The researchers also found that more than 40 percent of these cases were “unwitnessed,” meaning that because no one saw the victims collapse, rescuers had no information about how long they had been in arrest — a crucial factor when the odds of successful resuscitation diminish by 10 percent with each minute.

None of the elderly patients with non-shockable rhythms and unwitnessed arrests survived hospitalization. Yet about 44 percent of the surveyed clinicians thought those attempts, too, were appropriate. Nursing home residents had particularly dire outcomes.

The study had limitations, including its reliance on clinicians’ memories. But other research has documented bleak outcomes after CPR for out-of-hospital cardiac arrest at advanced ages, even though resuscitation results have improved over all. (For hospitalized patients, it succeeds more frequently.)

Swedish researchers using data from a national registry found that 30-day survival dropped from 6.7 percent for patients in their 70s to 4.4 percent for those in their 80s to 2.4 percent for those over 90.

In San Mateo County, Calif., researchers found that of patients over 80 with unwitnessed arrests and non-shockable rhythms, none survived to hospital discharge.

Among medical organizations, “the big ethics discussion has been around, ‘When do we stop?’” said Dr. Monique Starks, a cardiologist at the Duke University School of Medicine and an author of an editorial accompanying the study. “Only recently have there been discussions about, ‘When do we start it?’”

Emergency responders in the United States typically default to what one research team called a “maximalist” strategy. But as with any intervention, CPR has its own risks: broken bones, lacerated organs, trauma for the patient and family, expense, diversion of emergency services from patients more likely to benefit, even traffic deaths.