Extracorporeal

Shockwave LithotripsyDr.3amer 3alwan

3rd year board in urology

HISTORICAL OVERVIEW

High-energy shockwaves have been recognized for many years. Example of high-

energy shockwaves is the potentially window shattering sonic boom created when

aircraft pass beyond the speed of sound.

Engineers at Dornier Medical Systems during research on the effects of shockwaves

on military hardware to determine if the shockwaves striking the wall of a military

tank would damage the lungs of enemy member,discovered the possibility of safely

applying shockwave energy to human tissue when an engineer touched a target

body by chance. The engineer felt a sensation similar to an electric shock,

although the contact point at the skin showed no damage at all. In the course of

this effort the engineers discovered that shockwaves generated in water could

pass through living tissue (except for the lung) without damage but that brittle

materials in the path of the shockwaves would be fragmented.

Methods and Physical Principles

1-Electrohydraulic Generator:

• shockwave is generated by an

underwater spark discharge.

• For the shockwave to be focused

onto 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).

• Disadvantages are the

substantial pressure fluctuations

from shock to shock and a

relatively short electrode life.

2.Electromagnetic Generator:

An electrical current pass through conductors producing strong magnetic field moving the plate against the water and thereby generating a pressure wave.

The energy in the shockwave is concentrated onto the target by focusing it with an acoustic lens.

Advantage: Introduction of energy into the patient’s body over a large skin area, which may cause less pain.

3.Piezoelectric Generator:

These generators are made of a ceramic

elements each of which can be induced to

rapidly expand by the application of a

high-voltage pulse.The piezoelectric

elements are usually placed inside of a

spherical dish to permit convergence of

the shockfront.

The advantages include the focusing

accuracy, a long service life, and the

possibility of an anesthetic-free

treatment because of the relatively low

energy density at the skin entry point of

the shockwave.

Imaging Systems

1.Fluoroscopy Alone :

Advantages of fluoroscopy include its 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, and

the ability to display anatomic detail.

The disadvantages include the exposure of the staff and patient to

ionizing radiation, the high maintenance demands of the equipment, and

the inability to visualize radiolucent calculi without the use of

radiographic contrast agents.

2. Ultrasonography Alone

Advantages: is inexpensive to manufacture and to maintain compared

with fluoroscopic systems. Another advantage is in the treatment of

children and infants when one is concerned about the dose of ionizing

radiation. In addition, ultrasonography can localize slightly opaque or

nonopaque calculi.

Disadvantages: requires a highly trained operator. It is almost impossible

to view a 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.

3. Combination of Ultrasonography

and Fluoroscopy

In some cases combining ultrasonography and fluoroscopy for stone

localization are clearly advantageous but each system has a drawback

that limits one of the functions of the system.

Mechanisms of Stone

Comminution

A typical shockwave involves an initial

short compressive front with

pressures of about 40 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 µsec.The ratio of the

positive to negative peak pressures is

approximately 5:1.

1-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

2- 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

3- 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.

4- 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.

5- Cavitation

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.

6- Dynamic fracture

the damage induced by SWL

accumulates during the course of

the treatment, leading to the

eventual destruction of the

stone.

Bioeffects of SWL

1. Acute Extrarenal Damage

visceral injuries, such as perforation of the colon, hepatic hematoma,

splenic rupture, pancreatitis, and abdominal wall abscess.

Extrarenal vascular complications such as rupture of the hepatic artery,

rupture of the abdominal aorta, and iliac vein thrombosis.

Thoracic events, such as pneumothorax and urinothorax.

shockwaves could induce cardiac arrhythmia, an observation that led to

electrocardiographic synchronization with R-wave triggering on the

Dornier HM3 device.

The development of diabetes was related to the total number of

shockwaves and the power level of the lithotripter.

2. Acute Renal Injury

Hematuria is so common that it may be considered an incidental finding,

and its severity is rarely of concern. Although hematuria was initially

considered to be a consequence of irritation of the urothelium as stones

were fragmented by shockwaves, it is now known that shockwaves

rupture blood vessels and can damage surrounding renal tubules .

risk factors:

• unsatisfactory control of their hypertension at the time of SWL

• diabetes mellitus

• coronary artery disease

• Obesity.

3. Chronic Renal Injury

accelerated rise in systemic blood pressure because

1. subcapsular hematomas can induce hypertension, such changes are

generally transient.

