Janne,

How are the collection tubes evacuated?

With the same pump? If so, then irrespective of the size of the vessel, they will all have the same pressure inside them.

Hi James, thanks for the quick answer.

The blood collection tubes are from different vendors and I don´t know how the evacuate their tubes.

These tubes are not completely evacuated. For example if you have a 16x100mm tube with a filling volume of 5ml, the vendor evacuates this tube to an extent so that if you collect blood it stops at 5 ml. Then again there are tubes with same outer dimensions but with 8 or 9 ml filling volume (=higher vacuum).

The theory is that if the suction in these tubes is too high, the amount of ruptured red blood cells rise (=hemolyse). What I am trying to find out is: How does the vacuum pressure in a 16x100mm cylindric tube with 5ml filling volume differ compared to a tube with 13x75mm outer dimensions and 4.5ml filling volume.

I admit it is a bit puzzling - Please let me know if my explanation was not claer.

Thanks again for any help.

One potential way to evaluate differences is to use a 10 cc syringe with needle. Fill the syringe with air and insert the needle into the tubes to determine how much air they remove from the syringe. One could then use a vacuum or pressure equation to determine the pressure inside of each tube. Years ago, we simply used a vacuum pump to evacuate tubes that had lost their vacuum for various reasons and then reused the tubes. I do not recall any differences in hemolysis.

Hi Jack,

thank you for your very practical approach! I will try it immediately and respond to this thread, hopefully with a solution :)

Hmmmm... I don´t seem to get anywhere. I think I´ll just leave it with the following statement: If you fill 16x100mm/9ml tubes to the maximum, they are nearly completly filled. If you do the same with 13x75/4.5ml tubes, they are only filled to 50-75%. Subsequently the vacuum pressure in the latter tube has to be lower.

If anyone wants to disagree - please do so!

Thanks to Jack and James once again for your contribution!

Use the equations here to find the vacuum level in the tubes by knowing the amout of air they inspire from the syringe and the dimensions of the tubes.

http://www.eng-tips.com/viewthread.cfm?qid=288689

Hi Jack,

Thanks for your suggestion. I looked through the thread and tried the following formulas:

σ= P*D/(2*t)

and

σ = P x [ R^2 - (R-t)^2 ] / [ R^2 - (R-t)^2 ] + αΔTE

σ = Stress (psi)

P = external pressure (psi) - this should be 14.7

R = external radius (in) - this is 16mm/0.63in or 13mm/0.51in

t = thickness (in) - this is 1mm/0.04in

α = coefficient of thermal expansion (in/°F) - ??

ΔT = change in temp (°F) - ??

E = modulus of elasticity (psi) - ??

However I´m not sure how to put the numbers right. How do I use these formulas if I haven an cylindric tube 13mm in diameter and 75mm in length (Wall thickness 1mm) which is pre-evacuated - I know that the vacuum is capable of sucking 4.5ml of liquid and want to calculate the vacuum in negativ psi?

Sorry if I am a little slow on the uptake, but I´m a physician :)

Thanks again!

I have not done the calculations and am traveling, so it will be a few days before I can get to this. I'd say the thermal expansion and elasticity would be negligible. 

No hurry. I hope your travelling is for vacation purposes ... :)

I forgot to mention: I the first formula D=diameter of the tube

Instead of air, which is expandable, I would suggest using water with the method described by Jack. It has the advantage that you will notice any leakage in the system, and you will get less bias due to an unknown difference of pressure between the system and ambient pressure at the end of aspiration.

In fact, I would use a 10cc syringe filled with water, measure how much is aspirated, measure how much the tube can hold when totally full with water. The difference (Vresidual) will give you the quantity of air that was originally in the tube, at ambient pressure.

According to the Boyle-Mariotte law, Psearched=Patmx  Vresidual/ Vfull

The only thing you need to make sure is that the friction between piston and cylinder of the syringe does not hinder or bias the pressure equalisation when aspiring the liquid, i.e. the difficulty is to make sure you've got the same pressure inside and outside the tube-syringe system at the end of the aspiration.

With this method, you don't need to care about wall thickness, calculating cylinder volumes, etc.

Hi Adrian,

thank you so much for your approach.

So I have these three pre-evacuated blood collection tubes:

For a better understanding I attached an image of the three tubes. As already menttioned, the come pre-evacuated, so that when you attach the needle the tube will suck blood to a defined volume - this volume ist marked with the black small line.

I measured the Vfull which is 10.5 ml for the 16x100mm tubes and 5.9ml for the 13x75mm tube. Taking the predefined filling volume of the tubes of 8ml, 5ml and 4.5ml, this leaves Vresidual with 2.5ml,  5.5ml and 1.4ml respectively.

Patm should be 14.7 psi. When I put these numbers in the equation you posted I get for Psearched 3.5 psi, 7.7 psi and 3.5psi, respectively.

If I now want to define the negative pressure of the vacuum of an empty pre-evacuated tube, do I have to subtract Psearched from Patm? If so, I would end up with -7psi for tube #2 and -11.2psi for tubes #1 and #3. This would fit quite well to the theory that the vaccum pressure is lowest in the tube #2 (16x100mm/5ml).

Hi Janne,

The residual pressure in the tubes is the one you indicated. There is no such thing as a "vacuum pressure". Vacuum pressure is 0.

If you want to express it as a negative pressure in comparison with the atmospheric pressure, i.e. considering normal atmospheric pressure as "0", negative pressures would be their difference to 14.7 psi, hence -3.5, -7 and -11.2 psi respectively. The fuller the tube is supposed to be at the end of aspiration, the bigger the difference to the atmospheric pressure has to be.

Keep in mind that it's the atmospheric and venous pressures that push the blood into the tube, until the pressures inside and outside the tube are equal. Venous pressure may push a little more blood into the tube if you want to be very accurate, but then, the collapse of the vein and the blood viscosity should also be taken in consideration.

Depending on what you need the data for, the latter factors may be negligible.

Hi Adrian,

Thanks a lot. You really helped me out here.

I think that venous pressure is negligible in this case, since I just want to know the difference between tubes. All influencing factors (temperature, sea-level, venous pressure, ...) can be assumed equal for all three tubes.

Thanks again to Adrian, Jack and James !