Trends in acute myocardial infarction hospitalizations: Are we seeing the whole picture?

BACKGROUND: Payers and policy makers rely on studies of trends in acute myocardial infarction (AMI) hospitalizations and spending that count only hospitalizations where the AMI is the principal discharge diagnosis. Hospitalizations with AMI coded as a secondary diagnosis are ignored. The effects of excluding these hospitalizations on estimates of trends are unknown. METHODS: Observational study of all AMI hospitalizations in Fee-for-Service Medicare beneficiaries 65 years and older, from 2002 through 2011. RESULTS: We studied 3,663,137 hospitalizations with any AMI discharge diagnosis over 288,873,509 beneficiary-years. Of these, 66% had AMI coded as principal (versus secondary). From 2002 to 2011, AMI hospitalization rates declined 24.5% (from 1,485 per 100,000 beneficiary-years in 2002 to 1,122 in 2011). Meanwhile, the proportion of these hospitalizations with a secondary AMI diagnosis increased from 28% to 40%; by 2011 these secondary AMI hospitalizations accounted for 43% of all expenditures for hospitalizations with AMI, or $2.8 billion. Major changes in comorbidities, principal diagnoses and mean costs for hospitalizations with a non-principal AMI diagnosis occurred in the 2006-2008 timeframe. CONCLUSIONS: Current estimates of the burden of AMI ignore an increasingly large proportion of overall AMI hospitalizations and spending. Changes in the characteristics of hospitalizations that coincided with major payment and policy changes suggest that non-clinical factors affect AMI coding. Failing to consider all AMIs could inflate estimates of population health improvements, overestimate the value of AMI prevention and treatment and underestimate current and future AMI burden and expenditures.