Background Microsize myocardial infarction (MI) is a recently described phenomenon that meets rigorous criteria for MI with very low peak troponin elevations. We aim to compare the risk for cardiovascular events and mortality following microsize versus usual MIs.

Methods and results Prospective cohort analysis of REasons for Geographic And Racial Differences in Stroke (REGARDS) study participants without a history of coronary heart disease (CHD) who had an incident MI between 2003 and 2015. Incident MIs were classified as microsize MI (peak troponin <0.5 ng/mL) or usual MI (peak troponin ≥0.5 ng/mL). Participants were followed for a composite of cardiovascular events that included recurrent MI, coronary revascularisation, fatal CHD and heart failure, and all-cause mortality. Overall, 1024 participants with an incident MI were included in the analysis (328 with microsize MI and 696 with usual MI). Participants with microsize MI were more likely to be older and black. The multivariable-adjusted adjustment HR for cardiovascular events among participants with microsize versus usual MI after a median follow-up of 1.7 years was 1.11 (95% CI 0.86 to 1.44). The multivariable-adjusted HR for all-cause mortality after 28 days from incident MI among participants with microsize versus usual MI after a median follow-up of 3.6 years was 1.09 (95% CI 0.81 to 1.45).

Conclusion Microsize MIs have a prognostic value for future cardiovascular events and mortality comparable to usual MIs. These findings should encourage clinicians to initiate secondary prevention strategies in patients with microsize MI until this emerging clinical entity is better understood.