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1. A covering structure of connective tissue, usually of an elongated part, such as the membrane covering a muscle.
2. An instrument introduced into a vessel during angiographic procedures when multiple catheter changes are anticipated. It facilitates ease of change and decreases morbidity at the puncture site.

The sheath introduced into the femoral artery, the preferred vascular access route for percutaneous coronary intervention, is a 4 to 6 French (1.35 to 2 mm) in size. The sheath remains in place after completion of the procedure and removal of the catheter until anticoagulation is reversed or anticoagulants are below peak action. The sheath is connected to high-pressure tubing and a flushing system; manual or automatic flushing keeps the line patent. A stopcock connected to the system permits drawing of blood samples.

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