The current status of lithotripsy

Calcific Plaque Modification by Acoustic Shockwaves: Intravascular Lithotripsy in Cardiovascular Interventions

PURPOSE OF REVIEW

To provide a review of recent literature on the treatment of moderate-to-severe calcification in coronary and peripheral vasculature with intravascular lithotripsy (Shockwave Medical, Santa Clara, CA).

RECENT FINDINGS

Moderate-to-severe calcific plaques constitute a significant proportion of lesions treated with transcatheter interventions in the coronary and peripheral vascular beds and portend lower procedural success rates, increased periprocedural major adverse events, and unfavorable long-term clinical outcomes compared to non-calcific plaques. Intravascular lithotripsy (IVL) is a new technique that uses acoustic shock waves in a balloon-based system to induce fracture in the calcium deposits to facilitate luminal gain and stent expansion. IVL demonstrated high procedural success and low complication rates in the management of moderate-to-severe calcification in coronary and peripheral vascular beds and led to large luminal gain by modification of calcific plaque as assessed by optical coherence tomography. Further studies will determine the role of IVL in an integrated, protocolized approach to the treatment of severely calcified plaques in the coronary and peripheral vascular beds.

Single-shot measurements of the acoustic field of an electrohydraulic lithotripter using a hydrophone array

Piezopolymer-based hydrophone arrays consisting of 20 elements were fabricated and tested for use in measuring the acoustic field from a shock-wave lithotripter. The arrays were fabricated from piezopolymer films and were mounted in a housing to allow submersion into water. The motivation was to use the array to determine how the shot-to-shot variability of the spark discharge in an electrohydraulic lithotripter affects the resulting focused acoustic field. It was found that the dominant effect of shot-to-shot variability was to laterally shift the location of the focus by up to 5 mm from the nominal acoustic axis of the lithotripter. The effect was more pronounced when the spark discharge was initiated with higher voltages. The lateral beamwidth of individual, instantaneous shock waves were observed to range from 1.5 mm to 24 mm. Due to the spatial variation of the acoustic field, the average of instantaneous beamwidths were observed to be 1 to 2 mm narrower than beamwidths determined from traditional single-point measurements that average the pressure measured at each location before computing beamwidth.

Treatment of calculi in kidneys with congenital anomalies: an assessment of the efficacy of lithotripsy

We studied the effectiveness of extracorporeal shock wave lithotripsy (ESWL) in the treatment of stones in kidneys with congenital anomalies to determine factors that may affect the results. Patients found to have renal calculi in kidneys with different types of congenital anomalies were treated using ESWL. All patients were investigated by intravenous urography (IVU) to confirm the diagnosis. J stents were inserted prior to therapy in renal units with calculi exceeding 1.5 cm in diameter. Complications encountered and factors affecting success using this treatment modality were analysed. Twenty-five patients (18 males, 7 females) were studied between August 1988 and July 2005. There were nine patients with horseshoe kidneys, eight with ectopic kidneys, three with malrotated kidneys, two with duplex renal system, and one patient each with polycystic kidneys and hypoplastic kidney. The IVU showed 31 isolated calyceal or renal pelvic stones with mean stone burden of 1.44cc. All 25 patients were treated by lithotripsy. Twenty-four (77.4%) renal units (in 19 patients) were completely cleared of stones, 2 (6.5%) renal units (2 patients) were partially cleared of calculi and the procedures failed in 5 (16.1%) renal units (4 patients). Out of five renal units in which the procedures failed, open surgery was performed in three renal units and percutaneous nephrolithotomy (PCNL) was performed in two. None of the 25 patients developed any major complications. No significant adverse changes in renal function tests were observed at 3-month follow-up. The stone-free rate was influenced and reduced by stone size and location in the pelvi-calyceal system. Calculi in kidneys with congenital anomalies may be treated successfully by ESWL as a first-line therapy in the majority of patients. With position modifications, localization of stones may be facilitated and disintegrated. The outcome in patients so treated does not differ significantly from that in those with normal kidneys.

