Acute MI risk-standardized hospitalization and mortality rates significantly declined among Medicare beneficiaries from 1999 to 2013, according to an observational study.

During that same time period, the mean hospitalization rates were significantly higher in low-income counties compared with high-income counties. However, the one-year mortality rates were similar in all groups.

Lead researcher Erica S. Spatz, MD, MHS, of the Yale University School of Medicine in New Haven, Connecticut, and colleagues published their results online in JAMA Cardiology on May 11.

“Successes in decreasing [acute MI] hospitalizations and mortality have reached communities of different income levels, yet disparities persist, suggesting the need for more targeted research and investment in low-income counties,” the researchers wrote.

The researchers used the Medicare denominator files to identify more than 60 million adults who enrolled in the Medicare fee-for-service program for at least one month from Jan. 1, 1999 through Dec. 31, 2013. They also used U.S. Census bureau data to determine the median income and percentage of people living below the federal poverty level in each county during that time period. They found a high correlation between county-level income and county-level poverty rates.

The researchers also stratified the counties into three groups based on their annual median income. They considered counties as low income if their median income was less than the 25^th percentile of the national level; high income if their median income was greater than the 75^th percentile of the national level; and average income for all other income levels.

Between 1999 and 2013, the consumer price index-adjusted median incomes decreased from $37,271 to $34,593 in the low-income group, $46,344 to $43,966 in the middle-income group and $60,495 to $57,559 in the high-income group.

In 1999, the mean hospitalization rates were 1,353 per 100,000 person-years among the low-income counties and 1,123 per 100,000 person-years among the high-income counties. In 2013, the mean hospitalization rates were 853 per 100,000 person-years and 648 per 100,000 person-years, respectively.

Low-income counties had the highest number of acute MI hospitalizations in 1999, 2006 and 2013, while high-income counties had the lowest number of acute MI hospitalizations in all three years that the researchers examined. They added that the rate of decline was consistent in the low-, middle-, and high-income groups. 

“There was an interaction between time and the average-income group but no interaction between time and the low-income group, indicating that the slope of decline in the low-income group was similar to the slope of decline in the high-income group,” the researchers wrote.

In 1999, the one-year mortality rates after hospitalization for acute MI were 31.5 percent among the low-income counties, 31.4 percent among the middle-income counties and 31.1 percent among the high-income counties. In 2013, the one-year mortality rates after hospitalization for acute MI were 26.2 percent, 26.1 percent and 25.4 percent, respectively.

The researchers cited a few limitations of the study, including that it only used median county household income to measure socioeconomic status. They said considering other variables such as educational level, employment and household crowding may have provided better information on socioeconomic status.

In addition, the study only included Medicare fee-for-service beneficiaries, so the results may not be generalizable to other patient populations. Each county also had heterogeneity in annual household income. Further, the study only focused on hospitalization and mortality rates associated with acute MI.

“These rates do not take into account prehospital sudden cardiac death due to [acute MI],” the researchers wrote. “Further studies are needed to determine whether trends by county-level income are consistent for sudden cardiac death due to [acute MI], as well as for non-[acute MI] conditions.”