Doctors have put thousands of heart defibrillators in patients who may not need the devices and could even been harmed by them according to a new study of how and when the high-tech implants are used.

Implantable cardioverter-defibrillators monitor a person’s heart rate and rhythm for dangerous patterns and can send an electric jolt to the heart to reestablish a normal beat when needed.

Although the devices can be life-savers for some, the study showed that many doctors do not follow guidelines for their use. More that 25,000 people receiving the implants tracked by the National Cardiovascular Data Registry, or 22.5 percent, did not meet evidence-based criteria for implantation. In some hospitals, more than 40 percent of the defibrillators implanted were placed in patients not meeting the guidelines.

The implant procedure can cost more than $35,000, according to reports. But more importantly, people implanted with defibrillators that don’t meet the criteria for the devices’ use have a higher chance of dying in the hospital or suffering from post-procedure complications than people who do meet the criteria.

The study, published in The Journal of the American Medical Association, also found that in many cases, black and Hispanic patients were more likely to be implanted with defibrillators without sufficient evidence for their use.

If properly applied, suggest the authors of an editorial accompanying the study, the findings “may improve practice patterns and outcomes, with the unique opportunity to do so while lowering healthcare costs.”

Kenneth Stein, chief medical officer of cardiac rhythm management for Boston Scientific, which sponsored many of the studies referenced in the report, said in reality, only a fraction of eligible patients actually get the devices. “While ICDs are at times used outside the guidelines, the more fundamental problem is that on the whole, this life-saving therapy is under-utilized, rather than over-utilized.”