No Injury Observed in Kidneys Treated with Burst Wave Lithotripsy in Therapeutically Anticoagulated Pigs

Abstract

Introduction:

Current American Urological Association guidelines recommend that patients with acute obstructive kidney stone requiring continuous anticoagulation/antiplatelet therapy should not be treated by shockwave lithotripsy or percutaneous nephrolithotomy because of the risk of catastrophic renal hemorrhage possible with those techniques. Currently, ureteroscopy is the only recommended surgical treatment. We evaluated if burst wave lithotripsy (BWL) could be used in these cases by treating pigs with BWL while undergoing anticoagulation therapy.

Materials and Methods:

Six pigs (31–37 kg) were given 200 units/kg porcine heparin, and then the right kidney of each animal was treated with a dose of BWL (18,000 ultrasound pulses at 10 Hz, 20 cycles/pulse, peak positive pressure of 12 MPa) known to fracture kidney stones. The contralateral kidney served as a control. Therapeutic anticoagulation was confirmed by evaluating activated partial thromboplastin time (aPTT). Microhematuria was checked by urine test strips. Kidneys were subsequently evaluated for hemorrhagic injury via MRI.

Results:

aPTT was significantly elevated (>100 seconds) throughout the entire treatment period (p = <0.001) indicating therapeutic anticoagulation. Gross hematuria was not observed in any of the pigs. After BWL, microhematuria averaged 145.8 red blood cells (RBC)/μL in the BWL-treated kidney and 48 RBC/μL in the control kidneys, and there was no statistically significant difference noted in microhematuria rates between the two groups (p = 0.16). Hemorrhagic injury measured only 0.10 ± 0.02% of the tissue in the BWL-treated kidneys and 0.12 ± 0.04% in the control side, with a paired difference of −0.03 ± 0.05%, showing no significant difference (p = 0.58).

Conclusions:

A typical clinical dose of BWL causes no hemorrhagic injury to the kidney even during therapeutic anticoagulation therapy. This result suggests that BWL should be safe to use in patients with stone undergoing anticoagulation/antiplatelet therapy.

Introduction

Nephrolithiasis is a common disease in the United States—with an estimated lifetime prevalence of nearly 9%.¹ For symptomatic stones with a low chance of spontaneous passage, intervention is indicated. Current American Urological Association guidelines recommend that patients with acute obstructive kidney stone requiring continuous anticoagulation/antiplatelet therapy should not be treated by shockwave lithotripsy (SWL) or percutaneous nephrolithotomy (PCNL) because of the risk of catastrophic renal hemorrhage possible with those techniques. Currently, ureteroscopy is the only recommended surgical treatment.

Burst wave lithotripsy (BWL) is a novel form of extracorporeal lithotripsy for the treatment of nephrolithiasis. Utilizing periodic bursts of focused ultrasound waves, BWL can effectively fragment renal calculi.² Several studies have focused on the optimal treatment parameters and the risk of renal injury in BWL. Sorensen et al. first demonstrated that BWL induces no detectable tissue injury in a porcine model when used at clinically effective parameters in 2018, and since then further studies have confirmed this finding.^3,4 Furthermore, human patients treated with BWL have also shown no signs of renal injury.⁵ However, no study to date has focused on the effects of BWL in an anticoagulated state. This study evaluates if hemorrhagic injury occurs when BWL is used with standard clinical trial parameters in an anticoagulated pig model.

Materials and Methods

Animal procedures

The surgical and animal treatment protocols used to assess hemorrhagic renal injury and morphology changes in this study were carried out in accordance with the National Institutes of Health Guide for the Care and Use of Laboratory Animals and were approved by the Institutional Animal Care and Use Committee of the Indiana University School of Medicine (approval #21060).

The design of the experiment and all surgical procedures employed during the experiment followed methods used in previously published studies.^4,6–8 Six female pigs (Yorkshire × Landrace mix), weighing 31–37 kg each (Oak Hill Genetics, Edwin, IL), were anesthetized with a combination of ketamine and xylazine, intubated, and then maintained on isoflurane anesthesia. Both flanks of each animal were shaved to remove hair, and the pigs were placed on a surgical table in a supine position. Respiration was spontaneous.

A catheter was placed in an ear vein for infusion of isotonic saline at a rate of 1% body weight per hour. Next, catheters were placed in a femoral artery, for blood pressure monitoring and blood sampling, and in both ureters for timed urine collection. Urine samples were collected from the pig before (for baseline) and after BWL treatment and were analyzed for the presence of blood (Chemstrip 10 with SG, Roche Diagnostics, Indianapolis, IN).

A therapeutic dose of 200 units/kg heparin was administered intravenously to achieve a fully anticoagulated state. Activated partial thromboplastin time (aPTT) was measured just before BWL treatment was started and at the end of BWL treatment to confirm appropriate anticoagulation (Antech Diagnostics, Brownsburg, IN).

