French catheter scale
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The French scale or French gauge system is commonly used to measure the size of a catheter. It is most often abbreviated as Fr, but can often be seen abbreviated as Fg,Ga, FR or F. It may also be abbreviated as CH or Ch (for Charrière, its inventor).
D (mm) = Fr / 3
Fr = D (mm) * 3
For example, if the French size is 9, the diameter is 3 mm.
From the basic math equation C = πd, it follows that the catheter's circumference in mm is only slightly (about 4.7%) greater than the French size.
An increasing French size corresponds to a larger external diameter. This is contrary to needle-gauge size, where an increasing gauge corresponds to a smaller diameter needle.
The French size is a measure of the external diameter of a catheter (not internal drainage channel). So, for example, if a two-way catheter of 20 Fr is compared to a 20 Fr three-way catheter, they both have the same external diameter but the two-way catheter will have a larger drainage channel than the three-way. Three-way catheters accommodate an extra channel for irrigation within a similar external diameter.
The French gauge was devised by Joseph-Frédéric-Benoît Charrière, a 19th-century Parisian maker of surgical instruments, who defined the "diameter times 3" relationship.
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