Christian Volpe was shopping with his wife when an alarm started beeping to warn that only 15 minutes of battery power were left on the implanted heart pump that was keeping him alive.

Mr. Volpe, 67, a slight, gray-haired man, looked in his car for the bag he always keeps nearby with spare batteries. But, no bag. In his mind’s eye he saw exactly where he had left it, to make sure he would not forget it, on a chair near the door back home  an hour and a half away. He thought of the clever little hand pump he had been given to keep his mechanical heart going in an emergency. It, too, was in the missing bag. Standing in the parking lot, he could hear one thing. Beep. Beep. Beep.

“I have to admit, I panic,” he said.

Mr. Volpe is one of thousands of Americans who have had these pumps, called left ventricular assist devices, surgically implanted to help their failing hearts. Former Vice President Dick Cheney is another. Sometimes the pumps are used to keep people alive until a transplant becomes available, but in other cases they are meant to remain as long as the patient lives.

Mr. Volpe, a retired subway conductor who had had two heart attacks and two bypass operations, had an assist pump implanted in October 2009 by Dr. Yoshifumi Naka at NewYork-Presbyterian/Columbia hospital.