A 52-year-old man awaiting a heart transplant was being kept alive Wednesday with a first-of-its-kind portable heart pump, an implantable device connected to a battery pack that can be slung over the shoulder.

The patient, whose identity was withheld at his family's request, was in critical condition at St. Luke's Episcopal Hospital, said Marc Mattsson, spokesman for the Texas Heart Institute, which is based at St. Luke's.The operation, the first of its kind, was performed Thursday. The device was functioning without technical problems, said Victor Poirier, president of Thermo Cardiosystems Inc. in Waltham, Mass., which makes the pump.

The Texas Heart Institute won approval from the Food and Drug Administration in January for a program to test the HeartMate device, which is designed to keep patients alive and mobile while they await donated hearts.

The patient's failing heart remains in place, but the implant boosts or takes over the pumping function of the left ventricle - the biggest of the four chambers of the heart and the one that pumps oxygen-rich blood to the body.

The pump, 4 inches in diameter and 11/4 inches thick, is implanted in the abdomen and a tube connects it to the left ventricle.