Discuss the principles of ESWL. (TU 2071-5)
- A source external to the patient’s body generates a shock wave. Specifically, the energy source rapidly deposits pulses of energy into a fluid environment, which results in the generation of a shock wave.
- Shock waves are surfaces that divide material ahead, not yet affected by the disturbance, from that behind, which has been compressed as a consequence of energy input at the source.
- These waves move faster than the speed of sound, and the stronger the initial shock, the faster the shock wave moves.
- Their behavior is characteristic of the propagation of nonlinear waves.
- Although the shock waves in lithotripters generate large pressures, they are relatively weak in that they induce only slight compression and deformation of a material.
- The uniqueness of the shock wave lithotripter is in its exploitation of shock wave focusing. Relatively weak, nonintrusive waves are generated externally and transmitted through the body.
- The shock waves build to sufficient strength only at the target, where they generate enough force to fragment a stone
Generator types in ESWL
- Electrohydraulic (Spark Gap) Generator
- Electromagnetic Generator
- Piezoelectric Generato
Electrohydraulic (Spark Gap) Generator
- - a spherically expanding shock wave is generated by an underwater spark discharge
- - For the spherically expanding shock wave to be focused on a calculus the electrode is placed at one focus (termed F1) of an ellipsoid and the target (the kidney stone) is placed at the other focus (termed F2).
- - effectiveness in breaking kidney stones
- - substantial pressure fluctuations from shock to shock and a relatively short electrode life
- - Another issue to consider is that as the electrode deteriorates, it wears down, and a 1-mm displacement of the electrode tip off F1 can shift F2 up to 1 cm off the initial target.
- - Whereas the electrohydraulic lithotripter produces focused shock waves by bouncing spherically expanding shocks off an ellipsoid reflector, the electromagnetic generators produce either plane or cylindrical shock waves.
- The plane waves are focused by an acoustic lens; the cylindrical waves are reflected by a parabolic reflector and transformed into a spherical wave.
- - more controllable and reproducible than electrohydraulic generators because they do not incorporate a variable in their design such as the underwater spark discharge.
- - introduction of energy into the patient’s body over a large skin area, which may cause less pain.
- - a small focal point can be achieved with high-energy densities, which may increase its effectiveness in breaking stones.
- - This generator will deliver several hundred thousand shock waves before servicing, thereby eliminating the need for frequent electrode replacement
- - small focal region of high energy results in an increased rate (3.1% to 3.7%) of subcapsular hematoma formation.
- - The piezoelectric lithotripter also produces plane shock waves with directly converging shockfronts.
- - These generators are made of a mosaic of small, polarized, polycrystalline, ceramic elements (barium titanate), each of which can be induced to rapidly expand by the application of a high-voltage pulse
- - The piezoelectric elements are usually placed on the inside of a spherical dish to permit convergence of the shockfront.
- - The focus of the system is at the geometric center of the spherical dish.
- - focusing accuracy
- - long service life
- - possibility of an anesthetic-free treatment because of the relatively low-energy density at the skin entry point of the shock wave. For this reason, piezoelectric lithotripters in general tend to produce less discomfort than do lithotripters with other energy sources.
- - insufficient power it delivers, which hampers its ability to effectively break renal stones
An imaging system in ESWL?
- Three basic designs
- - fluoroscopy alone
- - ultrasonography alone
- - combination of ultrasonography and fluoroscopy
- Fluoroscopy alone -
- Advantages of fluoroscopy
- - familiarity to most urologists
- - the ability to visualize radiopaque calculi throughout the urinary tract
- - the ability to use iodinated contrast agents to aid in stone localization
- - the ability to display anatomic detail
- Disadvantages of Fluoroscopy
- - exposure of the staff and patient to ionizing radiation
- - high maintenance demands of the equipment
- - inability to visualize radiolucent calculi without the use of radiographic contrast agents
- Ultrasonography aloneAdvantages - no radiation
- - Sonographic localization of a kidney stone requires a highly trained operator.
- - It is almost impossible to view a kidney stone in areas such as the middle third of the ureter or when there is an indwelling ureteral catheter.
- - Once a stone is fragmented it is difficult to identify each individual stone piece
Character of pressure pulse generated by an electrohydraulic shock wave lithotripter?
- - initial short and steep compressive front with pressures of approximately 40 megapascals (MPa) that is followed by a longer, lower amplitude negative (tensile) pressure of 10 MPa, with the entire pulse lasting for a duration of 4 microseconds
- - the ratio of the positive to negative peak pressures is approximately 5 : 1.
Explain the stone disintegration mechanism in ESWL. (TU 2068,70-5)
Mechanism of stone fragmentation by lithotripter?
