Smallest gauge for blood draw?

[KLouHerout]

Hi there,

I'm not a nurse, but a vet tech. I need to practice my venipuncture technique and was wondering about whether an insulin needle could be used? Or would the gauge need to be larger due to the viscosity of blood?

Thanks for any advice, my potential guinea pigs will appreciate it!!

[adventure_rn, MSN, NP]

Hmmm, this pediatric gauge size has really gotten me thinking....

Perhaps it depends on how much pressure you are using to withdraw the blood. For instance, we use 25g (or even 27g) needles in neonates, however we never attach them to vacutainers, but rather draw back slowly with a syringe (then squirt the sample into the vacutainer).

If you were to stick an adult with a 27 gauge needle and attach a vacutainer, it seems like the blood would become ridiculously hemolyzed since you'd have a ton of pressure creating a bottleneck through a tiny gauge needle. I suppose you could stick an adult with a 27 gauge needle and slowly draw back a couple of mLs of blood into a syringe (as we do with neonates), but there wouldn't really be any point to obtaining such a tiny sample in an adult. I'm guessing that the vacutainer manufacturers have a recommendation for minimum gauge size to prevent hemolysis based on the mmHg of pressure in the sealed tube.

It's kind of like the rate discussion regarding running blood products in neonates (i.e. we can run blood through a 24 g IV since our rates are slower), but in reverse.

Now, if you're manually drawing the blood into a syringe at a fairly slow rate, it seems like you could use whatever size is most appropriate for the animal. With a larger gauge, it seems that you could apply more pressure without the sample hemolyzing, therefore it you could draw it faster....

Someone, please rescue me from the rabbit hole....

[SummitRN, BSN, RN]

You don't have access to a needle that is too small.

Average erythrocyte has a diameter of 7μm

Largest cell you are going to find in the blood is a 20μm monocyte

Inner diameter needles:

27ga - 210μm

25ga - 260μm

24ga - 311μm

22ga - 413μm

You can see cells fit just fine.

It's about pressure vs diameter to get acceptable flow rates without lysing due to excessive flow velocity: smaller lumen -> higher flow velocity for a given rate -> turbulence slamming cells into the lumen wall and each other hard and repeatedly.

Rule of thumb, less likely to lyse using a larger volume syringe (or letting vascular/vacuum pressure work its job)

I can post equations if desired.

[LadysSolo]

I worked oncology, and after several rounds of chemotherapy, if the patient refused a port, often the largest gauge you could get in them was a 24. Blood infused fine, and sometimes we were pushing the limit on how long blood could hang, but it worked. I worked as a vet tech one summer, and dogs and cats have HUGE (for their body size) veins in their front legs! I was stunned how big they are.

[bgxyrnf, MSN, RN]

To be fair, I doubt adult volumes would run through a 24g.

The gravity flow rate of a 24ga BD Insyte Autoguard is 20 mL/min... so you could let blood hang and run the whole 330mL (or thereabouts) in well under an hour.

[canoehead, BSN, RN]

When I put 24G on a pump they start to back out of the vein because of pressure at 500ml/h, 400ml/h is iffy, but 300ml/h goes in fine. A unit of blood over 4 hours is no problem.

[SummitRN, BSN, RN]

When I put 24G on a pump they start to back out of the vein because of pressure at 500ml/h, 400ml/h is iffy, but 300ml/h goes in fine. A unit of blood over 4 hours is no problem.

So the question is, backpressure or pressure exerted directly by the infusion?

You don't need fluid dynamics equations to see that a 24G that backed out while infusing doesn't whip around spraying blood like a firehose. But, how much force does the infusion really exert?

First, we need to know the average velocity of the fluid flowing through the IV.

V=A/Q (velocity = cross sectional area / flow rate)

4*500(ml/hr)/pi/(311μm)^2

That gets you a flow velocity of about 1.8m/s (about 4mph)

But how much force?

F=m(*) * V (force=mass flux* velocity)

We need density (specific gravity) to get mass, SG for PRBCs is ~1.06

1.06(g/ml)*500(ml/hr)*1.8(m/s)=265μN

265μN of force is exerted by the flow of PRBCs at 500m/hr through a 24G catheter.

This a small fraction of the force exerted by gravity on the catheter. Thus, the flow rate is not pushing the catheter out of the vein due to the size of the catheter.

If it is not direct pressure, it would have to be backpressure. Backpressure in the vein is not directly related to catheter guage. Backpressure is related to infusion rate vs vein size/proximal structure, blood pressure, and blood viscosity. Does this only happen in 24G inserted in to the tiniest of distal veins?

