The assembly has an in-situ implantation accessory (26) comprising a probe body (28) with a spiral conductor (30) coated with sheath (32) to carry a guide formed of a helix spring (36) and a tube to form a support, guiding and driving unit of standalone leadless capsule (10). The guide extends the body. Inner diameter of the guide is same as outer diameter of a cylindrical tubular body (12) to accommodate the guide. Distal and proximal coupling fingers (20, 22) project between turns of the guide, where helical direction of the guide is opposite to that of an anchoring screw (14).
