ICD-10 Code I40.9 – Acute myocarditis, unspecified (2026): Diagnosis, Symptoms & Billing Guide
2026 ICD-10-CM Diagnosis Code I40.9 – Acute myocarditis, unspecified
What it is
I40.9 identifies acute inflammation of the heart muscle when the provider does not specify a cause or type. Use this code for a new, active myocarditis diagnosis without a more detailed subtype.
Clinical signs
Clinical features vary; refer to documentation. Common findings may include chest pain, shortness of breath, palpitations, fatigue, or evidence of myocardial injury on cardiac testing.
When to use this code
Use I40.9 when the record states acute myocarditis but gives no further detail about etiology or subtype. It also fits when the clinician documents myocarditis and the chart does not support a more specific code. Check documentation if the cause is known, such as infectious or autoimmune myocarditis.
Do not use for
Do not use I40.9 for chronic myocarditis, cardiomyopathy, or nonspecific chest pain without a documented myocarditis diagnosis. If the provider specifies a different acute myocarditis type, code that more specific diagnosis instead.
Coding tip
Code the documented specificity first; if the note only says acute myocarditis, I40.9 is the correct default.