Heart-attack patients whose ambulances were diverted from crowded emergency rooms to hospitals farther away were more likely to be dead a year later than patients who weren’t diverted, according to a recent study published in the journal Health Affairs and reported by Kaiser Health News.

The study, conducted by researchers at the University of California-San Francisco and the National Bureau of Economic Research, looked at ambulance diversions affecting nearly 30,000 Medicare patients in 26 California counties from 2001 to 2011.

The study adds to a growing body of research nationally showing that temporary diversions of ambulances from the nearest hospital can harm patients with life-threatening conditions, including heart attacks and stroke.

Some hospitals see diversion as a necessary safety valve for full-up emergency rooms. But emergency care experts say they push the crowding problem to nearby hospitals and can compromise patient care, especially in life-threatening cases.

“This setup is absolutely a disaster for these kinds of patients,” Mike Williams, president of the Abaris Group, a Martinez, Calif-based health care consulting firm that specializes in emergency medical services, told Kaiser Health News. Williams was not involved in the study.

The researchers found that heart-attack patients whose ambulances had been diverted to an emergency room farther away were nearly 10 percent more likely to be dead one year later than those whose ambulances were not diverted. They were also slightly less likely to get the treatment they needed to restore blood flow to major organs, such as an angioplasty or coronary artery bypass graft.

That’s primarily because diverted patients were taken to ERs with less advanced cardiac care technology.

“This study is telling us that as we continue to saturate our emergency care system, [diversions] affect everybody,” said University of California-San Francisco researcher Dr. Renee Hsia, one of the study’s authors. “We tend to think that it’s only people who aren’t that sick who don’t get treated quickly in emergency rooms. Diversions affect patients who are really sick, too.”

The study found, however, that patients of color were more likely to be diverted. Statewide, about half of heart attack patients were diverted to other hospitals on the day of admission between 2001 and 2011.

Diversion policies varied from one county EMS agency to another, and some places were far more affected than others. In LA County, as many as three-quarters of heart-attack patients were diverted in some years, the study showed.

Experts said that the higher mortality rate is particularly troubling because it is only the most extreme indicator of how patients fare. Other possible outcomes include non-fatal strokes, bleeding complications or heart arrhythmias, all of which can require re-hospitalization later on.

“Those are hidden behind the higher mortality rate,” said Dr. James Augustine, director of clinical operations for Emergency Medicine Physicians, a national network of emergency doctors based in Ohio. “You’re only looking at the worst outcomes when you do these studies.”