The numbers may seem confusing but I’ve always wondered something and when I found a few expired saline bags in the storage closet I decided to do a completely scientific and infallible test.

Now that Rogue Medic is sitting up in his chair I will confess this is in no way an “accurate” demonstration, but all elements were equal so I can only assume that the same thing would happen proportionately under different situations.

Like, say, actually starting lines on people.

In the past I have made the in field decision, like many of us do, to get the 18g when others may try for the 16g.  But if I know I can get the 20g, but maybe get the 18g, shouldn’t I err on the side of some access is better than no access?

Maybe, maybe not.

So I went out to the workbench, hooked up a 20g angiocath and opened the line wide open.

How long do you think it took to empty a 1000ml bag?  More on that later.

Taking into account the widespread myth that D50 can’t fit through a 20g needle (then where does it go?) what is the difference between a 20g and an 18g?

“Large bore.”

In my wonderfully arranged protocol manual there is an entry under abdominal pain and multi-systems trauma that mentions starting “2 large bore IVs…” but does that mean a 16g? 18g?  Certainly not a 20g, right?

But it seems like such a small difference to step from the 18g to the 20g and slamming fluid into these kinds of patients can be debated all year long, but I have to follow my protocols.  So what is the difference in fluid flow between those two catheters?


The 20g drained in 18:08.

The 18g drained in 12:31.

That’s a considerable difference and far more than I would have expected.  Maybe a few minutes, but 6?  So now wondering how fast a 16g can go, alas, I am out of expired bags.  But if the trend continues I would expect to be able to drain that 1000ml bag in 6 minutes or so.

6 minutes to run in a liter of fluid or 18 minutes just from going 16g to 20g?  That’s huge.

We all know that in a bumpy rig with other concerns that line can get away from us sometimes and you look up at what you thought was a slow bolus and see an empty bag, so I used to err on the side of time instead of volume, just in case.

But that was back when I had 45 minute transport times.  Now that I’m just a few minutes out should I change my way of thinking of getting access?

We’ll see, but I just wanted to pass along my experiment.

Stay safe,


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