Technical
Generation of external shock waves which are transmitted through the body and focussed on stone causing fragmentation.
Short steep positive pressure wave (shock) is followed by a longer negative pressure period. Total wave is 4 microseconds.
The shock waves are designed to build to sufficient strength only at the target (the stone).
Shock waves do not travel well through air – hence need good coupling mechanism between machine and patient.
Stone fragmentation occurs directly by the shock wave hitting the stone or indirectly by the collapse of bubbles.
- Spall fractures – microcracks caused at the entry and exit points of the wave i.e. stone/fluid interface – direct fragmentation from positive pressure wave
- Squeezing-splitting (circumferential compression) – shock wave in stone and in fluid surrounding travels at different speeds, causing compression force and stress at either end
- Shear stress – shear waves or transverse waves generated when initial wave hits stone
- Superfocusing – amplification of stresses inside the stone due to its geometry
- Cavitation – formation and subsequent collapse of bubbles
- Negative pressure in second part of the wave causes bubble formation, which collapse violently, which in liquid forms a ‘microjet’
Parts of ESWL machine:
- Energy source
- Coupling mechanism (water bath or gel)
- Focussing device
- Localisation / imaging (ultrasound or fluoro)
Types of shockwave generators:
Optimising success for ESWL:
Success is dependent on:
Patient factors:
- Obesity (skin to stone distance)
- Movement / pain control
Stone factors:
- Density and composition
- Location (lower pole worst, IP angle)
- Size
Machine/procedural factors:
- Machine efficacy
- Targeting and coupling
- Single vs multiple treatment sessions
Techniques:
- Optimal rate is 60 shocks / min – slower rate increases SFR but increases duration of treatment
- Number of shock waves dependent on lithotripter and power – no consensus – I use 4000 shocks as a maximum
- Stepwise increasing power can induce vasoconstriction and reduce haematoma rates and injury
- No role for routine pre-ESWL stent placement
- Antibiotics generally unnecessary in the absence of instrumentation or risk factors
- If ectopic beats are an issue – the shockwaves can be ‘gated’ to the ECG (R wave)
- Post ESWL MET may improve SFR and may reduce pain
- EUA – pacemaker patients may be treated with ‘appropriate technical precautions’, AICD must be re-programmed
Contraindications to ESWL
Absolute:
- Distal obstruction
- Active UTI
- Coagulopathy
- Pregnancy
- Calcified renal artery aneurysm in vicinity of stone
Relative:
- Pacemaker / AICD
- Severe obesity
- Calcified pancreas
- AAA
- Hard stones i.e. cystine
- Spinal deformities or other issues with positioning
- Severe hypertension
Complications of ESWL
- Renal haematoma or renal bleed (manage conservatively a la renal trauma)
- Infection
- Steinstrasse and obstruction
- Pain
- Adjacent organ injury – pancreatitis
- Hypertension in long term
- Cardiac ectopic beats and rarely arrythmia
- Failure to fragment or pass stone
- Skin bruising
Steinstrasse
Accumulation of gravel or small fragments in the ureter after ESWL, which may cause obstruction and pain.
4 – 7 % of ESWL cases.
Major risk is stone size.
Management:
- Conservative +/- MET if asymptomatic or pain controlled
- If febrile – standard infected obstructed kidney management (? nephrostomy preferred over stent in EAU guidelines)
- Up front ureteroscopy an option in afebrile patient
- ESWL is mentioned as an option in guidelines
How can ESWL outcomes be improved?
Pre-op / patient selection:
- Non obese patient
- Single stone, upper pole or renal pelvis
- Easily targetable
- Not too big, not too hard
Intra-op:
- GA preferred, increases success and minimises respiratory movement
- Good coupling agent
- Good localisation and targeting
- Avoid ureteric stent
- Ramp up power to induce vasoconstriction
- Keep frequency low around 60 – 80 / min
Post-op:
- Consider alpha-blockers
- Consider percussion inversion and diuresis