U pitanju je sistem za obeležavanje prečnika katetera. Pogledajte referencu: The French scale or French gauge system (most correctly abbreviated as Fr, but also often abbreviated as FR or F) is commonly used to measure the size (diameter) of a catheter. 1 Fr = 0.33 mm, and therefore the diameter of the catheter in millimeters can be determined by dividing the French size by 3:
D (mm) = Fr/3
or
Fr = D (mm) × 3
For example, if the French size is 9, the diameter is 3 mm.
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.
In some countries (especially French-speaking), this unit is called Charriere and abbreviated as Ch.

Nisam sto posto sigurna kako se to sve kod nas obeležava, ali znam sigurno za F, pa nećete pogrešiti ako ostavite tako.