Abstract 15871: Trends in Myocardial Infarction as a Secondary Discharge Diagnosis

Background: Little research has focused on myocardial infarctions (MIs) that are not the primary illness, and thus MI is listed as a secondary hospital discharge diagnosis. Objective: We examined secular trends in the proportion of MI hospitalizations where MI was a secondary diagnosis and changes in the primary reasons for hospitalization associated with secondary diagnosis of MI. Methods: Using data from the Medicare national 5% sample for 1999-2011, we identified MI hospitalizations as inpatient claims with primary or secondary discharge diagnoses of International Classification of Diseases, Ninth Revision, Clinical Modification, 410.xx, excluding 410.x2 which represents subsequent episodes of care. This approach has previously been validated. For each year, we calculated the proportion of MI hospitalizations where the MI diagnosis appeared in a secondary position. For hospitalizations with MI as a secondary diagnosis, we examined the trends in the most common primary diagnoses. Results: The number of MI hospitalizations ranged from 20,821 in 2003 to 17,640 in 2011 among Medicare beneficiaries in the 5% sample. In 1999, 21% of MI diagnoses were secondary diagnoses, and by 2011, this proportion had risen to 36% (Figure). Over this time period, the proportion of MI hospitalizations with congestive heart failure and coronary atherosclerosis as primary diagnoses declined (16% to 9%, and 11% to 3%, respectively) and the proportion with septicemia as the primary diagnosis increased (3% to 17%). Conclusions: Our results suggest that a growing number and percentage of MI diagnoses are occurring among patients hospitalized for other reasons. The causes of this increase require further investigation. Because individuals whose primary illness is not MI are underrepresented in research studies, the appropriateness and effectiveness of secondary prevention of cardiovascular disease in this patient population are not well described.