Medicare will expand coverage for surgically implanted heart-shocking devices for people with weakened hearts, a move that is expected to cost an estimated $3 billion.

The decision could represent the most expensive decision in the history of Medicare, a program that is also being considered a top target for significant cutbacks. The Medicare Payment Advisory Commission recently issued recommendations to Congress that included freezing payments to nursing homes in 2006 and lowering Medicare’s inflationary payment increase to hospitals.

Medicare’s plan on defibrillators means that more than half a million Americans with congestive heart failure could be eligible for the battery-powered implants and accompanying surgery under the plan.

The devices sense heart rhythm abnormalities and deliver shocks to the heart when potentially fatal flutters occur.