Hospitalizations and Outcomes of T1MI Observed Before and After the Introduction of MI Subtype Codes

Background: International Classification of Disease (ICD)-10 coding of type 1 myocardial infarction (MI) is used for reimbursement, value-based programs, and clinical research.

Objectives: This study sought to determine whether the introduction of ICD-10 codes for type 2 and types 3-5 MI was associated with changes in hospitalizations for ICD-10 codes now attributed to type 1 MI.

Methods: Using the Nationwide Readmissions Database, we identified patients with ICD-10 codes now attributed to type 1 MI between January 2016 and December 2018. Patients were stratified according to the timing of their event in relation to the introduction of the type 2 and types 3-5 MI codes on October 1, 2017.

Results: There were 2,680,323 hospitalizations for ICD-10 codes now attributed to type 1 MI; after adjustment for seasonality, there was a 13.7% decline in hospitalizations after the introduction of the new subtype codes. Patients with ICD-10 codes now attributed to type 1 MI after the coding change were less likely to be female, had lower prevalence of several cardiovascular and noncardiovascular comorbidities, and had higher rates of coronary angiography and revascularization. After introduction of the new codes, there was a positive deflection in the slope of risk-adjusted in-hospital mortality (0.007%; P <0.001) and a negative deflection in risk-adjusted 30-day readmission (-0.002%; P = 0.05) for patients with ICD-10 codes now attributed to type 1 MI.

Conclusions: The introduction of ICD-10 codes for type 2 and types 3-5 MI was associated with a decrease in hospitalizations for ICD-10 codes now attributed to type 1 MI and changes in the observed characteristics and treatment patterns of these patients.