The study looked at data from a total of 416 hospitals, but not at how changes in readmission rates were related to mortality rates at specific hospitals, researchers note online in JAMA Cardiology.

One study that did focus on hospital-level data, published in JAMA in July, got different results, according to its lead author, Dr. Kumar Dharmarajan, chief scientific officer at Clover Health, a San Francisco-based health insurance startup company.

“While we too found slight increases in mortality for heart failure over time, we found that hospitals lowering readmissions tended to lower mortality despite this finding,” Dharmarajan said by email. “Given our results, it is very unclear to me how efforts to lower readmissions could have resulted in harm.”

Other factors during the study period, like hospital closures and an economic downturn that’s made it harder for many people to afford medications, might also explain some of the change in mortality rates, said Dr. Karen Joynt Maddox of Barnes-Jewish Hospital and the Washington University School of Medicine in St. Louis.

“It’s important to note that we’re looking at patterns over a decade across the country, where a lot of things are happening at once,” Maddox, who wasn’t involved in the study, said by email. “We really need more research to understand these findings so we can figure out what to do.”