I40.9Acute myocarditis, unspecified

I40.9 is the ICD-10-CM diagnosis code for acute myocarditis, unspecified. It belongs to category I40 (acute myocarditis), block I30-I5A (other forms of heart disease) and chapter 9 (diseases of the circulatory system). It is billable, so it can be reported on its own for dates of service from October 1, 2026 through September 30, 2027.

Within I40, choose I40.9 (unspecified) only when documentation doesn't support a more specific option: infective myocarditis (I40.0), isolated myocarditis (I40.1) and other acute myocarditis (I40.8).

As a secondary diagnosis it is an MCC (major complication or comorbidity), which can move an inpatient stay into the highest-paying "with MCC" MS-DRG of its family, unless the principal diagnosis excludes it.

In MS-DRG v44.0, it is part of the grouping logic for DRG 314 (Other Circulatory System Diagnoses with MCC, relative weight 2.0873), DRG 315 (Other Circulatory System Diagnoses with CC, relative weight 0.9508), DRG 316 (Other Circulatory System Diagnoses without CC/MCC, relative weight 0.6677), DRG 974 (HIV with Major Related Condition with MCC, relative weight 3.0506), DRG 975 (HIV with Major Related Condition with CC, relative weight 1.3252) and DRG 976 (HIV with Major Related Condition without CC/MCC, relative weight 0.8124), in MDC 05 (Diseases and Disorders of the Circulatory System) and MDC 25 (Human Immunodeficiency Virus Infections), as a principal or secondary diagnosis. The DRG a claim lands in depends on the principal diagnosis, procedures and any CC/MCC secondary diagnoses.

It has been part of ICD-10-CM since the code set took effect on October 1, 2015 and hasn't changed since.

Before ICD-10, this condition was coded in ICD-9-CM as 422.90 (acute myocarditis, unspecified).