2026 ICD-10-CM Diagnosis Code I51.4Myocarditis, unspecified

ICD-10-CM Codes›I00–I99›I30-I5A›I51

ICD-10-CM I51.4
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

I51.4 is a billable ICD-10-CM diagnosis code for myocarditis, unspecified. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026) and groups to MS-DRG 314 through 316. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Myocarditis and cardiomyopathy.

For Medicare Advantage risk adjustment, I51.4 maps to CMS-HCC Category 227 (Cardiomyopathy/Myocarditis) under the V28 model, adding a risk factor of about 0.189 for a community, non-dual, aged beneficiary in payment year 2026.

Code Identity

ICD-10-CM Code
I51.4
Billable Status
Yes — Valid for Submission
Code Describes
Myocarditis, unspecified
Short Description
Myocarditis, unspecified
Same as the full description in the CMS dataset.
Parent Code
I51
Complications and ill-defined descriptions of heart disease

Code Classification

ChapterI00–I99Diseases of the circulatory system
SectionI30-I5AOther forms of heart disease
CategoryI51Complications and ill-defined descriptions of heart disease
This CodeI51.4Myocarditis, unspecified

Medicare Risk Adjustment (HCC)Billing

I51.4 maps to a payment category in the CMS-HCC model used to risk-adjust Medicare Advantage payments. Weights are the published community factors for payment year 2026.

CMS-HCC V28 Category (Payment Model)
HCC 227— Cardiomyopathy/Myocarditis
Payment HCC · PY 2026 one of 14 ICD-10-CM codes in this category
Risk Adjustment Factor (RAF) Weight
+0.189
community, non-dual, aged · ranges 0.145–0.200 across segments
Hierarchy
Superseded by HCC 221, HCC 222, HCC 223, HCC 224, HCC 225, and HCC 226
a more severe related category takes the payment when both are reported
Prior Model (CMS-HCC V24)
HCC 85
V24 retired V28 pays 100% of MA risk scores since PY 2026
Other CMS Models
PACE (CMS-HCC V22): HCC 85 · ESRD (V21): HCC 85 · ESRD (V24): HCC 85
ESRD V21 weights: 0.082 dialysis, 0.186–0.336 functioning graft · ESRD V24 weights: 0.063 dialysis, 0.169–0.302 functioning graft
Part D (RxHCC)
RxHCC 186 — Heart Failure
also risk-adjusts in the Part D prescription drug model (V08)

Source: CMS Payment Year 2026 risk adjustment mappings and model software. Weights are relative factors, not dollar amounts; a beneficiary's total RAF also includes demographics and interactions. Browse all CMS-HCC categories.

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Acute HIV infection
  • Acute myocarditis
  • Chronic interstitial myocarditis
  • Chronic myocarditis
  • Fibroid myocarditis
  • Heart disease caused by ionizing radiation
  • Heart disease co-occurrent with human immunodeficiency virus infection
  • Heart failure with normal ejection fraction
  • Heart failure with reduced ejection fraction
  • Heart failure with reduced ejection fraction due to myocarditis
  • Interstitial myocarditis
  • Left ventricular failure with normal ejection fraction due to myocarditis
  • Lymphocytic myocarditis
  • Myocardial dysfunction with sepsis
  • Myocarditis
  • Myocarditis caused by physical agent
  • Myocarditis caused by radiation
  • Myocarditis co-occurrent with human immunodeficiency virus infection
  • Myocarditis with AIDS
  • Senile myocarditis

Tabular List NotesGuidance

Coding notes and annotation back-references applicable to this code.

Inclusion Terms

  • Chronic (interstitial) myocarditis
  • Myocardial fibrosis
  • Myocarditis NOS

Type 1 Excludes

  • acute or subacute myocarditis I40

Index to Diseases and InjuriesGuidance

Alphabetical index entries that point to this code.

Clinical ClassificationClinical

AHRQ’s CCSR groups this code into broader clinical categories.

CCSR CIR005
Myocarditis and cardiomyopathy
Default principal diagnosis: inpatient Yes · outpatient Yes

Clinical InformationClinical

  • Chagas Cardiomyopathy

    a disease of the cardiac muscle developed subsequent to the initial protozoan infection by trypanosoma cruzi. after infection, less than 10% develop acute illness such as myocarditis (mostly in children). the disease then enters a latent phase without clinical symptoms until about 20 years later. myocardial symptoms of advanced chagas disease include conduction defects (heart block) and cardiomegaly.

  • Myocarditis

    inflammatory processes of the muscular walls of the heart (myocardium) which result in injury to the cardiac muscle cells (myocytes, cardiac). manifestations range from subclinical to sudden death (death, sudden). myocarditis in association with cardiac dysfunction is classified as inflammatory cardiomyopathy usually caused by infection, autoimmune diseases, or responses to toxic substances. myocarditis is also a common cause of dilated cardiomyopathy and other cardiomyopathies.

  • Grade 1 Myocarditis, CTCAE 6.0

    asymptomatic with lab or ecg findings only

  • Grade 2 Myocarditis, CTCAE 6.0

    symptoms with moderate activity or exertion

  • Grade 3 Myocarditis, CTCAE 6.0

    severe with symptoms at rest or with minimal activity or exertion; intervention indicated; new onset of symptoms

  • Grade 4 Myocarditis, CTCAE 6.0

    life-threatening consequences; urgent intervention indicated (e.g., continuous iv therapy or mechanical hemodynamic support); hemodynamically unstable requiring non-invasive or invasive ventilation; complete or high grade av block; significant ventricular arrhythmia

  • Grade 5 Myocarditis, CTCAE 6.0

    death

  • Myocarditis, CTCAE 6.0

    a disorder characterized by inflammation of the muscle tissue of the heart.

