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Just a Big Prick

A patient brought in by a BLS crew is rushed to your ER after sustaining a gunshot wound to the abdomen. The patient is alert, oriented and slightly diaphoretic. Bleeding however is profuse. You know he will need volume resuscitation and thus an IV. What is the best IV cannula size to obtain on this patient? 18G, 16G, 14G? WRONG. The best IV size is the ONE YOU CAN OBTAIN THE FASTEST.

IV placement is a skill every medic and bedside nurse must master, but the act of inserting an IV is only half the battle. Knowing the science behind the procedure and what size to use is the other. Establishing a free flowing, adequate, and reliable point of access to a patient’s circulatory system can make the difference between life and death. Yet I have seen too many Ricky and Rhonda Rescues’ reach for that 14G and waste valuable time inserting and reinserting, fishing, and destroying precious sites on a patient who in the past likely had PICC line due to such poor vasculature.

PIVs lined up in order of size. A 14G is similar in size to a household toothpick

We all want to be the one to yell out “14G Left forearm”, and feel like Billy Badass, but is it worth risking valuable time? That 20G in the hand you obtained in a mere 45 seconds just provided valuable blood work and is infusing, which means you can now channel your inner @thetraumaguy, place that 14G in the greater saphenous, chug a beer, grunt and take a victory lap.

Many of the Ricky Rescues are now rolling their eyes. Some are scoffing, some are just closing the page, hell, some are tossing their whiskey spiked coffee at their poor EMT in disgust. “I would rather have no access than have a 24G, its useless, there is no point, I would just get an IO, blah blah blah blah”. THIS IS IMPORTANT TO NOTE, I am NOT saying large bore IVs should not be used. They most definitely have their place and are necessary in many emergent situations, but let me explain some of the reasoning behind my thought process.

First, not every patient is a trauma patient. In fact, most of your patients won’t be trauma patients. Medical emergencies dominate the field! Strokes, Seizures, hypoglycemic events, STEMIs, etc. These patients DO NOT NEED RAPID INFUSION OF VOLUME! A 24G will just as effectively administer tPa to a stroke, deliver Ativan to stop a seizure, resuscitate a diabetic coma, or providee morphine (not to inferior MIs) and troponin values from a STEMI.

Second, to say no access is better than small access is immature and dead wrong. I touched on this above, but even with the smallest IV, critical information can be obtained from the blood. I now can get a point of care CHEM7, CBC, Lactate, VBG, and more. I can give clotting factors, platelets’, cryoprecipitate, pressors, and perhaps most importantly…ACLS DRUGS…which if you are a prehospital provider (aside from military and some critical care programs) is really all that matters because you aren’t giving blood en route.

Third, IOs are amazing, and in the situation described in the initial scenario, there is no contraindication to obtaining one. But they aren’t without their complications, and they don’t always FLOW well! Which is the whole purpose of this article…FLOW. Have you ever had an IO in a patient and just couldn’t flush saline…or draw blood…or use it on a pump/rapid transfuser without the alarms constantly stopping the infusion stating “downstream occlusion”? Ya, it happens. Sure, perform some quick carpentry skills, get the IO, and then get a peripheral too. Cause you know…options!

Ok, you’ve made it this far…your EMT is still sobbing from second degree burns to the crotch, but perhaps you are less frustrated. Good! That’s really all that matters! Now that you have refilled your Irish coffee, let’s look at the physics of IV/IO access.

All PIV packages have the gauge, length and the flow rate. The flow rate of this 18G is 110ml/min

IV cannulas differ in size for one reason, to provide various flow rates. In situations that call for rapid volume resuscitation, the American College of Surgeons recommends two large bore (14G or 16G) IVs be placed. Why so big? Well let’s look at the usual flow rate of these IVs (which, fun fact, is printed on every IV cannula package!)

A standard length 14G can infuse up to 345ml/min. A 16G at the same length can pump in 215ml/min. Compare this to the most commonly used size, the 20G, at the same length will infuse 63ml/min. Those decreases in flow are quite literally exponential and were discovered by a French dude named Jean Léonard Marie Poiseuille.

Poiseuille wanted to better understand human blood flow within the body, and thanks to his diligent work, we can also! Poiseuille's law (see the figure) states that in a uniform pipe, flow is a product of pressure X radius^4 divided by viscosity X length. IVs manipulate this via radius (gauge) and length. So, the bigger and shorter the cannula, the faster the flow…or as I like to say “short and thick does the trick”.

Let’s break down the IV variables a little further. Radius is perhaps the most important characteristic to manipulate when trying to increase flow. In the equation it is to the fourth power, so every small change in the radius is exponential. Of course, just changing the radius and not changing the other variables won’t always have the desired effect, but the bigger the tube, the more liquid you can place through it.

The longer the fluid is in contact with the walls of a tube, the less flow. This is why length must be noted. A 16G that is ¾” will have a higher flow rate than a 16G that is 1¾” when all other variables are constant. This is why adding multiple extension sets to an IV line will decrease the flow.

As seen in the equation, other variables aside from IV radius and length are at play here. Pressure also plays a large part. The greater the pressure difference, the greater the flow! This, is why in emergent situations, infusions are hung high, in a pressure bag, or in a rapid transfuser.

Viscosity is also important to note. Think of viscosity as how thick the liquid is. Lactated ringers will be a lot less viscous than whole blood which is less viscous than D50. The more viscous or “thicker’ the fluid the harder it is to push through the tube, and thus, the less flow.

And finally, we must remember that Poiseuille's law applies to laminar flow, or non-turbulent flow of liquids through pipes of uniform section. When we start adding extension sets, piggybacks, and needless endcaps to IV cannulas, we are creating turbulent flow as well as adding pipes of differing radius’! This will significantly hinder flow.

In fact a study performed by Anesthesia and Analgesia showed that crystalloid flow rates were reduced by 29% to 85% from control in catheters >18 gauge, while red blood cell flow reductions ranged from 22% to 76% when a needless leur lock adapter was attached to the hub of an IV cannula. That's is an insanely significant decrease in flow! This is why in situations where volume replacement is critical, IV tubing should be connected DIRECTLY to the cannula hub and NOT to an extension set or to a needless leur lock adapter. These will just restrict flow and can make your 14G about as useful as an 20G!

The last two photos are examples of leur lock needless adapters. The first has an extension set attached that not only is adding length, but has a smaller radius than most IV tubing leading to less overall flow and increased turbulence!

Now look at the second, Think of all the turbulence occurring inside of this needless adapter as blood or saline attempts to circumvent the valve. This is why patients from the field, ER or OR will often have IV tubing connected directly to the hub,... so don't assume your coleauges were just lazy!


A Lehn, Robert & B Gross, Jeffrey & H McIsaac, Joseph & E Gipson, Keith. (2015). Needleless Connectors Substantially Reduce Flow of Crystalloid and Red Blood Cells During Rapid Infusion. Anesthesia and analgesia. 120. 10.1213/ANE.0000000000000630.