A transcutaneous energy transfer system is provided whichis operable to vary an amount of power transmitted from an externalpower supply to an implantable power unit in accordance with amonitored condition of the implantable power unit. In such way, theamount of power supplied to the implantable power unit for operating apump, for example, can be varied in accordance with a cardiac cycle,so as to maintain the monitored condition in the power circuit withina desired range throughout the cardiac cycle.
