An implantable medical device comprising a pulse generator, a power source, an hermetically sealed capsule that contains the pulse generator and the power source, and an electrode assembly, the electrode assembly comprising a longitudinal axis, a helical structure coupled to the capsule and extending distally from the capsule and around the longitudinal axis in a first direction being either clockwise or counter-clockwise, a first electrode surface formed on the helical structure, and a second electrode surface electrically isolated from the first electrode surface, the helical structure including a piercing distal tip located adjacent the first electrode surface; and characterised by the device comprising at least one fixation member, wherein the or each fixation member comprises: a fixed end coupled to the capsule; and a tissue engaging portion terminated by a piercing distal tip, each tissue engaging portion extending from the corresponding distal tip to the corresponding fixed end along a circular path around the longitudinal axis, the helical structure being located within a perimeter of the circular path; and wherein each tissue engaging portion extends from the piercing distal tip thereof in a direction along the circular path that is the same as the first direction in which the helical structure extends from the piercing distal tip thereof, either clockwise or counter-clockwise; wherein the fixed end of each fixation member of the electrode assembly comprises a post that extends approximately parallel with the longitudinal axis, between the distal portion of the device and the tissue engaging portion of the corresponding fixation member, and wherein the tissue engaging portion of each fixation member of the electrode assembly includes a first segment and a second segment, each first segment extending along the circular path and bending proximally from the corresponding post, toward the capsule of the device, and each second segment extending along the circular path and distally from the corresponding first segment, away from the capsule of the device. a fixed end coupled to the capsule; and a tissue engaging portion terminated by a piercing distal tip, each tissue engaging portion extending from the corresponding distal tip to the corresponding fixed end along a circular path around the longitudinal axis, the helical structure being located within a perimeter of the circular path; and wherein each tissue engaging portion extends from the piercing distal tip thereof in a direction along the circular path that is the same as the first direction in which the helical structure extends from the piercing distal tip thereof, either clockwise or counter-clockwise; wherein the fixed end of each fixation member of the electrode assembly comprises a post that extends approximately parallel with the longitudinal axis, between the distal portion of the device and the tissue engaging portion of the corresponding fixation member, and wherein the tissue engaging portion of each fixation member of the electrode assembly includes a first segment and a second segment, each first segment extending along the circular path and bending proximally from the corresponding post, toward the capsule of the device, and each second segment extending along the circular path and distally from the corresponding first segment, away from the capsule of the device.