A dual passive cavitation detector for localized detection of lithotripsy-induced cavitation in vitro

A passive cavitation detector (PCD) identifies cavitation events by sensing acoustic emissions generated by the collapse of bubbles. In this work, a dual passive cavitation detector (dual PCD), consisting of a pair of orthogonal confocal receivers, is described for use in shock wave lithotripsy. Cavitation events are detected by both receivers and can be localized to within 5 mm by the nature of the small intersecting volume of the focal areas of the two receivers in association with a coincidence detection algorithm. A calibration technique, based on the impulse response of the transducer, was employed to estimate radiated pressures at collapse near the bubble. Results are presented for the in vitro cavitation fields of both a clinical and a research electrohydraulic lithotripter. The measured lifetime of the primary growth-and-collapse of the cavitation bubbles increased from 180 to 420 microseconds as the power setting was increased from 12 to 24 kV. The measured lifetime compared well with calculations based on the Gilmore-Akulichev formulation for bubble dynamics. The radiated acoustic pressure 10 mm from the collapsing cavitation bubble was measured to vary from 4 to 16 MPa with increasing power setting; although the trends agreed with calculations, the predicted values were four times larger than measured values. The axial length of the cavitation field correlated well with the 6-dB region of the acoustic field. However, the width of the cavitation field (10 mm) was significantly narrower than the acoustic field (25 mm) as bubbles appeared to be drawn to the acoustic axis during the collapse. The dual PCD also detected signals from "rebounds," secondary and tertiary growth-and-collapse cycles. The measured rebound time did not agree with calculations from the single-bubble model. The rebounds could be fitted to a Rayleigh collapse model by considering the entire bubble cloud as an effective single bubble. The results from the dual PCD agreed well with images from high-speed photography. The results indicate that single-bubble theory is sufficient to model lithotripsy cavitation dynamics up to time of the main collapse, but that upon collapse bubble cloud dynamics becomes important.

In vivo pressure measurements of lithotripsy shock waves in pigs

Stone comminution and tissue damage in lithotripsy are sensitive to the acoustic field within the kidney, yet knowledge of shock waves in vivo is limited. We have made measurements of lithotripsy shock waves inside pigs with small hydrophones constructed of a 25-microm PVDF membrane stretched over a 21-mm diameter ring. A thin layer of silicone rubber was used to isolate the membrane electrically from pig fluid. A hydrophone was positioned around the pig kidney following a flank incision. Hydrophones were placed on either the anterior (shock wave entrance) or the posterior (shock wave exit) surface of the left kidney. Fluoroscopic imaging was used to orient the hydrophone perpendicular to the shock wave. For each pig, the voltage settings (12-24 kV) and the position of the shock wave focus within the kidney were varied. Waveforms measured within the pig had a shape very similar to those measured in water, but the peak pressure was about 70% of that in water. The focal region in vivo was 82 mm x 20 mm, larger than that measured in vitro (57 mm x 12 mm). It appeared that a combination of nonlinear effects and inhomogeneities in the tissue broadened the focus of the lithotripter. The shock rise time was on the order of 100 ns, substantially more than the rise time measured in water, and was attributed to higher absorption in tissue.

Improved focusing for extracorporeal shock wave lithotripsy of ureteral calculi

Extracorporeal shock wave lithotripsy of ureteral calculi, which is usually performed with the patient in the prone position, may be difficult or even impossible under certain conditions because x-ray visibility is poor or the calculus is out of range of the shock wave focus. Treatment conditions can be improved by a simple change in the positioning of the patient, using the shock wave head from the contralateral side. This method is applicable for the common Siemens Lithostar device.

Severe exacerbation of post traumatic syringomyelia after lithotripsy. Case report

A case of post traumatic thoracic syringomyelia is reported which presented with an acute and severe worsening of the spinal cord condition after extracorporeal shock wave lithotripsy was performed for ureteral stone. The worsening was probably caused by the shock waves reverberating the fluid within the intramedullary cavity producing further damage to the spinal cord.

In situ ESWL for ureteric calculi: the optimum treatment?

In situ ESWL for ureteric calculi is associated with good stone clearance rates but is it without significant morbidity? A review of 189 patients with single ureteric calculi (150 upper ureter, 39 lower ureter calculi) revealed an 89% stone clearance for upper ureteral calculi and 80% stone clearance for lower ureteral calculi. However 11% of patients with upper ureteral calculi and 20% of patients with lower ureteral calculi required additional intervention for complications or failed treatment. In situ ESWL may not be the optimum therapy for all ureteric calculi especially those in the lower ureter.