An external drive unit for an implantable heart assist pump is provided. The drive unit comprises a motor housing, a transcutaneous drive shaft and a motor for driving the heart assist pump. The motor is connectable to the heart assist pump via the drive shaft, and the motor is arranged inside the motor housing. The drive unit further comprises a catheter surrounding the drive shaft and a purge line for injecting a purge medium into a lumen of the catheter or into a space between the catheter and the drive shaft. The purge line is in thermal contact with an outer surface of the motor housing and/or with an outer surface of a proximal section of the catheter. Due to the thermal contact heat is transferred from the outer surface of the catheter in the proximal section and/or from the outer surface of the motor housing to the purge medium.