After baseline urine collections were completed, each pig was placed on its left side. Next, Scan ultrasound gel (centrifuged to remove bubbles; Parker Laboratories, Fairfield, NJ) was layered onto the flank, and a water bath was secured in place and then filled with degassed water until the water bath flexible membrane touched the ultrasound gel layer coupling the tub to the pig, and air bubbles in the gel were removed from the membrane–skin interface to maximize acoustic coupling.⁶ When this was completed, the BWL treatment head and ultrasound imaging probe were both immersed in degassed water. The water in the tub was continuously degassed during the treatment period. This arrangement allowed for maximal delivery of BWL energy into the body of the animal.

Ultrasound imaging was then used to locate the right kidney of each animal. From there, a lower pole calix of the right kidney was selected and aligned with the focus of the BWL. The kidneys received treatment with BWL using the parameters of 18,000 pulses at a pulse repetition rate of 10 pulses/seconds with 20 cycles/pulse and a positive pressure amplitude of 12 MPa and negative pressure amplitude of −7 MPa.

After completion of BWL treatment, the pigs were again placed supine and urine collection was repeated 30 minutes after treatment. Additional details on preparation of the animals and on the BWL equipment can be found in a previous publication.⁴

Lesion analysis

At the end of the experiment, the kidneys were perfusion-fixed in situ with 2.5% glutaraldehyde in 0.1 M cacodylate buffer (pH = 7.4). After the kidneys were removed, they were submerged in fresh fixative for subsequent determination of lesion size. The treated kidneys were analyzed for morphological injury using MRI scans as done previously.⁴

The MRI images were then used to determine lesion volume and expressed as a percent of the functional renal volume (FRV) of renal parenchyma damaged for each kidney. Mean lesion size ± standard error of mean was calculated for the different groups, and a one-way analysis of variance with Tukey-Kramer Honest Significant Difference Test was used to compare the lesion sizes between the heparinized BWL group and the heparinized control group, as well as one additional group (SWL-treated animals) from a previous publication.⁴ These additional animals are of similar size and were treated with the comparable procedures. Two-sided p-values <0.05 were considered significantly different.

Results

aPTT rose from 10.9 seconds to >100 seconds after heparin injection and remained elevated throughout the entire treatment period (p = <0.001) indicating therapeutic anticoagulation. Urine samples were collected from the experimental and control kidneys both before and after BWL treatment, and we did not observe gross hematuria in any urine sample. We did observe microscopic hematuria in five of the six heparin-BWL kidneys and in two of the six heparin-control kidneys after BWL. In the heparin-BWL group, red blood cells (RBC) in the urine reached 145.8 RBC/μL (immediately post-BWL), whereas in the heparin-control group, microscopic hematuria reached 48 RBC/μL. There was no significant difference noted in microhematuria rates between the BWL and control groups (p = 0.16).

Hemorrhagic injury to the kidneys was assessed with 3.0 Tesla MRI. Previous studies by our group have established methods for assessing SWL injury in a non-anticoagulated state, and these data were included as an additional comparison to the information from this study as shown in Figure 1.⁴ There was no difference noted between the heparin-BWL and heparin-control kidneys in this study (paired t-test, p = 0.58). Additionally, there was a difference between the lesion sizes in the heparinized kidneys and the SWL kidneys from the previously published study (0.10% vs. 1.55%, p = <0.0001).

FIG. 1.

BWL hemorrhagic lesion size comparisons, with data using SWL taken from Connors et al.⁴ For statistical comparison, data were log-transformed and compared using analysis of variance and the Tukey–Kramer Honest Significant Difference test. BWL = burst wave lithotripsy; SWL = shockwave lithotripsy.

Discussion

BWL is a technology developed to address many of the safety concerns associated with SWL. SWL uses high peak pressure shock waves (30–100 MPa) with single cycle pulses at low rates (2 Hz), but BWL uses ultrasonic waves with bursts of sinusoidal pulses at much lower peak pressures (<12 MPa) and higher rates (<200 Hz).⁷

Multiple preclinical studies have focused attention on BWL and the risk of renal injury.⁸ As treatment parameters and exposure durations have become better understood, renal injury appears to be avoidable without compromising treatment effectiveness. This advances the idea that BWL could serve as a preferred alternative to more invasive procedures for anticoagulated patients. This is a relevant issue to the practicing urologist, as the prevalence of oral anticoagulant usage in the United States has increased steadily in the past decade. Colacci et al. published that in 2017, over 4 million Medicare beneficiaries were prescribed oral anticoagulants.⁹ Although it is possible to perform ureteroscopic stone operation while on anticoagulation, a 2017 study demonstrated that patients continuing anticoagulation during the perioperative period had a 15.4% significant bleeding-related event, compared with 9% for enoxaparin-bridging and 3% for patients who suspended their anticoagulation therapy.¹⁰ Active anticoagulation is a contraindication for SWL and PCNL. The limited treatment options for anticoagulated patients mean that BWL has the potential to improve the quality and safety of care for this patient population.

