Evaluation of shock wave lithotripsy injury in the pig using a narrow focal zone lithotriptor

Abstract

What's known on the subject? and What does the study add? Of all the SW lithotriptors manufactured to date, more research studies have been conducted on and more is known about the injury (both description of injury and how to manipulate injury size) produced by the Dornier HM-3 than any other machine. From this information have come suggestions for treatment protocols to reduce shock wave (SW)-induced injury for use in stone clinics. By contrast, much less is known about the injury produced by narrow-focus and high-pressure lithotriptors like the Storz Modulith SLX. In fact, a careful study looking at the morphology of the injury produced by the SLX itself is lacking, as is any study exploring ways to reduce renal injury by manipulating SW delivery variables of this lithotriptor. The present study quantitates the lesion size and describes the morphology of the injury produced by the SLX. In addition, we report that reducing the SW delivery rate, a manoeuvre known to lower injury in the HM-3, does not reduce lesion size in the SLX.

Objective: • To assess renal injury in a pig model after treatment with a clinical dose of shock waves using a narrow focal zone (≈3 mm) lithotriptor (Modulith SLX, Karl Storz Lithotripsy).

Materials and methods: • The left kidney of anaesthetized female pigs were treated with 2000 or 4000 shock waves (SWs) at 120 SWs/min, or 2000 SWs at 60 SWs/min using the Storz SLX. • Measures of renal function (glomerular filtration rate and renal plasma flow) were collected before and 1 h after shock wave lithotripsy (SWL) and the kidneys were harvested for histological analysis and morphometric quantitation of haemorrhage in the renal parenchyma with lesion size expressed as a percentage of functional renal volume (FRV). • A fibre-optic probe hydrophone was used to determine acoustic output and map the focal width of the lithotriptor. • Data for the SLX were compared with data from a previously published study in which pigs of the same age (7-8 weeks) were treated (2000 SWs at 120 or 60 SWs/min) using an unmodified Dornier HM3 lithotriptor.

Results: • Treatment with the SLX produced a highly focused lesion running from cortex to medulla and often spanning the full thickness of the kidney. Unlike the diffuse interstitial haemorrhage observed with the HM3, the SLX lesion bore a blood-filled core of near-complete tissue disruption devoid of histologically recognizable kidney structure. • Despite the intensity of tissue destruction at the core of the lesion, measures of lesion size based on macroscopic determination of haemorrhage in the parenchyma were not significantly different from kidneys treated using the HM3 (2000 SWs, 120 SWs/min: SLX, 1.86 ± 0.52% FRV; HM3, 3.93 ± 1.29% FRV). • Doubling the SW dose of the SLX from 2000 to 4000 SWs did not significantly increase lesion size. In addition, slowing the firing rate of the SLX to 60 SWs/min did not reduce the size of the lesion (2.16 ± 0.96% FRV) compared with treatment at 120 SWs/min, as was the case with the HM3 (0.42 ± 0.23% FRV vs 3.93 ± 1.29% FRV). • Renal function fell significantly below baseline in all treated groups but was similar for both lithotriptors. • Focal width of the SLX (≈2.6 mm) was about one-third that of the HM3 (≈8 mm) while peak pressures were higher (SLX at power level 9: P+≈90 MPa, P-≈-12 MPa; HM3 at 24 kV: P+≈46 MPa, P-≈-8 MPa).

Conclusions: • The lesion produced by the SLX (narrow focal width, high acoustic pressure) was a more focused, more intense form of tissue damage than occurs with the HM3. • Slowing the SW rate to 60 SWs/min, a strategy shown to be effective in reducing injury with the HM3, was not protective with the SLX. • These findings suggest that the focal width and acoustic output of a lithotriptor affect the renal response to SWL.

FIG. 1

Subcapsular haematomas in kidney treated with the Storz SLX (2000 SW, 120 SWs/min). Photographs show haematomas (arrows) on the anterior (A) and posterior (B) surfaces of kidney, corresponding to entry and exit points on the SW axis. The images show the haematoma pattern found in nine of 21 kidneys treated with the SLX.

FIG. 2

Path of lesion in pig kidney treated with the Storz SLX (2000 SWs, 120 SWs/min). These macroscopic images are of four sections (nos 6, 26, 79, 101) taken from 157 serial sections spanning the full thickness (anterior to posterior) of the kidney. The lesion can be tracked beginning at the posterior side (no. 6) where the parenchymal lesion (flanked by arrows) is continuous with an indentation caused by a subcapsular haematoma (blood washed out during processing). The lesion in sections 79 and 101 includes a region (arrowhead) in which haemorrhage was displaced during processing leaving a plug of embedding wax. This could only occur with complete ablation of tissue. In some cases such a channel could be tracked to the surface of the kidney. The lesion size for this kidney was determined to be 3.15% FRV.

