A question for the (hydraulic) engineer type people

[Radshooter]

When we do CT Scans at work, we use IV contrast a lot of times. This is injected with a power injector at a rate of anywhere from 0. 5 cc/second to as high as 5. 0 cc/second, depending on what type of exam we do. We typically use 3 sizes of IV catheters, again depending on the type of exam we are doing. They are 18 guage (internal diameter of 1. 35 mm), 20 guage (internal diameter of 1. 15 mm), and 22 guage (internal diameter of 0. 95 mm).



Here is my question: How can I determine the PSI coming out the various sizes of catheters based on flow rate and the size of the IV? In other words, if I am pushing 4. 0 cc/sec through the 1. 15mm diameter IV, what is the pressure of the contrast coming out of the catherer and into the vein?



Hope this makes sense. It is something I have always wondered about.



Thanks... ... . Steve

 

[Jacobs]

I am not a hydraulic engineer type people, but here goes.



Pressure is the result of resistance to flow. At the end of your catheter where the fluid enters the vein, the pressure is the same as the pressure in the vein. As you go upstream the pressure increases due to resistance to flow. Given a constant flow rate, flow velocity out of the catheter will increase as catheter diameter decreases and pressure at the pump will increase as well. If catheter diameter is increased for a given flow, velocity decreases and pressure at the pump does likewise.



You never develop more pressure than it takes to overcome resistance to flow.



Hope this helps



Bruce

 

[1stgen4evr]

Pressure question

The contrast pressure entering the vein is more than the pressure in the vein or it would not flow. I would imagine that the pump is a positive displacement type of pump which lends itself to volumetric calibration. Thus you can set the pump to a given amount. The pump will push the set volumn and the pressure will necessarily be higher for the smaller catheter for the reasons stated by Jacobs. For practical purpose, you can put a pressure guage at or very near the catheter and get a reading which will show the pressure without involving resistance through whatever tubing you use. Again, for parctical purpose, I think you will find that a pressure guage at the pump will show the result with negliable influence as to resistance. This will be relative to the fact that pressure is unchanged throughout the system, which is a basic law of fluid dynamics.



Are you concerned that the smaller catheter would raise the stream velocity such that there could be erosion in the vein?



James

 

[Radshooter]

"Are you concerned that the smaller catheter would raise the stream velocity such that there could be erosion in the vein?"



Yes James, this is exactly why we (I) worry about the pressure. We try to never push more than 2cc/sec through a 22 ga. IV. Most places I work have a policy of having at least a 20 ga. IV if pushing for a 4cc/sec test, even if it means another stick for the patient. If the vein blows and it is not caught quickly and the test halted, the contrast will extravisate into the surrounding tissue, and this can lead to serious problems.



The injector looks like a giant syringe that can hold up to 200cc of fluid. Instead of a steel needle on the end, there is a flexible small diameter tube about 4 foot long that connects the injector to the patient's IV. The plunger is motor driven and programmable for both flow rate and volume injected. Hope this makes things a little clearer.



Is there a formula that uses diameter and flow rate to calculate pressure? Putting a guage on the line is not practical in most clinical settings.



Jacobs..... you said "You never develop more pressure than it takes to overcome resistance to flow. " Does this mean that if I push 2cc/sec through a 20 ga. IV, it will have the same pressure as if I was pushing 4cc/sec through the same size IV? Wouldn't the 20 ga. IV have the same resistance in both cases? How does velocity relate to pressure? If velocity increases, wouldn't pressure increase also, if all other factors (diameter) remained the same?



Thanks guys. I am not trying to be stupid here, but I have asked many people this same question in hospitals, and have never gotten a satisfactory answer. Usually I get that "why would you care" look.



Steve

 

[1stgen4evr]

Pressure/volocity

Does this mean that if I push 2cc/sec through a 20 ga. IV, it will have the same pressure as if I was pushing 4cc/sec through the same size IV? Wouldn't the 20 ga. IV have the same resistance in both cases? How does velocity relate to pressure? If velocity increases, wouldn't pressure increase also, if all other factors (diameter) remained the same?



Wrong and partly wrong. If you push the same VOLUMN of product over the same time interval through the two different size catheters, the larger catheter will exhibit/allow lower pressure. Pressure is going to be the driving influence of velocity. That is, as the pressure goes up, so will the volocity. So if you use the larger dia catheter, you will be able to infuse more product at lower pressure. Again, I imagine that the pump is positive displacment so it does whatever you set as to volumn. It WILL push it at a higher pressure with the smaller catheter and can have the fire hose effect in the vein that you are concerned with. I also suspect that there is some result from the condition of the patient. That is, one patient might tollerate a greater pressure where another would show serious damage from the same size/flow parameter. Just as in older individuals, a minor bump will cause serious hematoma where a young lad can take much abuse and shrugg it off.



Your concern is ligitimate and I also know that there are people in the system who don't know enough to realize there is a difference. And that covers the ones who give a D--- in the first place. Too many don't even give a D---.



James

 

[WDaniels]

RADSHOOTER,

While I was taking my EMT class I read an article about the doctors in "NAM" when the severly wounded came with extream blood loss. They would take the largest big bore needles they could find and put one in each arm and take blood (right out of the fridge) and both doctors would squeez the bags as hard and fast that could to replace the blood. They would put the blood in so fast that the patients would often go hypothermic because the cold blood would go in so fast that the body couldn't warm it. I would say that the pressure exerted by the battlefield doctors was quite high. ????

WD

 

[Radshooter]

WDaniels... . That's amazing. That would be a lot of pressure. We keep our contrast warm to decrease viscosity and make it flow easier. I have always respected people that can get a big bore IV in a patient that has no veins like those severe trauma cases. I have enough trouble with a 22 ga. sometimes!!

 

[DaveN]

I'm not a doctor, but I play one on tv... .



I think that what you're concerned about is the velocity of the fluid coming out of the tube, this is what does the damage. The pressure of the fluid at the end of the tube is technically the same as the blood pressure. However, the velocity is what will do the damage. You stated that you try to limit your cc per second. You can calculate the velocity by dividing your cc/second by the cross sectional area of your tube (in cm^2) this will give you your velocity of the fluid in centimeters per second.



Assuming you feed the same amount of fluid (cc/sec), the larger tube would obviously have a reduced velocity since it has a larger cross sectional area. Conversely, if you were pushing double the cc/sec through the same size tube, then your velocity would double.



Now that you have your velocity, how fast is too fast? Not sure. This probably depends on alot of things, the distance of the end of the tube from the vein wall, the vein size, etc etc etc.

 

[WDaniels]

I'm not a doctor either, but I did stay at a Holiday Inn last night. :-laf :-laf

WD

 

[Radshooter]

Thanks DaveN.

I had never thought of it in terms of velocity. Interesting concept, but makes perfect sense. Just when I thought I was getting it figured out, here comes another variable to the equation!



Thanks

Steve