Boston Scientific

Overview
A variety of the techniques are available for dealing with nephrolithiasis. These include laser, electrohydraulic, pneumatic, ultrasonic, electromagnetic, piezoelectric, and shockwave lithotripsy. This article reviews the techniques for the primary forms of lithotripsy utilized in urology.

Extracorporeal Shockwave Lithotripsy (ESWL)
ESWL works by producing shockwaves from an external source aimed and focused on an intracorporeal source. The shockwaves build strength and force as they approach the stone to cause fragmentation. The shockwaves are produced from either an electrohydraulic, electromagnetic, or piezoelectric source. The patient is placed on the ESWL table and fluoroscopy is used to place the stone close to the crosshairs, indicating the point of maximal energy delivery. Once appropriately positioned the procedure can begin. Most ESWL machines have a standard protocol for energy delivery, all of which start with minimal energy delivery that can then be increased during the procedure. The shocks per minute can begin at range of 60-90 shocks/minute and are often increased to up to 120 shocks per minute. There is some evidence that a slow shock rate (60-80 shocks/minute) is more effective than fast shock rates (eg. 120 shocks/minute). Throughout the procedure, fluoroscopy should be intermittently used to ensure that the stone is in the crosshairs. Once the stone begins to break, the energy no longer needs to be increased. ESWL can induce cardiac arrhythmias by way of energy delivery to the heart. If the patient starts to have premature ventricular contractions (PVCs) during the case, he/she should be gated. This allows the shockwave production to correlate with the R wave of the QRS complex of the heart rate, thus delivering the shock during the resting phase of the cardiac cycle. Leaving a patient that is beginning to show signs of cardiac instability (eg. PVCs) ungated is dangerous as these patients can progress to ventricular tachycardia, supraventricular tachycardia, or other cardiac arrhythmias. Other potential complications of ESWL include perinephric hematoma, bleeding, obstruction from stone fragments, and pain (Figure 1).

Figure 1: ESWL Machine

Laser lithotripsy
Lasers produce energy by exciting electrons that can then release the energy in the form of light. Most lasers function by turning this energy into a plasma bubble that produces a shockwave upon collapse. The most widely used laser lithotripter is the holmium:YAG laser and is available in 200, 365, 550, and 1000 micrometer fibers. The holmium:YAG laser has a pulse duration of 250-350 microseconds, functions at a wavelength of 2140 nm, and a depth of penetration of 0.5-1.0 mm. This particular laser works by causing stone vaporization through a photothermal mechanism, rather than by producing a shockwave. When performing laser lithotripsy, the fiber should be placed in contact with the stone and irrigation should be available as fragmentation may cause decreased visibility. The tip of the laser must be visible at all times while activated as it will fragment whatever is in front of it, including a wire or the ureteral wall. It is best to avoid drilling a hole through the stone. In general, the best technique is to start on the center of the stone and work outward, vaporizing the stone. By the end of the procedure, there should only be one fragment left that may be removed with a basket. If being placed through a flexible instrument then a 200 or 365 mm fiber should be used. The procedure should begin with 0.6 joules and a pulse rate of 6 hertz. If need be, the pulse rate can be increased for quicker fragmentation (Figure 2).

Figure 2: Laser Lithotripsy

Electrohydrolic Lithotripter (EHL)
The EHL uses an underwater spark discharge to generate a plasma channel that vaporizes the water near the electrode and produces a hydraulic shockwave. The hydraulic shockwave impacts and fragments the stone. If the shockwave misses the stone, it will release energy on whichever portion of the urothelium encountered and can cause ureteral or bladder perforation. Also, if the stones are loose, the shockwave can propel stones in a retrograde fashion. The probe should be placed >2 mm from the end of the endoscope and within 1 mm of the stone. The procedure should begin with 50-60 volts in short or single bursts. The energy can be increased as need to effectively fragment the stone. If the insulation from the tip of the catheter loosens, a new probe should be used (Figure 3).

Figure 3: EHL

Pneumatic Lithotripters
A pneumatic lithotripter works by essentially jackhammering the stone by direct contact. This mechanism is very effective in fragmenting stones and is also very safe, unless the lithotripter probe is moved off of the stone, potentially damaging whatever is in front of the probe. Success rates are 73%- 100%, with ureteral perforation rates up to 2.6%. However, given the mechanism of action, the chance of retrograde migration is greater than with other modalities. The probe is placed in direct contact with the stone, a clear visual is imperative, and the stone is pinned against the bladder, ureteral wall, or kidney prior to activating the probe. Again, the goal is to fragment the stone into multiple small fragments that can be removed or pass spontaneously.

Figure 4: Pneumatic Lithotripter

Figure 5: Stone with Indentation and fragments from pneumatic lithotripter

Ultrasonic Lithotripsy
Ultrasonic lithotripters work by using ultrasound waves at 23,000 to 25,000 Hz to cause stone fragmentation by vibration with minimal effect on surrounding tissues. These lithotripters also have a suction port that allows small fragments to be removed during lithotripsy. Using these lithotripters in the ureter is not as effective as aforementioned lithotripters. However, they are very effective when utilized via percutaneous access for treatment of renal stones.

The probe is applied directly onto the stone surface, but done gently to prevent the stone from moving. It is best to pin the stone against the urothelium and fragment the stone until it can be removed or the fragments can pass.

There are a combination of Pneumatic/Ultrasonic lithotripters available.

Cystolithalopaxy forceps
Cystolithalopaxy forceps are used to crush stones in the bladder. The forceps will crush anything in them, including the bladder wall. The bladder must be distended so bladder wall isn’t involved in the forceps. This is an effective technique; however, visualization can become difficult with multiple uses secondary to urothelial trauma.

Figure 6: Cystolithalopaxy

Andrew Harris, MD
Cheif Resident, University of Pennsylvania