Search

OBJECTIVE To determine how the size and location of coupling defects caught between the therapy head of a lithotripter and the skin of a surrogate patient (acoustic window of a test chamber) affect the features of shock waves responsible for stone breakage. METHODS Model defects were placed in the coupling gel between the therapy head of a Dornier Compact-S electromagnetic lithotripter and the Mylar window of a water-filled coupling test system. A fiber-optic hydrophone was used to measure acoustic pressures and map the lateral dimensions of the focal zone of the lithotripter. The effect of coupling conditions on stone breakage was assessed using Gypsum model stones. RESULTS Stone breakage decreased in proportion to the area of the coupling defect; a centrally located defect blocking only 18% of the transmission area reduced stone breakage by an average of almost 30%. The effect on stone breakage was greater for defects located on-axis and decreased as the defect was moved laterally; an 18% defect located near the periphery of the coupling window (2.0 cm off-axis) reduced stone breakage by only ~15% compared to when coupling was completely unobstructed. Defects centered within the coupling window acted to narrow the focal width of the lithotripter; an 8.2% defect reduced the focal width ~30% compared to no obstruction (4.4 mm versus 6.5 mm). Coupling defects located slightly off center disrupted the symmetry of the acoustic field; an 18% defect positioned 1.0 cm off-axis shifted the focus of maximum positive pressure ~1.0 mm laterally. Defects on and off-axis imposed a significant reduction in the energy density of shock waves across the focal zone. CONCLUSIONS In addition to blocking the transmission of shock wave energy, coupling defects also disrupt the properties of shock waves that play a role in stone breakage, including the focal width of the lithotripter and the symmetry of the acoustic field; the effect is dependent on the size and location of defects, with defects near the center of the coupling window having the greatest effect. These data emphasize the importance of eliminating air pockets from the coupling interface, particularly defects located near the center of the coupling window. INTRODUCTION The quality of acoustic coupling in shock wave lithotripsy (SWL) is often overlooked and may be one of the most important factors affecting treatment outcomes (1,2). SWL can be very effective in breaking stones but only if the shock waves (SWs) can get to the target. In early lithotripters such as the Dornier HM3 the patient was immersed in a water bath, providing an ideal medium for SW propagation. Modern lithotripters on the other hand are dry-head devices in which the cushion of the treatment head must be coupled, usually with gel or oil, to the skin of the patient. Unfortunately, air can get trapped at the coupling interface and this interferes with SW transmission to the patient (3,4). Reports have suggested that newer lithotripters are not nearly as effective as the Dornier HM3 (1, 5–7). Clearly there are multiple factors that distinguish one lithotripter from the next so it is difficult to know what contributes to higher success rates with the HM3. The HM3 is not the most powerful lithotripter nor does the acoustic output or dimensions of the focal volume distinguish this lithotripter from most others. The HM3 is, however, the only lithotripter that employs a complete immersion water bath, the only lithotripter where the quality of coupling is not potentially problematic, and this could be the primary reason the HM3 has proven to be more effective than newer machines. In previous studies with dry-head lithotripters we have shown that air pockets caught at the coupling interface between the cushion of the treatment head and the acoustic window (surrogate skin) of the test tank interfere with the transmission of SW energy (8). As the area occupied by air pockets increased, acoustic pressure at the focal point of the lithotripter decreased, as did the efficiency in breakage of model stones. There was considerable variability in the system in that every coupling attempt yielded a different pattern of air pockets with defects of different shape, size and location depending on how the gel was handled and applied. This was found to be the case for tests using a Mylar membrane as surrogate skin, but also when a treatment cushion affixed to a viewing port was pushed against the skin of a volunteer. It was also observed that coupling attempts having a similar total area occupied by air pockets could yield stone breakage values differing by greater than 30%, suggesting that not only does the area of coupling defects matter, but perhaps that the location of the air pockets is also important (9). Air pockets caught at the coupling interface are acoustically opaque and block the SW transmission path, but they also have smooth or regular edges that could create diffraction with the potential to further disrupt the acoustic field at the target (10). Since the mechanisms of SW action in stone breakage and tissue damage are dependent on the acoustic output and dimensions of the focal zone of the lithotripter there is value in learning more about the potential mechanistic effects of defects at the coupling interface. Therefore, we undertook a study to assess the role that size, shape and location of coupling defects may play in lithotripter performance.

