Evaluation of the LithoGold LG-380 lithotripter: in vitro acoustic characterization and assessment of renal injury in the pig model

Abstract

Purpose: Conduct a laboratory evaluation of a novel low-pressure, broad focal zone electrohydraulic lithotripter (TRT LG-380).

Methods: Mapping of the acoustic field of the LG-380, along with a Dornier HM3, a Storz Modulith SLX, and a XiXin CS2012 (XX-ES) lithotripter was performed using a fiberoptic hydrophone. A pig model was used to assess renal response to 3000 shockwaves (SW) administered by a multistep power ramping protocol at 60 SW/min, and when animals were treated at the maximum power setting at 120 SW/min. Injury to the kidney was assessed by quantitation of lesion size and routine measures of renal function.

Results: SW amplitudes for the LG-380 ranged from (P(+)/P(-)) 7/-1.8 MPa at PL-1 to 21/-4 MPa at PL-11 while focal width measured ~20 mm, wider than the HM3 (8 mm), SLX (2.6 mm), or XX-ES (18 mm). For the LG-380, there was gradual narrowing of the focal width to ~10 mm after 5000 SWs, but this had negligible effect on breakage of model stones, because stones positioned at the periphery of the focal volume (10 mm off-axis) broke nearly as well as stones at the target point. Kidney injury measured less than 0.1% FRV (functional renal volume) for pigs treated using a gradual power ramping protocol at 60 SW/min and when SWs were delivered at maximum power at 120 SW/min.

Conclusions: The LG-380 exhibits the acoustic characteristics of a low-pressure, wide focal zone lithotripter and has the broadest focal width of any lithotripter yet reported. Although there was a gradual narrowing of focal width as the electrode aged, the efficiency of stone breakage was not affected. Because injury to the kidney was minimal when treatment followed either the recommended slow SW-rate multistep ramping protocol or when all SWs were delivered at fast SW-rate using maximum power, this appears to be a relatively safe lithotripter.

FIG. 1.

Peak positive (P⁺, top) and negative (P⁻, bottom) pressure as a function of power level (PL1–11) for the LG-380 lithotripter. Each point shows the mean and standard deviation for 10 sets of SWs (20–30 SWs per set) from pooled data collected using five different LG-380 lithotripters. Shockwaves were measured at the target point. Average P⁺ at the highest power level (PL-11) was ∼21 MPa.

FIG. 2.

Acoustic output over electrode lifetime for the TRT LG-380 and Dornier HM3 lithotripters. Each point is the average of 100 shockwaves (SWs). Whiskers indicate standard deviation of the mean. Trace for LG-380 begins after 365 SWs had been fired, while data for HM3 begin with SW#1 and are aligned opposite LG-380 points for convenience. Mean peak positive (P⁺, top) and negative (P⁻, bottom) pressures for the LG-380 (uses a self-adjusting spark-gap electrode) were consistent over 6000 SWs. Mean P⁺ for the HM3 fell ∼25% over 2200 SWs. Recommended lifetime for Dornier-style electrodes is 1000 to 2000 SWs, depending on kV.

FIG. 3.

The lateral distribution of peak positive pressure (P⁺) for the TRT LG-380 (at PL-9), the Dornier HM3 (at 18 kV), and Storz-SLX (PL-9) lithotripters. Measures were in the plane of the treatment focal point of each lithotripter (target point for LG-380, F2 focal point for HM3, focus of SLX). The focal width (i.e., −6dB width; pressure half maximum amplitude; radius indicated by intersection of dashed lines with x-axis) was ∼20 mm for the LG-380, ∼8 mm for the HM3, and ∼2.6 mm for the SLX. Data points are averaged values for at least 10 SWs collected at steps lateral to the axis of SW propagation.

FIG. 4.

Peak positive (P⁺) and negative (P⁻) pressures for SWs collected at steps along the acoustic axis (Z-axis) for the LG-380 and HM3 lithotripters. Data are for SWs (mean of 30–40 SWs) fired during the working lifetime of the electrodes (SWs 900–1500, PL-9 for LG-380; SWs 500–900, 18 kV for HM3). Zero on the Z-axis marks the target point of the LG-380 and the F2 focal point of the HM3. Maximum P⁺ (∼30 MPa) and P⁻ (∼−4.5 MPa) for the LG-380 was located ∼30–35 mm postfocal. Maximum P⁺ (∼38 MPa) for the HM3 was at the F2 focal point, while the greatest negative pressure (∼−7.5 MPa) was located ∼20–25 mm prefocal. This illustrates key differences in the acoustic properties of the two lithotripters.

FIG. 5.

LG-380 waveforms at selected locations relative to the target point. SWs were collected during mapping along the acoustic axis (Z-axis, see inset) early in the lifetime (SWs 900–1500, PL-9) of the electrode, and waveforms (mean of 30 SWs) are aligned so that the transition from positive to negative pressure is at the zero point on the time scale. This illustrates the evolution of the focused shockwave along the Z-axis, showing that SWs are relatively low amplitude (∼20 MPa) at the point where the stone is targeted (target point, Z=0 in inset), but achieve much higher amplitude (∼30 MPa) at ∼35 mm beyond the target point.

FIG. 6.

Stone breakage for wide and narrow focal zone lithotripters. Gypsum model stones held in a 2-mm mesh basket were positioned on-axis and at 5 mm and 10 mm off-axis in the plane of the target point of the lithotripter. SWs were fired at settings in the range of clinical treatment for the LG-380 (PL-9, 19 MPa), XX-ES (9.3 kV, 17 MPa), and Storz SLX (PL-9, 90 MPa) at 60 SW/min until no fragments remained in the basket. The Storz SLX was more effective at breaking stones positioned on-axis than either the LG-380 or the XX-ES (P<0.01). Breakage efficiency fell faster, however, for the narrow focal width SLX than for either of the two wide focal width lithotripters, with the SLX needing three times as many SWs to break stones positioned 5 mm off-axis as at the focus (P<0.001 vs on-axis). For the XX-ES, stones had to be moved to 10 mm off-axis for a significant increase in number of SWs to breakage (P<0.05), but this needed on average 34% more SWs than on-axis. The LG-380 needed a similar increase in number of SWs at 10 mm, on average 31% more than on-axis. *P<0.05, **P<0.001 vs SWs on-axis (distance=0).

FIG. 7.

Renal parenchymal lesion in pig kidneys treated using the TRT LG-380 and Dornier HM3 lithotriptors. Frame A shows small foci of hemorrhage (arrows) in the lower pole medulla of a pig kidney treated using the multistep power ramping protocol (PL 1–8 @50, PL-9 2600 SW, 60 SW/min) recommended by TRT. Lesion volume in this group of animals measured <0.1% FRV (n=4). Frame B shows lower pole medullary lesions (arrows) in the kidney of a pig treated using the Dornier HM3 fired at 60 SW/min (2000 SW, 24 kV), while frame C shows a larger region of hemorrhage (arrows) extending from the cortex to medulla in a kidney treated at the same dose (2000 SW, 24 kV) but at 120 SW/min. Lesion volume of renal parenchyma, expressed as functional renal volume, in these representative sections measured 0.42% and 3.9% FRV, respectively. Data for renal injury with the Dornier HM3 have been reported previously.^–,