FIG. 3

Comparison of lesion in pig kidneys treated at 60 SWs/min with Dornier HM3 and Storz SLX lithotriptors. Frames A–D show macroscopic images of four tissue sections (nos 22, 48, 87, 113) among 160 serial sections from a kidney treated at 60 SWs/min using the HM3 (2,000 SWs, 24 kV). The parenchymal lesion is limited to discrete regions in sections 48 and 87 (arrows). The lesion size in this kidney treated at slow SW rate measured 0.5% FRV compared with a mean value of 3.93% FRV for kidneys treated at 120 SWs/min as previously reported (Connors et al. [10]). Frames E and F show two sections (nos 88, 112) from a kidney likewise treated at 60 SWs/min but using the SLX (2000 SWs, PL-9). The lesion (flanked by arrows) is seen as a region of concentrated haemorrhage surrounded by seemingly unaffected parenchyma. The lesion volume in this kidney was 4.1% FRV.

FIG. 4

Histology of renal cortex from a pig kidney treated using the Storz SLX (2000 SWs, 120 SWs/min). (A) The haemorrhagic lesion (*) surrounded by intact kidney parenchyma. The portion of the lesion shown in this frame measures ≈ 2.5 mm × 3.0 mm. (B) The margin of the lesion, showing an abrupt transition between intact renal tubules and the core of the lesion largely devoid of organized structures. The transition from intact parenchyma to zone of complete tissue disruption occurs over a distance of just a few tubule diameters (square bracket). (C) Continuity can be seen between the parenchymal lesion (dashed line) and a subcapsular haematoma (SH). Several areas of intense haemorrhage are evident in the lesion (arrows). Bars, 0.5 mm (A), 0.05 mm (B), 0.5 mm (C).

FIG. 5

Lesion size in pig kidneys treated using the Storz SLX and Dornier HM3 lithotriptors. Data are presented as percentage of functional renal volume (% FRV) determined by morphometric analysis of parenchymal haemorrhage (excluding the renal calyceal system). The lesion sizes for 2000 SWs at 120 SWs/min were not significantly different between the SLX and the HM3. Doubling the dose to 4000 SWs did not increase the lesion size with the SLX. Slowing the SW rate did not reduce the lesion size, as happened with the HM3. The hash (#) indicates a significant difference between groups. N indicates number of individual kidneys sectioned and quantified in each group. Data for the Dornier HM3 were published previously by Connors et al. [10].

FIG. 6

Glomerular filtration rate at 1 h after treatment with the Storz SLX or Dornier HM3 lithotriptor. There was no significant difference in measurements of GFR among groups. N indicates the number of animals per group. The asterisk (*) indicates that post-SWL renal function is significantly different from pre-SWL baseline measurements for the group. Data for the Dornier HM3 were previously published by Connors et al. [10].

FIG. 7

PAH extraction (E_PAH) at 1 h after treatment with Storz SLX or Dornier HM3 lithotriptor. There was no significant difference in measurements of E_PAH among groups. N indicates number of animals per group. The asterisk (*) indicates that post-SWL renal function is significantly different from pre-SWL baseline measurements for the group. Data for Dornier HM3 were previously published by Connors et al. [10].

FIG. 8

Renal plasma flow at 1 h after treatment with the Storz SLX or Dornier HM3 lithotriptor. There was no significant difference in measurements of RPF among groups. N indicates number of animals per group. The asterisk (*) indicates that post-SWL renal function is significantly different from pre-SWL baseline measurements. Data for Dornier HM3 were previously published by Connors et al. [10].

Fig. 9

Peak positive pressure (P+) at PL-9 as a function of lateral distance off the acoustic axis. The − 6 dB width (i.e. pressure half-maximum amplitude; dashed lines) of the acoustic field was ≈ 2.6 mm (1.3 mm radius from the axis). The inset shows temporal profiles of SWs at various distances from the acoustic axis. Waveforms are aligned so that the transition from positive to negative pressure is at the zero point on the timescale. The duration of the positive-pressure phase is longer for waveforms collected off axis. Comparing waveforms at 0 and 1.25 mm, the P+ declines quickly off-axis: the P+ at 1.25 mm off-axis (≈ 45 MPa) was about half the amplitude recorded at the focus of the lithotriptor (90 MPa).