At four U.S. medical centers, surgeons, nurses and anesthesiologists are quietly scrubbing in for the return of one of the most vilified medical devices ever conceived-the artificial heart. Each of the four teams was tapped a year ago by Abiomed, a little-known Danvers, Mass., company whose engineers have worked for more than a decade to build a 900-gram electromechanical pump they call the PulsaCor. Surgeons at the Texas Heart Institute and Massachusetts General Hospital, among others, are now practicing putting the synthetic heart into calves. David Lederman, Abiomed's CEO, says they'll be performing the surgery on a human before 2000 is out.

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