A new study finds that one in five heart defibrillators may be implanted for questionable reasons without solid evidence the devices will help.

The implants shock the heart back into a normal rhythm when it starts beating irregularly. They can prevent sudden death in people with advanced heart failure. But researchers haven’t found a benefit for other patients.

The new study looked at four years of national data and found that 22 percent of the surgeries were in patients who didn’t fit research-backed guidelines.

The patients who got implants according to guidelines were less likely to die in the hospital than the patients whose surgeries clearly fell outside the guidelines.

The study appears in Wednesday’s Journal of the American Medical Association.