A kit for intravascular implantation of an implantable medical device (IMD) within a patient includes the IMD, an elongated shaft, and a locking mandrel. The IMD comprises a fixation assembly comprising a loop and defines at least one longitudinal lumen and a port in fluid communication with the lumen. The shaft is sized to traverse a vasculature of the patient. The port is sized to receive at least a portion of the loop. The locking mandrel is configured to be positioned within the at least one lumen of the shaft and to pass through the loop within the lumen at the port. A reduced profile portion of the shaft defines a reduced profile with respect to at least one other portion of the shaft. At least a portion of the reduced profile portion is configured to be adjacent to the IMD when the IMD is positioned on the shaft.