How are catheter and sheath sizes expressed, and why is this important?

• A. Sizes are expressed in millimeters; outer diameter determines vascular trauma risk and device compatibility.
• B. Sizes are expressed in French (Fr); outer diameter determines vascular trauma risk and compatibility with devices; improper sizing increases complication risk.
• C. Gauges are used to measure catheter length rather than diameter.
• D. Sizes are expressed in pounds; larger size reduces risk.

Answer: (B)

Sizes for catheters and sheaths are expressed in French (Fr). This unit encodes the outer diameter of the device, which matters because the entry channel must fit through the vessel and accommodate other tools. One French equals about 0.333 mm, so a larger Fr number means a larger external diameter. That outer diameter directly influences how much vascular trauma can occur during access and how well other devices (guidewires, dilators, sheaths, catheters) will fit and pass through. If sizing is too large, you increase the risk of arterial injury, dissection, or bleeding; if too small, devices may not advance or seal properly, leading to complications. Gauges aren’t used to measure catheter length, and measuring in pounds isn’t relevant, while the key point is that French sizing reflects outer diameter and its impact on safety and compatibility.