A cardiac assist system includes a pneumatic effector which is implanted beneath a pericardial sac and over a myocardial surface overlying the patient's left ventricle. A port is implanted and receives a percutaneously introduced cannula. The port is connected to supply a driving gas received from the cannula to the pneumatic effector. An external drive unit includes a pump assembly and control circuitry which operate the pump to actuate the pneumatic effector in response to the patient's sensed heart rhythm. A connecting tube has a pump end connected to the pump and a percutaneous port-connecting end attached to the implantable port.