This Is MS Multiple Sclerosis Knowledge & Support Community

LaPlace's law has come up before
http://www.thisisms.com/forum/chronic-c ... ml#p154607
but I still don't understand fully. Pressure is the same everywhere inside the balloon. But there are differences in wall tension. At the shoulder of the balloon, wall tension is at half. I don't think I am understanding the difference between wall tension and pressure. The shoulder is gentler on the vein.

Here is some more on this:
http://books.google.com/books?id=pncT2b ... on&f=false

It is important for one to understand the physical principles that govern the effects of balloon dilation. Dilating force is the outward force exerted against a luminal stenosis. It is dependent on balloon diameter, the balloon's inflation pressure, compliance of the balloon material, balloon length, and the degree of the stenosis. The law of Laplace states that the force of tension (T) exerted on the wall of the inflated balloon is directly proportional to the pressure (P) within the balloon and the radius (R) of the balloon (T=PxR). Thus, a balloon with twice the radius of a smaller balloon will exert twice the tension on the wall of a balloon for a given inflation pressure. If the diameter of the balloon is kept constant, the tension on the wall of the balloon will increase linearly with increases in inflation pressure. Because the tension on the balloon wall translates into dilating force, the dilating force generated by a balloon is directly proportional to the balloon's diamter and inflation pressure. Therefore, larger balloons will require less pressure than smaller balloons to generate substantial dilating force. Similarly, larger vessels such as the abdominal aorta or the common iliac arteries will require less pressure to dilate and rupture. The pressure within an angioplasty balloon is universally measured in "atmospheres."

A longer lesion has a larger total area of stenosis than a shorter one does. As a result, the dilating force created by a balloon is greater in a longer lesion. In addition, the dilating force genereated in a lesion is directly proportional to the degree of stenosis for a fixed balloon diameter and inflation pressure. As such, more dilating force will be generated in a tight, high-grade stenosis than in a shallow stenosis. This principle is governed by the so-called clothesline effect. That is, the radial force vector generated by a balloon in a stenosis is greated when the balloon has more of a "waist" on it. This outward force vector decreases as the waist on the balloon diminishes. Therefore, continuing to increase balloon inflation pressure to eliminate a minimal, residual waist will result in relatively small incremental changes in dilating force and is more likely to rupture the balloon. In this setting, low-profile, "high-pressure" balloons offer an advantage over low-pressure balloon catheters. In the past, if a residual waist could not be resolved, the procedure was either terminated or a larger balloon was used. With the use of larger-diameter balloon, more dilating force can be generated, but the risk of rupturing the vessel or creating a dissection "flap" is also increased.

Knowing the balloons in this detail is absolutely not necessary as a patient going for the procedure. But as a TiMSer trying to keep up with the doctors' discussion, it might be necessary....

So wall tension is the dilating force, and the dilating force is halved on the shoulder compared to the middle of the balloon.

Pressure is from the inside of the balloon, as a force pressing equally on all parts of the balloon, but up by the shoulder, the radius is diminished, and pressure times radius gives wall tension, thus reducing the wall tension, which is the force that actually pushes against the vein.

High pressure balloons will increase wall tension significantly.

When a high grade stenosis is ballooned, the waist might give way but a low grade or residual stenosis remains. The dilating force is greater against a high grade stenosis than a low grade stenosis. Thus, paradoxically, it might be easier to treat a high grade stenosis than to treat a low grade stenosis. That is interesting, if it is true.

I think this is suggesting that high pressure balloons are of use against residual waisting, and that the benefit of a high pressure balloon is that it creates greater dilating force without going up to a larger balloon, and by eliminating the need to go to a larger balloon size, it eliminates the increase in risk of rupture or dissection that go along with an increase in balloon size. But the high pressure might carry risks of its own. More to come....

Cece, you are amazing. Any chance of you heading to med school? For certain, you'd be accepted!

I thought this was med school???

Cece wrote:I thought this was med school???

HA HA! Too funny!

Does everyone understand the implications for balloon placement? The method in question is to place the balloon mostly in the lower vein, which goes into the innominate vein, which is larger and can take it. By placing the shoulder exactly so, then the valve stenosis gets the full dilating force from the mid-part of the balloon while the healthy vein above is spared.

