In the years after health insurance coverage under the Affordable Care Act unfurled in Multnomah County, Oregon, cardiac arrests among those newly covered fell 17 percent, researchers report this week in the Journal of the American Heart Association.

The pilot study, led by researchers at Oregon Health & Science University and the Heart Institute of Cedars-Sinai Medical Center in Los Angeles, is just an observational study—it can’t determine causation—and it only looked at the one county. But, the authors argue, the data begs for follow-up.

“Despite general agreement that expanded insurance coverage leads to positive health effects, the mechanism of this benefit and effect on health outcomes remains poorly understood,” they note. In past small studies, findings have been murky on whether health insurance significantly alters major health events—such as cardiac arrests.

For the new study, the researchers pulled census data of Multnomah’s population (around Portland) of about 636,000, as well as EMS reports of cardiac arrests. They looked at reports of those deadly events from 2011 to 2012, which was pre-ACA insurance expansion, and from 2014 to 2015, which was post-ACA expansion. They considered 2013 a transition period.

The researchers focused on data on residents 45 years old and up because they have the majority of cardiac arrests. In the two time periods, this age group grew from about 274,000 to about 291,000. Overall, the number of cardiac arrests didn’t change much—there were 844 in the earlier time period and 834 in the latter.

But when they stratified the data by age—and health coverage—there was a difference. The elderly (65 years old and up), were near-universally insured in both time points. Their coverage was stable throughout and they did not see any changes in incidence of out-of-hospital cardiac arrests.

The middle-aged 45 to 64 group, on the other hand, saw an abrupt increase in their insurance rate between the two time periods. The percentage of uninsured dropped from near 16 percent to around 7 percent. Expanded coverage on Medicaid was the biggest driver of this change. And in step with the insurance increase, cardiac arrests dropped 17 percent between the two time periods.

The correlation doesn’t mean that the insurance coverage caused the change, the authors stress. And even if it did, it’s not clear from the data how insurance affected cardiac arrest rates. It’s possible that with improved diagnoses and preventative care, heart health could improve in this age range. Interventions like smoking cessation programs and medications for cholesterol and atherosclerosis could all improve health, the authors speculate. But more and larger studies are needed to determine if this is true.

Journal of the American Heart Association, 2017. DOI: 10.1161/JAHA.117.005667 (About DOIs).