Effect of initial shock wave voltage on shock wave lithotripsy-induced lesion size during step-wise voltage ramping

Abstract

Objective: To determine if the starting voltage in a step-wise ramping protocol for extracorporeal shock wave lithotripsy (SWL) alters the size of the renal lesion caused by the SWs.

Materials and methods: To address this question, one kidney from 19 juvenile pigs (aged 7-8 weeks) was treated in an unmodified Dornier HM-3 lithotripter (Dornier Medical Systems, Kennesaw, GA, USA) with either 2000 SWs at 24 kV (standard clinical treatment, 120 SWs/min), 100 SWs at 18 kV followed by 2000 SWs at 24 kV or 100 SWs at 24 kV followed by 2000 SWs at 24 kV. The latter protocols included a 3-4 min interval, between the 100 SWs and the 2000 SWs, used to check the targeting of the focal zone. The kidneys were removed at the end of the experiment so that lesion size could be determined by sectioning the entire kidney and quantifying the amount of haemorrhage in each slice. The average parenchymal lesion for each pig was then determined and a group mean was calculated.

Results: Kidneys that received the standard clinical treatment had a mean (sem) lesion size of 3.93 (1.29)% functional renal volume (FRV). The mean lesion size for the 18 kV ramping group was 0.09 (0.01)% FRV, while lesion size for the 24 kV ramping group was 0.51 (0.14)% FRV. The lesion size for both of these groups was significantly smaller than the lesion size in the standard clinical treatment group.

Conclusions: The data suggest that initial voltage in a voltage-ramping protocol does not correlate with renal damage. While voltage ramping does reduce injury when compared with SWL with no voltage ramping, starting at low or high voltage produces lesions of the same approximate size. Our findings also suggest that the interval between the initial shocks and the clinical dose of SWs, in our one-step ramping protocol, is important for protecting the kidney against injury.

Figures

FIG. 1

Gross appearance of kidneys treated with 2000 SWs at 24 kV (standard treatment), 100 SWs at 18 kV followed by 2000 SWs at 24 kV, or 100 SWs at 24 kV followed by 2000 SWs at 24 kV with an unmodified Dornier HM-3 lithotripter. The white circles show the approximate location of the SW focus (F2) on the lower pole of each kidney. Note that no sites of hemorrhage are evident on the kidneys using the one-step ramping protocol of 18 kV or 24 kV, while a large subcapsular hematoma (asterisk) is located on the kidney following standard treatment. Beneath the gross view of each kidney is a lower pole section showing the typical lesion found using each protocol and the average lesion size calculated in each group (expressed as mean (SEM) of percent of FRV). The lesion has been segmented and pseudo-colored (red), so that the size of the SWL-induced injury can be appreciated. Single arrows point to papillae showing evidence of hemorrhagic injury. Double arrows indicate an area where the injury extended up into the cortex, a common finding in the kidneys from pigs after the standard treatment.