Catheter Balloon Inflation 1

[sph99]

Hi

I'm new to bioengineering but I've been asked to calculate the theoretical inflation time for a catheter balloon. Can any one point me in the right direction? Any guidance appreciated.

 

[BigInch]

V = Q * t
V = volume, Q is flowrate, t is time.

What would you be doing, if you knew that you could not fail?

 

[sph99]

Thanks. However I should have mentioned that it must be inflated to a specified pressure.

 

[MiketheEngineer]

And how big is the balloon and what pressure??

 

[sph99]

The balloon is 60mm long, and 2mm dia. the inlet pipe/supply pipe is 0.7mm dia. I want to inflate the balloon to 6atm.

 

[C2it]

I'm guessing you will use a hand pump. With the volumes you are dealing with, I would expect inflation to be practically instantaneous.6 atm sounds a bit painful too. What if it bursts ??

 

[sph99]

Yes I am using a hand pump. It has a flow rate of 1ml/sec in inflation and 5ml/sec in deflation. I have attached a portion of my code below. I find that the inflation rate is to slow. In practice this can be inflated in 4.6 sec. The deflation time is too fast, as again in practice the deflation time is 1.3 sec. Have I an error in my units or is there something I am missing?

Thanks for your help.

 

[sph99]

Hopefully this explains the problem better:

I need to calculate the time to fill and then deflate a catheter balloon. This consists of a hand pump connected to a long tube which then widens out to the balloon vessel. I need to reach a certain pressure in the balloon. The balloon is already expanded and it is assumed no further expansion of the material will occur.

I know the volumetric flow rate (Q) from the hand pump. The length (L) of the tube is 1400mm. I have calculated the pressure drop across the tube (delP).

How to I determine how long it will take to fill both the tube and the balloon from vacuum to a given pressure and then how long to pull a vacuum on the system? Any help appreciated.

I previously used Pressure = mass*Gas Constant*Temperature/Volume. I did this at intervals until the pressure in the vessel equaled the pressure required, giving me the time required. However I found that this did not match my experiment. (The calculated times were much greater than the actual)

 

[IRstuff]

Where are you getting the flow rate numbers? I'm having difficulty comprehending how the flow rate could be independent of pressure differential. Is this "hand pump" something your hand actually compresses at some repetition rate, i.e., like a blood pressure cuff pump? At a minimum, your deflation flow rate must be proportional to the pressure differential, and constrained by the resistance of the tube.

As general rule, you should refrain from double posting: thread755-322700 However, given that no one else replied to the other thread, going here is probably reasonable.

TTFN
faq731-376

 

[IRstuff]

You use ml here, but your code appears to refer to grams, and the two combined suggests water, i.e., 1gm/ml, but you are obviously talking about air.

TTFN
faq731-376

 

[sph99]

The flow rate numbers are got from tests I have done on the hand pump. It is like a syringe except that instead of pushing the plunger it is twisted. I have calculated the change in pressure across the length of long tube (Hagen Poiseuille) using this flow rate. I have done this since posting the code. But how should I proceed?

In the code (which is not giving me the correct answers) I have used the mass flow rate and to calculate the mass in the balloon and from that calculate the pressure in the balloon. I have converted the 1ml/sec to 1000mm3/sec and I am checking this model with water but need to ultimately do it with saline.

 

[IRstuff]

Everything is unclear. Your code mentions "saline" yet you're apparently using ideal gas law. Saline is a liquid; the ideal gas law does not apply.

TTFN
faq731-376

 

[sph99]

As I said I am new to this area and now I understand better about the gas law. But what should I use instead?

 

[sph99]

Another way of explaining what I want to do is:

I have a pump, which will pump water through a long length of pipe ( long in relation to its diameter) into a tank. it is a closed system. there will initially be a vacuum in the pipe and tank. i want to find out how long it will take to fill the tank to a prescribed pressure.

Thanks for your feedback.

 

[vpl]

sph, Do you have a copy of Crane's Technical Paper 410 Flow of Fluids? If not, you probably need to buy one. What you're wanting to do is described fairly well in there. You can get from Amazon or from Crane: Flow of Fluids



Patricia Lougheed

******

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[sph99]

Thanks vpl, I'll get a copy.

 

[IRstuff]

Is this for school? Applicability of the ideal gas law to only gases is covered in high school physics, and would have certainly been covered in your engineering curriculum in college.

TTFN
faq731-376

 

[sph99]

It is not for school. Up to now I have avoided all forms of fluid dynamics and am not at hot on them as I should be. I got away with this when I was working in structures but not so now in biomedical.

 

[IRstuff]

Crane410 will certainly get you started. However, I don't see that Crane necessarily covers something that resembles catheter balloon inflation. The scenario is either a balloon-like structure that resists inflation because of its skin's opposition to stretching, or the balloon is pushing against tissues that are resisting the advancement of the balloon, or both.

In any case, you've not stipulated anything that resembles a resistance to inflation

TTFN
faq731-376

 

[vpl]

You're absolutely right IRStuff. However, it is a cheap enough reference that will at least give him some basics on fluid flow that might get him closer to the ball park. I don't have time to read attachments such as his code, and was just picking up on the comments about using the ideal gas law to model a liquid such as water or saline.

Since we don't know if the cathetar is going to be placed inside the body and we also don't know the properties of the "balloon" fabric -- which really does play a role -- there's only so much that can be done. We can't really teach sph99 fluid flow over these forums -- and to be honest, he couldn't teach me (at least) structural engineering either. The book will give him a practical starting point. Between that, his test data, and his coworkers, hopefully it will be enough to solve his problem. If not, well, then the usual advice is to hire the right expertise, isn't it?

Patricia Lougheed

******

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.