I don't know if any other IRs are doing this besides Dr. Sclafani, but it is a valid question to ask your doctor. If you understand LaPlace's law, which is pretty well illustrated in the diagram above, you can see how this works.

The more typical approach would be to center the balloon over the stenosis, which means that the healthy vein above the stenosis will get ballooned with the full dilating force. If the healthy vein gets ballooned with the full dilating force, it might be fine especially if the balloon size was well chosen or it might be one of the unlucky ones that scars or clots. There are many other factors that go into whether a vein is damaged or not, but I think it is logical to consider that this is one of the factors, and we all want to get through the procedure with undamaged veins and healthy flow.

Cece wrote:When a high grade stenosis is ballooned, the waist might give way but a low grade or residual stenosis remains. The dilating force is greater against a high grade stenosis than a low grade stenosis. Thus, paradoxically, it might be easier to treat a high grade stenosis than to treat a low grade stenosis. That is interesting, if it is true.

I didn't define high grade stenosis vs low grade stenosis.... Personally I had two high grade stenoses (80% and 99% blocked) which just means big blockages. A 30% stenosis would have to be a low grade stenosis. Not sure where the line is for when it becomes high grade.

My left jugular was as high-grade a stenosis as you can get:

It's been said that second or third angioplasties might not be as successful as the first angioplasty and I wonder if this is a potential cause. During the first angioplasty, the stenosis is at its tightest and most high-grade. At a future angioplasty, it has been weakened by the first angio and it may not be as tight or the collagen as strong or the stenosis as high grade. Thus, paradoxically, when the balloon is inflated, it does not have as much dilating force against the stenosis as it did when it was a higher grade stenosis. This can be compensated for by increasing the atms or pressure of the balloon, but if high pressure balloons are not used, the stenosis will not get as much force against it as the first time.

To go back to the example of my left jugular, it was initially that extremely high grade 99% stenosis prior to being ballooned the first time. It was a lesser but still bad 70% stenosis just prior to being ballooned the second time, which was five months later. So in my example, the same size and pressure of balloon would have had less dilating force against the stenosis the second time as it did the first time.

Cece, you have a terrific sense of humor to go with all your smarts!

You've given a great presentation by making a difficult subject easy to understand. I never would have thought that I could ever understand LaPlace's law, but because of you, I do understand. Thank you!

That's a very interesting theory, it makes sense to me as does the placement of the shoulder of the balloon. Thanks for mentioning the difference between the high grade and low grade stenosis. I googled but didn't find anything that really explained it well

You might try a real animal-type balloon to see how much harder a slightly thicker one is to inflate. Try it with a small hose on your bathtub faucet, to see the effect of water-filling it. You can actually feel the difference diameter makes in how hard it is to deform the shape. I don't recommend overfilling it to bursting.

Remember if the balloon is closed, and full, the inside pressure is the same for every part of the surface. If you have stretched the wall completely everywhere, it will have the same thickness everywhere. So, with the same wall thickness, and the same pressure,why, when you twist the balloon to make an animal (air only!) does it get easier the more you twist?

If you thought there was a low probability of it bursting, you could pre-twist your balloon, insert it until the twist is right inside the valve, and then untwist it. As the twist gets bigger around, it would have more force, until it is the same diameter as the rest. The blood at either end might be affected. You might want to try it in vitro first...

1eye wrote:YSo, with the same wall thickness, and the same pressure,why, when you twist the balloon to make an animal (air only!) does it get easier the more you twist?

Dilating force is the outward force exerted against a luminal stenosis. It is dependent on balloon diameter, the balloon's inflation pressure, compliance of the balloon material, balloon length, and the degree of the stenosis.

When you twist a balloon, you've made the balloon shorter in length, at least for the portion you are working on. Do shorter balloons have less wall tension or more? If it's less, that would fit with your observation that it gets easier when the balloon is shorter.

