Ziad Ali Profile picture
13 Oct, 16 tweets, 10 min read
Mini-tweetorial on Shockwave MOA
1\ 3 types of lithotripsy
1)Extracorporeal (ESWL) – break kidney stones from the outside
2)Intracorpoeal (ISWL) – break kidney stones from the inside
3)Intravascular (IVL) – fracture calcium in the vasculature from the inside
2\ESWL
Because ESWL hits the stones from outside the body, it requires a lot more energy to work. Both the peak positive and negative energies are very high. To allow this depth of penetration, the energy has to be FOCUSED.
3/ISWL
On the contrary, ISWL is up close to the stone (via the ureter), requires less energy (10-fold less) because there is less distance to travel to the stone, and can be UNFOCUSED to dissipate some energy and prevent off target injury.
4\ Focused energy
Analogy is a flashlight. The flashlight reflector collects the light rays and FOCUSES them creating a hotspot, with minimal spill. This is ESWL. An electrode creates a short high voltage pulse that is focused through a reflector and is aimed at the stone.
5/ Unfocused energy
Analogy is a lightbulb. There is no reflector in a lightbulb and so the light rays are UNFOCUSED creating mostly spill, with minimal hotspot. This is how ISWL works. An electrode creates a short high voltage pulse that is disseminated very close to the stone.
5/ The more energy the better right?
Nope. Safety > Efficacy. If we used focused energy, we could pulverize the calcium but damage the soft tissue within the artery. Look at the magnitude of difference in energy between IVL and ESWL. Focused energy in the coronary = LAZER.
6\ How does it break the calcium?
Bunch of ways. The predominant IVL MOA is the 1st wave which hits the Ca like a hammer, compressing it, generating initial fractures. Subsequent pulses create more fx. There are other mechanisms, which are less prominent but still important.
7\ Can the MOA help?
The closer the electrode to the Ca the better. Size balloon 1:1 (use imaging to size). Place electrode perpendicular to the max Ca. This a) braces the Ca and b) maximizes the hotspot to the max Ca (blue arrows). Know where your emitters are on the balloon.
8/ Why is IVL so safe?
Shockwave through water doesn’t do much, and fortunately the impedance of soft tissue is pretty close to water. Similarly hitting a pillow with a hammer doesn’t do much. But, the impedance of Ca is high! So when the hammer hits the plate, it breaks.
9/ Why not just use an NC or ultra-high pressure balloon.
Analogy is a karate chop versus a weight lifter. Both can generate the same amount of force, but the karate chop, a sudden burst of energy is more likely to break concrete blocks than a weight-lifter pushing on them.
10\ How much calcium
Does Ca fracture with less than 180 degree of Ca with a balloon? With a balloon, Ca is like a wishbone. There needs to be enough wishbone for the balloon to expand within and break it. IVL uses a different MOA to create fx, 180 degrees is not necessary.
11\ Is one crack enough?
While we are all used to seeing linear fractures on OCT, that probably isn’t the only mechanism of fracture induced by IVL. In 6 I went through the different mechanisms of fracture.
12\There’s cracks and then there is cracks
- Cracks due to compression are parallel to the applied force.
- Tensile cracks are perpendicular to the applied force.
- Shear cracks are perpendicular to the tensile stress.
13\
IVL utilizes electrical energy to generate acoustic pressure waves which impact calcium.
Acoustic pressure impacts calcium by multiple mechanisms.
Compared to ESWL and ISWL, IVL energy is lower in keeping with its goal to fracture, but not pulverize calcium.
14\
Optimal balloon size is critical to effective IVL. Use imaging.
Understanding the spatial placement and separation of the IVL electrodes can guide pulse management.
The mechanism of calcium fracture is an acoustic pulse, short and sharp.

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