Thread by @rbarbosa91

🧵for the new residents and students on the meaning of 'French' and 'Gauge'.

We've covered this on here in the past, but a lot of new people are starting on surgical rotations, so let's look at them again.

As usual, we'll cover their history and what they mean.
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If you just want the short version:

The term 'French' is a unit of measurement of the *outer diameter* of a circular (or nearly circular) device.
1 'French' = 1/3 millimeters (or 0.33 mm if you prefer)

It's a direct measurement, so larger tubes have higher 'French' sizes.

'French' isn't named for France 🇫🇷 , the country.

It's (indirectly) named after a person -- Joseph-Frédéric-Benoît Charrière, a knifemaker who was born in Switzerland 🇨🇭 but moved to Paris later on. He went on to become the premier surgical instrument maker of his time.

When Charrière made instruments, he picked his own unit of measurement that corresponded to 1/3 of a millimeter...this was the 'Charrière'.

But English speakers struggled to pronounce 'Charrière', so the companies changed it to 'French'. So 1 'French' became 1/3 millimeter.

Only things that are circular tubes have a 'French'. So things like central lines, NG tubes, Foleys, etc. The 'French' (almost) always refers to the *outer diameter* of a tube. For example, an 18 Fr NG tube has an outer diameter of 6 mm.
(We’ll look at exceptions to this later).

Here is a 7 Fr triple lumen central line. Again, the '7 Fr' refers to the line's *outer diameter*.

It doesn't matter how many lumens it may have: that's a separate issue. It's 7 Fr, and therefore its *outer* diameter is 7/3 = 2.33 mm.

It's the same unit of measurement for (almost) all devices: A 12 Fr NG tube has the same outer diameter as a 12 Fr dialysis catheter, a 12 Fr Foley catheter, or a 12 Fr chest tube.

All of 12 Fr devices shown here will have an outer diameter of 12/3 = 4 mm.

I should point out that on the regular 'Argyle' type chest tubes, we're bending the rules a little bit.

These chest tubes are actually oval in shape, so we probably shouldn't use 'French', but we do. In this case the measurement is along the long axis of the chest tube.

On the other hand:
Vascular access sheaths, such as those used in cardiology or interventional radiology, are often described differently.

Here, it's often simpler to instead describe the sheath in terms of *what will fit through it*. After all, that's what the sheaths are for.

Here is a diagram of a '6 Fr' vascular access sheath.

Note that label is referring to the *inner* diameter here. The outer diameter is larger. In this case the '6 Fr' means that's what we can fit through the sheath.
It is the *opposite* of how 'French' is used elsewhere.

One thing about surgical drains often leads to confusion:

Jackson-Pratt drains are *not* circular, so they are *not* measured in French -- they are measured in mm.
-but-
Blake drains *are* circular and they *are* measured in 'French'.

This can be deceiving, as we'll see next.

This difference in the way JP's and Blake drains are measured means that a 'size 10' JP drain is actually considerably LARGER than a 'size 19' Blake drain.

Note that the 19 Blake has a 6.33 mm diameter, whereas the '10' JP drain is 10 mm wide. 🧐📏

In some countries were people have less difficulty pronouncing 'Charrière', the catheters instead often use the 'Ch' abbreviation instead of 'French'.

Here we see an '18 Ch' Foley, and we are reminded again that this means it's 6 mm in diameter.

On the other hand 'Gauge' is an entirely different kind of measurement.

'Gauge' refers to the Birmingham Gauge system, devised in the 1900's in England for manufacturing metal wire. It later became used for IV catheters.

There are a couple of major differences...

First, as the 'Gauge' number gets *larger*, the IV's get *smaller*. This is the opposite of the 'French' system (and most other systems).

As you can see here, a 14 Ga IV catheter is much LARGER than a 24 Ga catheter.

Also the 'Gauge' sizes are somewhat arbitrary...and the spacing between them isn't consistent.

So a 12 Gauge IV is *not* exactly twice the diameter of a 24 Gauge IV, for example.

If you do the math here, you see these relationships are neither linear nor consistent.

Finally, as a trauma surgeon, I have to point out one of my favorite teaching points: The flow rate of a 16 G IV is **twice** that of an 18 G IV.

So the difference between those two IV's may not sound much at first, but it makes a big difference for major resuscitations.
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Addendum:

Someone thankfully pointed out an unfortunate error in the thread...

'36 Fr' is actually 12 mm, whereas I had accidentally listed it in an earlier picture as '13 mm'. 😬

Addendum #2:

@TylerLarsenMD has a thread going into more detail about the gauge system:
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