OBJECTIVE To test the idea that a pause (~3-min) in the delivery of shock waves (SW) soon after the initiation of treatment is unnecessary for achieving a reduction in renal injury, if treatment is begun at a low power setting that generates low-amplitude SWs. MATERIALS AND METHODS Anesthetized female pigs were assigned to one of three SWL treatment protocols that did not involve a pause in SW delivery of more than 10 seconds (2000 SWs at 24 kV; 100 SWs at 12 kV + ~10-sec pause + 2000 SWs at 24 kV; 500 SWs at 12 kV + ~10-sec pause + 2000 SWs at 24 kV; all SWs delivered at 120 SWs/min using an unmodified Dornier HM3 lithotripter). Renal function was measured before and after SWL. The kidneys were then processed for quantification of the SWL-induced hemorrhagic lesion. Values for lesion size were compared to previous data collected from pigs in which treatment included a 3-min pause in SW delivery. RESULTS All SWL treatment protocols produced a similar degree of vasoconstriction (23–41% reduction in GFR and ERPF) in the SW-treated kidney. The mean renal lesion in pigs treated with 100 low-amplitude SWs delivered before the main dose of 2000 high-amplitude SWs (2.27% FRV) was statistically similar to that measured for pigs treated with 2000 SWs all at high-amplitude (3.29% FRV). However, pigs treated with 500 low-amplitude SWs before the main SW dose had a significantly smaller lesion (0.44% FRV) that was comparable to the lesion in pigs from a previous study in which there was a 3-min pause in treatment separating a smaller initial dose of 100 low-amplitude SWs from the main dose of 2000 high-amplitude SWs (0.46% FRV). Time between the initiation of the low- and high-amplitude SWs was ~4-min for these latter two groups compared to ~1-min when there was negligible pause after the initial 100 low-amplitude SWs in the protocol. CONCLUSIONS Pig kidneys treated by SWL using a 2-step low-to-high power ramping protocol were protected from injury with negligible pause between steps, provided the time between the initiation of low-amplitude SWs and switching to high-amplitude SWs was ~4-min. Comparison with results from previous studies shows that protection can be achieved using various step-wise treatment scenarios in which either the initial dose of SWs is delivered at low-amplitude for ~4-min, or there is a definitive pause before resuming SW treatment at higher amplitude. Thus, we conclude that renal protection can be achieved without instituting a pause in SWL treatment. It remains prudent to consider that renal protection depends on the acoustic and temporal properties of SWs administered at the beginning stages of a SWL ramping protocol, and that this may differ according to the lithotripter at hand. Keywords: kidney, lithotripsy, swine, tissue injury INTRODUCTION An undesirable side effect of SWL treatment is that SWs can injure renal and surrounding tissue [1]. The primary acute lesion is vascular trauma with breakage of blood vessels and pooling of blood within the parenchyma, which if extending to the kidney surface will result in subcapsular or perirenal hematomas [1]. Along with the vascular insult, there is damage to tubules and the production and release of proinflammatory cytokines and injurious agents (e.g. iron/reactive oxygen metabolites; vasoconstrictor peptides/ischemia; metabolic toxins) that can result in fibrosis and the loss of functional tissue [1,2]. Such SW-induced injury has been linked to adverse outcomes such as hypertension, diabetes and exacerbation of kidney stone disease [3–5]. This raises concern about the long-term safety of SWL, and developing SWL treatment strategies that reduce or prevent tissue injury would certainly help mitigate such concerns. One approach to reduce SWL-induced tissue injury has been to alter the manner in which SWs are delivered to the kidney [2,6], and in this regard we have reported that treatment of the pig kidney with low-amplitude SWs followed by a 3-min pause in treatment prior to applying high-amplitude SWs will reduce SWL-induced hemorrhagic lesion sizes by as much as 20-fold [7]. In fact, similar protocols in which low-amplitude SWs were substituted with a relatively small number of higher-amplitude SWs were also shown to reduce SW-induced tissue damage, implicating the 3-min pause in treatment to be a critical factor in the development of the renal protective response [8]. On the other hand, some clinical centers begin SWL treatment at a low power setting to condition the patient to treatment-related discomfort and then gradually ramp up to higher levels with continuous delivery of SWs. That is, there is typically no pause in treatment during the lithotripsy session [9–12]. It is unclear even with power ramping if continuous delivery of SWs can be used to protect the kidney from injury. Therefore, we sought to determine in our pig model, using a 2-step ramping protocol, whether a definitive pause in SW delivery is needed in order to protect the kidney from SWL-related tissue damage.

Study Type--Therapy (case series) Level of Evidence 4. What's known on the subject? and What does the study add? In shock wave lithotripsy air pockets tend to get caught between the therapy head of the lithotripter and the skin of the patient. Defects at the coupling interface hinder the transmission of shock wave energy into the body, reducing the effectiveness of treatment. This in vitro study shows that ineffective coupling not only blocks the transmission of acoustic pulses but also alters the properties of shock waves involved in the mechanisms of stone breakage, with the effect dependent on the size and location of defects at the coupling interface.• To determine how the size and location of coupling defects caught between the therapy head of a lithotripter and the skin of a surrogate patient (i.e. the acoustic window of a test chamber) affect the features of shock waves responsible for stone breakage.• Model defects were placed in the coupling gel between the therapy head of a Dornier Compact-S electromagnetic lithotripter (Dornier MedTech, Kennesaw, GA, USA) and the Mylar (biaxially oriented polyethylene terephthalate) (DuPont Teijin Films, Chester, VA, USA) window of a water-filled coupling test system. • A fibre-optic probe hydrophone was used to measure acoustic pressures and map the lateral dimensions of the focal zone of the lithotripter. • The effect of coupling conditions on stone breakage was assessed using gypsum model stones.• Stone breakage decreased in proportion to the area of the coupling defect; a centrally located defect blocking only 18% of the transmission area reduced stone breakage by an average of almost 30%. • The effect on stone breakage was greater for defects located on-axis and decreased as the defect was moved laterally; an 18% defect located near the periphery of the coupling window (2.0 cm off-axis) reduced stone breakage by only ~15% compared to when coupling was completely unobstructed. • Defects centred within the coupling window acted to narrow the focal width of the lithotripter; an 8.2% defect reduced the focal width ~30% compared to no obstruction (4.4 mm vs 6.5 mm). • Coupling defects located slightly off centre disrupted the symmetry of the acoustic field; an 18% defect positioned 1.0 cm off-axis shifted the focus of maximum positive pressure ~1.0 mm laterally. • Defects on and off-axis imposed a significant reduction in the energy density of shock waves across the focal zone.• In addition to blocking the transmission of shock-wave energy, coupling defects also disrupt the properties of shock waves that play a role in stone breakage, including the focal width of the lithotripter and the symmetry of the acoustic field • The effect is dependent on the size and location of defects, with defects near the centre of the coupling window having the greatest effect. • These data emphasize the importance of eliminating air pockets from the coupling interface, particularly defects located near the centre of the coupling window.