Interventional cardiology has steadily moved toward larger devices. Transcatheter valves, endovascular grafts, and mechanical circulatory support all require equipment far bulkier than a coronary balloon, and that equipment must reach the heart or aorta through an artery. The sheath is what makes that possible. It holds a channel open, protects the vessel wall from repeated device passage, and prevents bleeding during the procedure.
The artery itself is the limiting factor. A femoral artery has a finite diameter, and the relationship between sheath size and vessel size is what governs vascular complications at the access site. Too large a sheath in too small an artery risks dissection, occlusion, or rupture, and these complications most affect outcomes in large-bore procedures. Assessment before the case therefore involves measuring the vessel and comparing it against the sheath that will be used.
That is why outer diameter is the number that counts here, and why publishing it in millimetres rather than French alone is genuinely useful. Wall thickness matters for the same reason. Every fraction of a millimetre saved in the wall is a fraction less artery needed, which is why these sheaths are built with thin, reinforced walls rather than simple extrusions. Length follows the anatomy between the access site and the target. At the end of the case, the sheath comes out, and the arteriotomy has to be closed, usually with a dedicated large-bore closure device rather than manual pressure.
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