A little-known heart device, an implantable defibrillator, has been under a spotlight after the recent disclosure that a producer, the Guidant Corporation, did not tell doctors for years that one of its models had a defect that could render it useless. But well before that, medical experts had been increasingly debating other aspects of the device, namely its benefits and its costs.

No one disputes that many heart patients need a defibrillator, a device that sends out an electrical jolt to interrupt a chaotic heart rhythm that can cause sudden cardiac arrest, which is often fatal. But some doctors say the units, which are fast becoming one of the most costly medical devices in use today, are being implanted in many other patients who may not need them because they are either too well or too sick.

Both the medical and financial consequences of that debate have been growing as use of the device has soared. Last year, an estimated 135,000 devices were implanted in patients in the United States alone, a near tripling of the number in 2000. Meanwhile, the three major device producers, Guidant, Medtronic and St. Jude Medical, have reaped a financial bonanza as domestic sales rose during that same period to $3.5 billion, from $1.3 billion, according to Harris Nesbitt, an investment firm.

The overuse or inappropriate use of medical treatments is a familiar theme. But defibrillators pose unique issues because they are increasingly being used as a standard piece of safety equipment, like an air bag in an automobile, in an ever-growing number of heart patients whose individual need for the device is not clear. Defibrillator prices are also like those found on a new car, ranging from about $20,000 to $35,000 each.

"They are expensive and in theory almost anyone in the country could benefit," said Dr. Mark Hlatky, a professor of health research and policy at Stanford University School of Medicine. "So where do you draw the line?"

The dilemma reflects a growing medical conundrum. Defibrillators were first used in the 1980's only in patients who had survived an episode of sudden cardiac arrest. But since then, studies largely funded by the industry have resulted in a steady expansion of the device's use to ever-broader pools of heart patients with varying degrees of risk of cardiac arrest.

The problem, however, is that medical researchers have not figured out how to tell which patients in these broader groups are at high risk and which ones are at low risk. The result: they all qualify for a unit.

The problem is no small matter. Experts agree that many, if not the majority, of defibrillator candidates are at a low risk of cardiac arrest. "We can identify groups at risk, but we cannot really identify the individuals in those groups who are at higher risk," said Dr. Douglas P. Zipes, a cardiology professor at Indiana University School of Medicine.

Meanwhile, physicians like Dr. Zipes and health policy experts say that manufacturers have used a variety of strategies to increase profits by keeping device prices high. For example, rather than offering a low-cost unit that does the basic job of stopping a bad heart rhythm, defibrillator makers are engaged in a sort of medical arms race in which producers turn over models by adding new features like different ways to measure heart rhythms. Some of these additions are valuable, while others are just "bells and whistles," experts say.

"These companies don't compete on price, they compete on features," Dr. Hlatky said.

Doctors and patients also have no reason not to go for a top of the line model, said experts like Dr. Hlatky. Many physicians acknowledge that they do not consider product prices when deciding which model of defibrillator is best for their patient. And patients have little reason to care about cost because insurers like Medicare cover the cost of an implant procedure, which includes the device, regardless of its cost.

As in other areas of medicine, there have been claims that device makers retain the loyalty of physicians by providing them with consulting arrangements and other financial incentives. The companies insist that they have done nothing wrong and that their prices are fair. They also say they are responding to doctors' suggestions by producing the devices they want.