The invention provides an introducer sheath (1, 21, 51), whose distal end is to be positioned inside a vessel (10, 32, 62), at a predetermined distance from the wall (9, 31, 61) of the vessel (10, 32, 62). The introducer sheath (1, 21, 51) comprises a distal portion (2, 22, 52) with a first diameter and a proximal portion (3, 23, 53) with a second diameter, the first diameter being smaller than the diameter of the puncture hole and the second diameter being larger than the diameter of the puncture hole.