  • Myocarditis, CTCAE|Myocarditis|Myocarditis|Myocarditis, CTCAE 5.0

    a disorder characterized by inflammation of the muscle tissue of the heart.

  • Acute Myocardial Injury with Myocarditis Related to COVID-19|Acute myocardial injury with myocarditis

    acute myocardial injury with clinical, imaging, and or pathology evidence supporting myocarditis in a patient with probable or confirmed acute covid-19.

  • Acute Myocardial Injury without Ischemia, Heart Failure, Ventricular Dysfunction, or Myocarditis Related to COVID-19|Acute myocardial injury without ischemia, HF, ventricular dysfunction, or myocarditis

    acute myocardial injury, without evidence of acute ischemia, heart failure, a newly reduced left or right ventricular ejection fraction, cardiogenic shock, or myocarditis in a patient with probable or confirmed acute covid-19.

  • Acute Myocarditis

    the sudden onset of inflammation of heart muscle with myocellular necrosis; this is generally secondary to an infectious cause, and patients often have a recent history of a flu-like illness.

  • Acute Rheumatic Myocarditis

    inflammation of the myocardium in acute rheumatic heart disease.

  • Bacterial Myocarditis

    myocarditis that is caused by an infection with a bacterial agent.

  • Giant Cell Myocarditis

    an often fatal inflammatory disorder that affects the myocardium. morphologically, it is characterized by the presence of an inflammatory infiltrate in the myocardial tissue that includes multinucleated giant cells. it is often associated with the presence of an autoimmune disorder. patients present with arrhythmias and/or heart failure. heart transplantation is the only treatment option available.

  • Grade 2 Myocarditis, CTCAE|Grade 2 Myocarditis

    symptoms with moderate activity or exertion

  • Grade 3 Myocarditis, CTCAE|Grade 3 Myocarditis|Grade 3 Myocarditis

    severe with symptoms at rest or with minimal activity or exertion; intervention indicated; new onset of symptoms

  • Grade 4 Myocarditis, CTCAE|Grade 4 Myocarditis|Grade 4 Myocarditis

    life-threatening consequences; urgent intervention indicated (e.g., continuous iv therapy or mechanical hemodynamic support)

  • Grade 5 Myocarditis, CTCAE|Grade 5 Myocarditis|Grade 5 Myocarditis

    death

  • Immune Checkpoint Inhibitor-related Myocarditis|ICI-MC

    myocarditis associated with the therapeutic use of an immune checkpoint inhibitor.

  • Interstitial Myocarditis

    inflammation of the heart characterized by infiltration of the interstitial tissues by inflammatory cells, histiocytes, and the formation of granulomas. giant cells are often present.

  • Myocarditis

    inflammation of the muscle tissue of the heart.

  • Myocarditis, CTCAE 5.0|Myocarditis|Myocarditis

    a disorder characterized by inflammation of the muscle tissue of the heart.

  • Viral Myocarditis|Myocarditis

    myocarditis that is caused by an infection with a viral agent.

Patient EducationClinical

Cardiomyopathy

Cardiomyopathy is the name for diseases of the heart muscle. These diseases enlarge your heart muscle or make it thicker and more rigid than normal. In rare cases, scar tissue replaces the muscle tissue.

Read the full article at MedlinePlus

Courtesy of MedlinePlus, a service of the U.S. National Library of Medicine.

Convert I51.4 to ICD-9-CMHistory

The closest ICD-9-CM equivalents under the General Equivalence Mappings.

ICD-9-CM
429.0 Myocarditis NOS
Exact Match The mapping is direct, with no qualifiers.

Code HistoryHistory

FY 2016AddedAdded to the ICD-10-CM code setEffective October 1, 2015, the first year of ICD-10-CM.
FY 2017–2025No changes
FY 2026CurrentCurrent code set, no changesEffective October 1, 2025 through September 30, 2026.

Questions About I51.4Overview

What is the ICD-10 code for myocarditis, unspecified?

The ICD-10-CM code for myocarditis, unspecified is I51.4 (sometimes written as I514). It is billable on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

Is I51.4 (Complications and ill-defined descriptions of heart disease) a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report myocarditis, unspecified on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

What MS-DRG does I51.4 group to?

When myocarditis, unspecified is the principal diagnosis on an inpatient stay, it groups to MS-DRG 314, 315, 316, with relative weights from 0.6821 to 2.0852 depending on complications. Higher weights mean higher Medicare reimbursement.

What is the ICD-9 equivalent of I51.4?

Under the General Equivalence Mappings, myocarditis, unspecified converts to ICD-9-CM 429.0 (myocarditis NOS). The mapping is a direct match.

What HCC is I51.4?

I51.4 (myocarditis, unspecified) maps to CMS-HCC Category 227 (Cardiomyopathy/Myocarditis), commonly written as HCC 227, in the CMS-HCC V28 model used for Medicare Advantage risk adjustment in payment year 2026. It mapped to HCC 85 under the retired V24 model. It also maps in the PACE (CMS-HCC V22), ESRD (V21), and ESRD (V24) models. In the Part D prescription drug model it maps to RxHCC 186.

Does I51.4 risk-adjust for Medicare Advantage payment?

Yes. When documented and reported on a Medicare Advantage encounter, I51.4 adds a risk adjustment factor of about 0.189 to the beneficiary's RAF score for a community, non-dual, aged enrollee (published V28 weights range from 0.145 to 0.200 depending on the payment segment). A more severe related category (HCC 221, HCC 222, HCC 223, HCC 224, HCC 225, and HCC 226) supersedes it when both are reported. See the full factor table on the HCC 227 category page.