The application relates to an external drive unit (7) for an implantable heart assist pump (4). The proposed drive unit (7) comprises a motor housing (51), a transcutaneous drive shaft (3) and a motor (35) for driving the heart assist pump (4). The motor (35) is connectable to the heart assist pump (4) via the drive shaft (3), and the motor (35) is arranged inside the motor housing (51). The drive unit (7) further comprises a catheter (2) surrounding the drive shaft (3) and a purge line (53) for injecting a purge medium into a lumen of the catheter (2) or into a space (41) between the catheter (2) and the drive shaft (3). The purge line (53) is in thermal contact (54, 55) with an outer surface of the motor housing (51) and/or with an outer surface of a proximal section (52) of the catheter (2). Due to the thermal contact (54, 55) heat may be transferred from the outer surface of the catheter (2) in the proximal section (52) and/or from the outer surface of the motor housing (51) to the purge medium.