Let me credit the source of my materials:
Image-guided interventions, Volume 1 By Matthew A. Mauro is the book that's linked at the top of the page, from which I pulled a few passages. Typos in those passages are my own.
The diagram of the balloon came from here http://www.summitmd.com/pdf/pdf/080905_5.pdf which is a presentation from Seung-Jea Tahk, MD., PhD. at Ajou University Medical Center in Suwon, Korea.

edited to add: And let's toss out my response that the shortening of the balloon is what makes animal balloons easier as they go on.

The length of the balloon also has an effect on dilating force when the balloon material is compliant. Compliant balloons will continue to increase in diameter and deform with increasing inflation pressure. Therefore a shorter compliant balloon with a smaller surface will exert a more concentrated dilating force than a longer compliant balloon will. In contrast, the dilating force generated by a balloon constructed from noncompliant material is not affected by increasing balloon length.

Are air animal balloons compliant or noncompliant? The angioplasty balloons are noncompliant, so changes in length does not change the amount of dilating force that they generate. The definition of compliant is in here too:

Compliance is a measure of how much a balloon will stretch beyond a predetermined diameter when a force is applied to it. All balloon materials are elastic. Consequently, all balloons have some inherent degree of compliance. A completely noncompliant balloon will not stretch beyond a predetermined diameter – despite increases in inflation pressure – and maintains its profile and shape with repeated inflations.

You want a noncompliant balloon for angioplasty because otherwise you would not have as precise of control over how much the vein was stretched. With such things at stake as the possibility of undertreatment and recurring stenosis or overtreatment and vein damage, precision is king, which is also why IVUS is king, because IVUS gives exact measurements which improves the precision of the treatment.

munchkin wrote:That's a very interesting theory, it makes sense to me as does the placement of the shoulder of the balloon. Thanks for mentioning the difference between the high grade and low grade stenosis. I googled but didn't find anything that really explained it well

When I was out in Brooklyn, Dr. Sclafani gave me the words "high grade stenosis," in response to me talking about how big my blockages were.

Cece wrote:

1eye wrote:So, with the same wall thickness, and the same pressure,why, when you twist the balloon to make an animal (air only!) does it get easier the more you twist?

Dilating force is the outward force exerted against a luminal stenosis. It is dependent on balloon diameter, the balloon's inflation pressure, compliance of the balloon material, balloon length, and the degree of the stenosis.

When you twist a balloon, you've made the balloon shorter in length, at least for the portion you are working on. Do shorter balloons have less wall tension or more? If it's less, that would fit with your observation that it gets easier when the balloon is shorter.

I think the main source of the difference (I was kind of asking the question in a platonic way) is your Pascal/Laplace thingy.

The length of the balloon also has an effect on dilating force when the balloon material is compliant. Compliant balloons will continue to increase in diameter and deform with increasing inflation pressure. Therefore a shorter compliant balloon with a smaller surface will exert a more concentrated dilating force than a longer compliant balloon will. In contrast, the dilating force generated by a balloon constructed from noncompliant material is not affected by increasing balloon length.

You want a noncompliant balloon for angioplasty because otherwise you would not have as precise of control over how much the vein was stretched. With such things at stake as the possibility of undertreatment and recurring stenosis or overtreatment and vein damage, precision is king, which is also why IVUS is king, because IVUS gives exact measurements which improves the precision of the treatment.

I also respectfully disagree. I think the term compliant is usually used in reference to some man-made spec (compliant or non-compliant with the spec). In this case, comply may have a different technical meaning, referring to compliance as a physical property. My guess is that in plasty, you don't want the thing to either suddenly inflate, or burst, so non-compliance is a way of saying that if you blow it up past a certain length, and keep increasing the pressure, it won't inflate or explode, but will keep increasing outward force for some interval before it bursts, after the full length is reached. I think these things are not like party balloons at all, and will not even inflate the same way. Inflation usually is very sudden, and might not be what you want.

Anyway the phenomenon I was referring to, for the sake only of example, was all due to the dramatic change in tension with the sudden change in diameter, when you twist an (already inflated) animal type balloon, not length, which doesn't change much. I could be completely out to lunch on this. I am not a physicist, either. The animal artists leave an uninflated segment to accommodate some of the air when they twist. They are actually reducing the diameter of the inflated part, and they want the displaced air to inflate the end without bursting.

Too bad you can't have the IVUS and the balloon in there simultaneously.