifying the epidural space, only the smallest amount of air necessary should be injected. Air in the epidural space will provide an interface and cause dissipation of shock wave energy resulting in local tissue injury. Korbon et al 185 found a decrease in epidural compliance and pain on injection in repeat epidurals for subsequent lithotripsies in their patients. In animal experiments, they were able to show epidural tissue damage following injection of air and exposure to shock waves. 186 It is reassuring, however, that in the vast number of lithotripsies performed under epidural anesthesia worldwide, neurologic injury has not been a problem.
The main disadvantage of epidural anesthesia is its slow onset. Spinal anesthesia offers a reasonable alternative with its rapid onset. However, the incidence of hypotension (the patient is in sitting position for treatment) is higher. In one series, the incidence of hypotension with general, epidural, and spinal anesthesia was 13, 18, and 27 percent, respectively. 187 Local anesthetic infiltration of the flank with or without intercostal blocks provides adequate anesthesia when combined with intravenous sedation and avoids hypotension. 178 Intravenous analgesia-sedation in various combinations has been successfully used by many anesthetists. 180, 181, 182
The second- and third-generation lithotripters offer many advantages. First, there is no water bath; hence all the problems of immersion are avoided. Second, they tend to use multifunctional tables allowing other procedures such as cystoscopy and stent placement to be accomplished without moving the patient off the table. Third, the shock waves are focused, so that they cause less pain at the entry site. With the exception of the piezoelectric lithotripters (Wolf, EDAP, Diasonic) and the use of lowest energy levels with the other lithotripters, ESWL is not pain-free and intravenous analgesia-sedation is the mainstay of anesthesia with these newer lithotripters. Other incidental interventions such as cystoscopy, stone manipulation, or stent placement will alter anesthetic requirements. Many of these newer lithotripters have a much smaller focal zone for the shock waves. Hence, it is even more imperative with these to provide adequate analgesia and sedation so that the stone excursions with respirations are limited to the focal zone. Most analgesia-sedation combinations are adequate. Even patientcontrolled analgesia with alfentanil has been used. 188, 189
Pregnancy and untreated bleeding disorders and abdominally placed pacemakers are the only absolute contraindications to lithotripsy. Women of childbearing age must have a pregnancy test that is documented negative before lithotripsy. Standard tests of coagulation such as platelet count, prothrombin time, and partial thromboplastin time should be obtained. Other conditions that were labeled previously as absolute contraindications are no longer believed to be so provided that appropriate precautions are taken. These conditions include pacemakers, automatic implanted cardioverter defibrillators (AICD) abdominal aortic aneurysm, orthopedic prostheses, and obesity.
Patients with pacemakers can be treated safely if the pacemaker is pectorally placed and the following precautions are observed. 190, 191, 192 Pacemaker programmability should be established prior to the treatment, and a programmer should be available to switch the pacemaker to a nondemand mode should the shock waves interfere with pacemaker function. Alternating means of pacing should be available. Although most pacemakers located pectorally are at a safe distance from the blast path, some may be damaged. Weber et al 190 examined 43 different pacemakers and found that 3 were affected. Dual-chamber pacemakers tend to be more sensitive to interference. Treatment must be started at the lowest energy level and gradually increased while observing pacemaker function.
Manufacturers of AICDs and lithotripters generally consider an AICD a contraindication for lithotripsy. Patients with AICDs have been treated successfully with lithotripsy, however. 193 Transvenous AICDs are less of a concern than the older abdominally implanted defibrillators. AICD devices should be shut off immediately before lithotripsy and then reactivated immediately after the treatment.
Patients with small aortic aneurysms have been treated safely provided that the stone is not close to the aneurysm. Orthopedic prostheses such as hip prostheses and even Harrington rods are not a problem if they are not in the blast path, which is usually the case. Positioning of these patients may sometimes be problematic. Not only do extremely obese patients present with anesthetic challenges related to obesity, but focusing of the stone may be extremely difficult in the very obese, and it is not uncommon for the procedure to be abandoned in these patients because of inability to bring the kidney stone in the Fz focal zone. It is prudent, therefore, that focusing of the stone be attempted before administering any anesthetic in this high-risk population. With the newer lithotripters, some of these patients may have to be placed prone, a position that they may not be able to tolerate safely.
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