Although the data presented in our current study are promising, our study has several limitations related to the use of the pig model as a substitute for human patients. First, the treatment setup between the pig and the patients is slightly different. For the pig, the treatment head is suspended in a water bath, so the ultrasound pulses travel through water before they pass through a thin plastic membrane on route to the skin of the pig. For patients, the SonoMotion treatment head (currently in clinical trials) is placed directly against the patient’s skin with ultrasound gel.¹¹ Both setups are designed to be acoustically transparent to the ultrasound pulse. It is thought that the water bath setup attenuates the ultrasound pulse less than the materials in the SonoMotion transducer head so that our in situ pressures are slightly higher than what is experienced in patients at the same machine settings. Thus, our setup would be more likely to induce injury in the pig kidney compared with patients.

Second, we have not inserted stones into the kidneys of the pigs used in our experiments. In theory, stone fragments produced during any acoustics-based treatment could damage the urothelium of the kidney resulting in bleeding; thus, we would underestimate renal injury and hemorrhage in our stoneless kidneys. However, we do not expect stone fragmentation in the urine collection system would produce any damage to the urothelium during BWL. In SWL the pulse is an actual shock wave and is very forceful (30–100 MPa) and might propel a stone fragment into the urothelium. On the contrary, BWL operates with pulses of only 12 MPa. Also, a BWL pulse does not present as a shock front but rises relatively slowly to 12 MPa. Because of these differences, stone movement is slow and stone fragments should not achieve the velocity needed to cause urothelium injury.

And third, we used heparin in our experiments to achieve anticoagulation in the pigs. Most patients, on the contrary, are given oral anticoagulants as part of their medical therapy. It is difficult to determine if heparin anticoagulation in pigs is directly comparable with oral anticoagulants in humans. In addition, we are not aware of any studies directly comparing heparin versus oral anticoagulants in pigs. This leaves the question of comparability of these different approaches unanswered. However, we did achieve generalized anticoagulation by extending clotting times (>100 seconds) as shown by aPTT assays, and heparin has been shown by other investigators to increase bleeding and extend clotting times in pigs under experimental conditions.¹² With this information it seems reasonable to conclude that heparin use should have triggered additional bleeding if it were going to occur with BWL treatment.

This study provides the first insights into the safety profile of BWL in an anticoagulated state. Urinalyses in this study demonstrated no evidence of gross hematuria in either the heparin-BWL or heparin-control kidneys. There was no difference in levels of microscopic hematuria between these groups. More importantly, we found there was no difference in FRV lesion size between the heparin-BWL kidneys and the heparin-control kidneys. Lastly, there was a significant and sizeable difference between the non-heparin SWL lesion sizes compared with the heparin-BWL lesion sizes, with the SWL lesion being 16 times larger. This is consistent with previous data from our group characterizing SWL renal injury and the notably diminished tissue trauma induced by BWL.⁴

Conclusions

In this study, we demonstrate that BWL delivered at a typical clinical dose causes no hemorrhagic injury to the kidney even during therapeutic anticoagulation therapy. Although BWL remains in its clinical infancy, it continues to demonstrate tremendous promise as a novel therapeutic agent. Once it is introduced to clinical practice, the question of how to manage anticoagulated patients will become relevant. This study provides supporting evidence of safety in this clinical scenario. Future studies will aim to assess if there is an identifiable threshold where BWL can induce significant bleeding an anticoagulated state.

Abbreviations Used

aPTT

activated partial thromboplastin time

BWL

burst wave lithotripsy

FRV

functional renal volume

mPA

megapascal

MRI

magnetic resonance imaging

PCNL

percutaneous nephrolithotomy

RBC

red blood cell

SWL

shockwave lithotripsy

Authors’ Contributions

T.M.S.: Conceptualization, methodology, investigation, and writing—original draft preparation. B.A.C.: Methodology, validation, investigation, resources, writing—review and editing, and supervision. M.E.R.: Supervision. J.E.L.: Supervision and funding acquisition. M.R.B.: Methodology and supervision. J.C.W.: Methodology, formal analysis, validation, supervision, project administration, funding acquisition, visualization, and writing—review and editing.

Author Disclosure Statement

M.E.R. is a consultant for Boston Scientific. M.R.B. has equity in and a consulting agreement with SonoMotion, Inc., which is commercializing the BWL technology.

Funding Information

This work is supported by National Institutes of Health grant PO1 DK43881.