"It may be possible to deter malicious activities by making patients aware of those activities," Maisel, Tadayoshi Kohno of the University of Washington and colleagues wrote.

Their report, to be presented in May at a meeting of the Institute of Electrical and Electronic Engineers Symposium on Security and Privacy in Oakland, California, is available on the Internet at www.secure-medicine.org.

Maisel said more and more devices will use radio technology to communicate with physicians. "Right now these devices communicate over several feet most of time but it concerns us that in future they will communicate over longer and longer distances, so we want to initiate the discussion now," he said.

The defibrillators, known as ICDs, can deliver a shock to an out-of-rhythm heart, and can include a pacemaker to keep the heart beating properly. They also can keep a record of heart activity, transmit information to a bedside station and alert health workers to any dangerous or unusual heart activity.

Defending against attacks should not be difficult, the researchers said. "Our defenses do not require battery power and therefore may require only minimal design changes to future implantable devices," the researchers wrote.

Maisel and colleagues reported in 2006 that between 1990 and 2002, more than 2.6 million pacemakers and ICDs were implanted into patients in the United States. Among them is Vice President Dick Cheney, who is 67 and has survived four heart attacks.

By Maggie Fox, Health and Science Editor

(Reporting by Maggie Fox; editing by Julie Steenhuysen and Mohammad Zargham)