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, FR or F. It may also be abbreviated as CH or Ch (for Charrière, its inventor). However, simply gauge, G or GA generally refers to Birmingham gauge.
The French size is three times the diameter in millimeters. A round catheter of 1 French has an external diameter of 1⁄3 mm, and therefore the diameter of a round catheter in millimetres can be determined by dividing the French size by 3:
D (mm) = Fr / 3
Fr = D (mm) * 3
For example, if the French size is 9, the diameter is 9/3 = 3.0; mm. It is seen that the French unit is fully redundant with the metric system but introduces potential for rounding errors. This metrication problem is further complicated by the mixed use of metric and imperial units in medical professions using catheters. As such, it may be seen simply as an annoyance for an engineer designing catheters.
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. Even though this relation is not-even-wrong, one could just have defined the French as 1mm/π.
An increasing French size corresponds to a larger external diameter. This is contrary to Birmingham gauge, where an increasing gauge corresponds to a smaller diameter needle.
The French size is a measure of the outer diameter of a catheter (not internal drainage channel, or inner diameter). 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.
- Poll, J. S. (1999). "The story of the gauge". Anaesthesia. 54 (6): 575–581. doi:10.1046/j.1365-2044.1999.00895.x. ISSN 0003-2409.
- "1 French". Wolfram|Alpha. Retrieved 20 March 2012.
- Iserson, Kenneth V. (1987). "J.-F.-B. Charrière: The man behind the "French" gauge". Journal of Emergency Medicine. 5 (6): 545–8. doi:10.1016/0736-4679(87)90218-6. PMID 3323304.