- Spall fracture - Once the shockwave enters the stone it will be reflected at sites of impedance mismatch. One such location is at the distal surface of the stone at the stone- fluid (urine) interface. As the shockwave is reflected, it is inverted in phase to a tensile (negative) wave. If the tensile wave exceeds the tensile strength of the stone, there is an induction of microcracks that eventually coalesce, resulting in stone fragmentation, which is termed spallation.
- Squeezing-splitting or circumferential compression - The shockwave advances faster through the stone than in the fluid outside the stone. The shockwave that propagates in the fluid outside the stone produces a circumferential force on the stone, resulting in a tensile stress in the stone that is at its maximum at the proximal and distal ends of the stone.
- Shear stress - The shock waves propagate through the stone and will result in regions of high shear stress inside the stone. Many materials are weak in shear, particularly if they consist of layers, because the bonding strength of the matrix between layers often has a low ultimate shear stress.
- Superfocusing - The shockwave that is reflected at the distal surface of the stone can be focused either by refraction or by diffraction from the corners of the stone.
- During the negative pressure wave, the pressure inside the bubble falls below the vapor pressure of the fluid, and the bubble fills with vapor and grows rapidly in size (almost three orders of magnitude)and then collapse violently, giving rise to high pressures and temperatures.
- the damage induced by SWL accumulates during the course of the treatment, leading to the eventual destruction of the stone
Describe how to optimize stone disintegration. (TU 2071-5)
Ways to Improve Shock Wave Lithotripsy Outcomes
- Appropriate coupling
- Water-soluble lubricant applied by hand
- Decrease rate to low (60–70 shocks/min) or intermediate (80–90 shocks/min)
- Image frequently and stop shocking once fragmented
- Do not use a preset number of shocks Ramping protocol
- Treat with low power escalating to higher levels
- General anesthesia
- Do not use a ureteral stent
- Consider alpha-blockers for medical expulsive therapy
- Consider percussion, diuresis, inversion therapy
Bioeffects of ESWL?
- Acute Extrarenal Damage- trauma to organs such as the liver and skeletal muscle
- - visceral injuries, such as perforation of the colon, hepatic hematoma, splenic rupture, pancreatitis, and abdominal wall abscess
- - cardiac arrythmia
- Acute Renal Injury: Structural and Functional Changes
- - rupture blood vessels and can damage surrounding renal tubules
- - hematoma
- Chronic Renal Injury: Structural and Functional Changes - Clinically there are four potential chronic renal changes that may be associated with SWL treatment. They are an
- - accelerated rise in systemic blood pressure
- - decrease in renal function
- - an increase in the rate of stone recurrence
- - induction of brushite stone disease.
Reversible and irreversible changes in kidney after ESWL?
- Reversible changes -
- Mild tubular necrosis
- Casts and red blood cells in tubular lumen
- Vacuolar changes of tubular lumen
- Mild interstitial edema and hemorrhage
- Irreversible changes resulting in loss of renal tissue
- Disruption of nephrons
- Extensive interstitial edema
- Large hematomas of cortex and medulla
- Rupture and occlusion of veins and arteries
- Fracture of glomerular and peritubular capillaries
Explain the factors responsible for stone clearance. (TU 2070-5)
Factors Negatively Affecting ESWL?
- Stone composition (cystine, calcium phosphate especially brushite, calcium oxalate monohydrate, matrix)
- Stone attenuation ≥1000 HU
- Skin-to-stone distance >10 cm (morbid obesity)
- Renal anatomic anomalies (horseshoe kidney, calyceal diverticulum)
- Unfavorable lower pole anatomy (narrow infundibulopelvic angle, narrow infundibulum, long lower pole calyx
Discuss the indications, contraindications and complications of ESWL. (TU 2068,70-5)
Contraindication of ESWL?
- Uncorrected coagulopathy or bleeding diathesis
- Untreated urinary tract infection
- Arterial aneurysm near stone (renal or abdominal aortic aneurysms)
- Obstruction of urinary tract distal to stone
- Inability to target stone (skeletal malformation)
Procedural recommendation for ESWL? EAU 2020
- Ensure correct use of the coupling agent because this is crucial for effective shock wave transportation. Strong
- Maintain careful fluoroscopic and/or ultrasonographic monitoring during shock wave lithotripsy. Strong
- Use proper analgesia because it improves treatment results by limiting pain-induced movements and excessive respiratory excursions. Strong
Side effects of ESWL?
- Hematuria (up 44%)
- Subcapsular or perirenal hematoma
- Intermittent renal colic, emergency department visits for pain control, and steinstrasse.
- Difficulty with focusing SWL can lead to injury to adjacent structures including the colon, vasculature, lung, spleen, and pancreas
- SWL has been associated with an increased risk for hypertension.