2. Mesangial proliferation after SWL could induce hypertensive changes

decrease in renal function

increase in the rate of stone recurrence (Stone recurrence rates may be

higher after SWL because of residual stone debris)

induction of brushite stone disease.

Techniques to Optimize SWL

Outcome

wider focal width increase the likelihood of stone breakage because the kidney

tends to move, as a consequence of respiratory motion, the stone may move in

and out of a narrow focal zone.

Optimal coupling permits the efficient transfer of energy from the lithotripter

to the patient; poor coupling will reduce stone breakage. Most commonly,

energy transfer through a coupling medium is attenuated by air pockets in the

coupling interface itself.

Decrease the rate of shockwaves because the dynamic bubbles are given a

longer time interval to dissipate with a slower rate and therefore have less of a

shielding effect and energy draw from subsequent shocks.

Decrease the energy setting on the machine. Increasing the power setting on

most electromagnetic lithotripters actually narrows the focal zone which

decreases stone breakage and may also increase the risk of renal injury.

To reduce stone motion, urologists can perform SWL with general anesthesia,

which will control the patient’s respiratory rate and volume.

Anesthesia

1. in 1980s regional or general anesthesia was used in all instances because

the unmodified HM3 device produced a powerful shockwave and

treatment at recommended energy levels caused intolerable pain.

2. Short-acting agents, such as the narcotic alfentanil and the sedative-

hypnotics midazolam and propofol can be used in various combinations to

allow most SWL treatments with any lithotripter.

3. topical agents e.g.EMLA cream, a mixture of lidocaine and prilocaine, has

been shown to reduce anesthesia requirements during SWL . EMLA cream

should be applied at least 45 minutes before SWL.

4. Children and extremely anxious individuals may be served best by

general anesthesia. Patients who received general anesthesia

experienced a significantly greater stone-free rate than did those

patients who underwent intravenous sedation. Possible explanation for

this is the more controlled respiratory excursion that is conferred by the

general anesthetic.

Fragmentation

Safe shock wave dosage is unknown. Shock waves induce trauma,

including intrarenal and perirenal hemorrhage and edema, and thus the

minimal shocks needed to achieve fragmentation should be given.

Determination of adequate fragmentation during treatment may be

difficult. Initial sharp edges become fuzzy or blurred .Stones that were

initially visualized may disappear after successful fragmentation.

Intermittent visualization ensures accurate focusing and assessment of

progress and eventual termination of the procedure.

Preoperative antibiotic prophylaxis

IndicationsThe American Urological Association Stone Guidelines Panel has classified ESWL as a

potential first-line treatment for ureteral and renal stones smaller than 2 cm.

Indications

The reasons for poor clearance

of fragments from the lower pole

after SWL are unclear. The

gravity-dependent position of

the lower pole calyx may impede

the passage of stone

fragments,The examination of

the angle formed between the

lower infundibulum and the renal

pelvis and if the angle greater

than 90 degrees it should

facilitate drainage of fragments

from the lower pole.

Contraindications

Absolute

1. Pregnancy

2. uncorrected blood clotting disorders (including anticoagulation)

3. known renal artery stenosis

4. Acute UTI or urosepsis

5. Uncorrected obstruction distal to the stone

6. Abdominal aortic aneurysm

Relative

1. Uncontrolled HT

2. Morbid obesity

3. Renal ectopy or malformation

4. Renal insuficiency

5. Cardiac pacemaker

New applications

1. Heel spur

2. Pancreatic stone

3. Gall stone

4. Peyronie disease

5. Erectile dysfunction

Postoperative care

Patients should be encouraged to maintain an active ambulatory status

to facilitate stone passage.

Fluid intake should be encouraged.

Severe pain unresponsive to routine intravenous or oral medications

should alert the physician for perirenal hematomas. In such a situation,

CT scan should then be undertaken to stage the injury.

Steinstrasse (stone street) Asymptomatic individuals can be followed up

with serial KUBs and ultrasonography. Severe pain or fever requires

intervention. Percutaneous nephrostomy drainage is usually

uncomplicated owing to the associated hydronephrosis. Decompressing

the collecting system allows for effective coaptation of the ureteral walls

and encourages resolution of the problem. If steinstrasse does not resolve

,retrograde endoscopic manipulations is required.