A new study suggests that limiting the number of hours a medical resident can work may lead to slightly lower death rates in American teaching hospitals.

Since rules restricting the work hours of physicians-in-training came into effect four years ago, researchers at the Stanford University School of Medicine discovered one fewer death for every 400 patients admitted to select teaching hospitals.

"Before the regulations, 60 out of every 1,000 medical patients died. After the regulations, the number of deaths was 57 or 58 out of every 1,000," said the study's lead author, Kanaka Shetty.

The study, which is the largest examination of work-hour regulations on residents so far, compared death rates in teaching hospitals before and after August 2003, one month after the rules went into effect.

It examined the clinical outcomes of 1.5 million adult patients classified as high mortality — generally, older and sicker — including 1.3 million admitted for medical diagnosis.

Another 200,000 surgical patients were also examined, but the new rules did not seem to have an impact on their death rate.

"It's difficult to say, based on our findings, that the regulations are good for everyone," said Shetty. "But they do appear to have a modest impact on some."

There are more than 100,000 medical residents in the United States. Concern over their performance grew through the 1980s and 1990s after a patient's death was blamed on a medical resident's error and fatigue. In 2003, the Accreditation Council for Graduate Medical Education capped the number of hours a resident could work per week at 80, or 30 hours straight.

Shetty's research will appear in the July 17 issue of the Annals of Internal Medicine alongside a Yale University study that found the work-hour regulations improved outcomes for internal medicine patients on three of seven measures.

In 2004, a New England Journal of Medicine study found that interns working under a work-hours cap made fewer errors than a similar group working under the traditional system did.

But a Journal of General Internal Medicine study published the same year found no conclusive evidence that resident work-hour restrictions in New York improved outcomes among multiple patient groups.

Calling for more study on the regulations, Shetty suggested the improvements in care might also have resulted after transferring patients from inexperienced residents to seasoned doctors.

If so, that could mean residents aren't gaining enough experience and skills during training and the work-hours cap could be causing more harm than good, he said.