[Susie2310]

So the question is, backpressure or pressure exerted directly by the infusion?

You don't need fluid dynamics equations to see that a 24G that backed out while infusing doesn't whip around spraying blood like a firehose. But, how much force does the infusion really exert?

First, we need to know the average velocity of the fluid flowing through the IV.

V=A/Q (velocity = cross sectional area / flow rate)

4*500(ml/hr)/pi/(311μm)^2

That gets you a flow velocity of about 1.8m/s (about 4mph)

But how much force?

F=m(*) * V (force=mass flux* velocity)

We need density (specific gravity) to get mass, SG for PRBCs is ~1.06

1.06(g/ml)*500(ml/hr)*1.8(m/s)=265μN

265μN of force is exerted by the flow of PRBCs at 500m/hr through a 24G catheter.

This a small fraction of the force exerted by gravity on the catheter. Thus, the flow rate is not pushing the catheter out of the vein due to the size of the catheter.

If it is not direct pressure, it would have to be backpressure. Backpressure in the vein is not directly related to catheter guage. Backpressure is related to infusion rate vs vein size/proximal structure, blood pressure, and blood viscosity. Does this only happen in 24G inserted in to the tiniest of distal veins?

This is nice, but it would be actually useful to most of us if you could explain the physics concepts i.e. velocity, flow velocity, units of measurement, and calculations you referred to in your example. Not everyone has studied physics.

[SummitRN, BSN, RN]

This is nice, but it would be actually useful to most of us if you could explain the physics concepts i.e. velocity, flow velocity, units of measurement, and calculations you referred to in your example. Not everyone has studied physics.

Canoehead mentioned 24G backing out at infusion rates of 500ml/hr but not lower flows, which begs the question why?

You don't need any math to know the answer, just deductive reasoning and newton's second law: For every action, there is an equal and opposite reaction.

200,000,000 ml/hr and 4" hose

If the force exerted by 500mL/hr through a 24G catheter sitting on a bedside table is insufficient to move it around, it is insufficient to back it out of a vein. Therefore, (back)pressure in the vein caused by adding 500mL/hr of relatively viscous fluid (PRBCs) must be causing it (in what I assume are small distal veins).

The math was just an informal proof confirming that deductive logic... that it wasn't the reactive force of the infusion. I used simplified algebraic fluid dynamics which should be mathematically understandable to those who remember high school physics. That was sufficient for a logic check.

One could express this with complex calculus based fluid dynamics, attempt to account for fluid shear forces vs reactive forces, IV line weight, tissue friction, tape, or even utilize computer modeling that accounts for turbulence, viscosity, and suspended particulates (cells). One could even estimate shearing forces on the RBCs. There might even be literature that informs us of shearing forces required for lysis. But I don't see the use of that exercise...

[Julius Seizure]

So the question is, backpressure or pressure exerted directly by the infusion?

You don't need fluid dynamics equations to see that a 24G that backed out while infusing doesn't whip around spraying blood like a firehose. But, how much force does the infusion really exert?

First, we need to know the average velocity of the fluid flowing through the IV.

V=A/Q (velocity = cross sectional area / flow rate)

4*500(ml/hr)/pi/(311μm)^2

That gets you a flow velocity of about 1.8m/s (about 4mph)

But how much force?

F=m(*) * V (force=mass flux* velocity)

We need density (specific gravity) to get mass, SG for PRBCs is ~1.06

1.06(g/ml)*500(ml/hr)*1.8(m/s)=265μN

265μN of force is exerted by the flow of PRBCs at 500m/hr through a 24G catheter.

This a small fraction of the force exerted by gravity on the catheter. Thus, the flow rate is not pushing the catheter out of the vein due to the size of the catheter.

If it is not direct pressure, it would have to be backpressure. Backpressure in the vein is not directly related to catheter guage. Backpressure is related to infusion rate vs vein size/proximal structure, blood pressure, and blood viscosity. Does this only happen in 24G inserted in to the tiniest of distal veins?

You are my IV science hero.

[canoehead, BSN, RN]

If it is not direct pressure, it would have to be backpressure. Backpressure in the vein is not directly related to catheter guage. Backpressure is related to infusion rate vs vein size/proximal structure, blood pressure, and blood viscosity. Does this only happen in 24G inserted in to the tiniest of distal veins?

It's not necessarily distal, but definitely small veins. And only with high rates.