The invention relates to a self-expandable medical instrument (100) for treating defects in a patient's heart, in particular for the transvascular implantation of a prosthetic heart valve (30), wherein the medical instrument (100) is introducible into the body of a patient in a minimally-invasive procedure using a catheter system (40) and comprises a stent (1) made of a flexible mesh (2) of thin wires or filaments (2'). In order to realize a positioning of medical instrument (100) in the patient's heart which is as precise as possible and to securely anchor same there, it is provided for the stent (1) composed of the flexible mesh (2) to exhibit in the expanded state of the medical instrument (100) a distal retention area (10) with a laterally-inverted beaded portion (12) which is engageable in at least one pocket (51) of the patient's defective heart valve (50), a proximal retention area (20), and a center area (15) positioned between the distal and the proximal retention area (10, 20).
