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

Because first-generation lithotripters require partial immersion of the patient in a water bath, the immersion typically results in a significant redistribution of peripheral venous blood toward the central compartment, causing increased central venous and pulmonary artery pressures. The degree of these changes is related to the level of water immersion, and these effects are opposed by anesthesia (general or neuraxial) and the sitting position. Water immersion—along with the straps used to secure the patient in some devices—can contribute to a rapid, shallow breathing pattern, and functional residual capacity can be decreased by 20% to 30%. These changes, along with increased pulmonary blood flow, can lead to ventilation-perfusion mismatch and hypoxemia.

Patient selection

ESWL has been used successfully to manage urinary stones in infants, children, and adults. Absolute and relative contraindications to the procedure are listed in Box 167-1. Performing ESWL on patients with untreated urinary infection and urinary obstruction distal to the location of the target stone predispose the patient to the development of urosepsis. Pregnancy is considered an absolute contraindication, because the effects of shock waves on the fetus are unknown, although ESWL has been inadvertently performed on pregnant women without apparent adverse effects on the fetus. The calcified wall of an abdominal aortic aneurysm provides an acoustic interface that can result in liberation of shock-wave energy and aneurysm rupture. Various authors have recommended minimum safe aneurysm diameters (e.g., 5 to 5.5 cm) and aneurysm-to-stone distances (e.g., at least 5 cm) along with maximum voltage settings and number of shock waves that can be safely delivered. ESWL in patients who are morbidly obese can be technically challenging and have lower success rates. ESWL can be safely performed in patients with pectorally-located implanted cardiac devices (i.e., pacemakers and automated implantable cardioverter-defibrillators), provided certain conditions are met (Box 167-2). Performing ESWL in a patient with abdominally located cardiac devices is not recommended.

Complications

Common side effects reported in the immediate postoperative period include flank pain, nausea and vomiting, and hypertension. Skin bruising at the site of shock-wave entry and flank pain lasting several days are also common. Hematuria is almost universally present due to shock wave–induced urothelial or renal parenchymal injury. Subcapsular hematoma is uncommon, with an incidence of 0.5%. Bleeding complications are more likely to occur in patients with hypertension, diabetes, or coronary artery atherosclerosis; the elderly; and patients with altered coagulation. Bleeding significant enough to require transfusion is rare. Stone fragments are generally excreted in the urine but can, occasionally, accumulate in the ureter, resulting in total obstruction (1%-5%). Air-filled alveoli within the lung present an impedance interface, and, therefore, shock waves directed toward the lungs result in liberation of shock-wave energy, alveolar rupture, and hemoptysis. In children or adults of short stature (under 48 inches), styrofoam can be used to protect the lungs from shock waves. Cardiac arrhythmias—including atrial and ventricular premature complexes, atrial fibrillation, and supraventricular and ventricular tachycardias—have been reported. Arrhythmias were extremely common in patients who had been treated with the first-generation lithotripters but are now thought to be quite rare. Some lithotripters can be programmed to deliver shock waves using “electrocardiogram gating” in an attempt to minimize the risk of an R-on-T phenomenon and subsequent ventricular arrhythmia. Pancreatitis and bowel injury resulting in rectal bleeding have been reported. There is conflicting evidence on the long-term effects of ESWL, but some studies suggest that patients who undergo ESWL may develop increased blood pressure and decreased renal function when compared with patients undergoing other treatments or observation. Elderly patients appear to be at higher risk for developing these complications.

Anesthetic considerations

Pain experienced during ESWL has cutaneous, somatic, and visceral origins. The amount of pain is directly related to the energy density of the shock wave at the skin entry site and the size of the F₂ focal zone. Modern lithotripters generally deliver shock waves of lower energy, compared with first-generation machines, and result in less patient discomfort. Additionally, piezoelectric lithotripters have a wider aperture and lower energy density at the skin entry point.

A wide variety of anesthetic techniques have been employed alone and in combination for ESWL, including general, epidural, and spinal anesthesia; flank infiltration; intercostal and paravertebral nerve blocks; topical application of EMLA (eutectic mixture of local anesthetic) cream; and intravenously and orally administered sedative and analgesic agents. Patient-controlled analgesia has been used successfully. First-generation lithotripters generally required general or neuraxial anesthesia, whereas procedures using modern lithotripters may be completed with conscious sedation and, if the patient has comorbid conditions, monitored anesthesia care. Neuraxial techniques should be performed with care to avoid the injection of air, which could provide an acoustic interface, resulting in the release of energy and tissue destruction. When neuraxial blockade is utilized, a sensory level of T6 is required.