What is ICD-10 code I21.29?

I21.29 is the ICD-10-CM diagnosis code for ST elevation (STEMI) myocardial infarction involving other sites. A heart attack with ST segment elevation affecting other locations of the heart not specifically categorized, such as the right ventricle or posterior wall. I21.29 is a billable code, meaning it can be reported on a claim as a primary or secondary diagnosis when supported by documentation. I21.29 is part of the FY2026 ICD-10-CM code set published by the CMS National Center for Health Statistics and used on Medicare, Medicaid, and commercial claims in the United States.

Yes. I21.29 is a billable ICD-10-CM code that can be reported on professional and institutional claims as a primary or secondary diagnosis when the provider documentation supports the specific condition described. Billable codes in ICD-10-CM are coded to the highest level of specificity available for the documented diagnosis. I21.29 is current for fiscal year 2026 and remains in the active CMS ICD-10-CM code set published by the National Center for Health Statistics.

What is the V28 vs V24 difference for I21.29?

CMS phased in the V28 risk adjustment model over payment years 2024 (33%), 2025 (67%), and 2026 (100%), replacing V24. V28 consolidates HCC categories, removes some lower-acuity conditions, and recalibrates RAF weights across the entire model. For I21.29, V28 maps to Acute Myocardial Infarction at RAF 0.000 while V24 mapped to Acute Myocardial Infarction at RAF 0.274. Both models can still appear in legacy data and in payer-specific lookback periods.

What should coders know when reporting I21.29?

This code is used for STEMI in atypical locations or when the specific site is documented but doesn't fit other categories Ensure the documentation clearly identifies the location and culprit vessel before assigning this code Because I21.29 maps to a payment HCC, the documentation must also satisfy MEAT criteria (Monitor, Evaluate, Assess, or Treat) for the encounter to count toward the patient's risk adjustment score.

I21.29 ICD-10 Code: ST elevation (STEMI)

ICD-10-CM Code View

HCC Buddy Code Card

Digital ICD-10 code-book layout with official code detail, always-visible risk models, Code Trumping, and Buddy coding guidance.

Code lookupI21.29Effective periodFY2026 Apr Update (Apr 1-Sep 30)

FY 2026 Apr update / Diseases of the circulatory system (I00-I99) / Ischemic heart diseases (I20-I25)

I21.29

Billable / SpecificICD-10-CMOfficial ICD-10-CMCodebook guidance

ST elevation (STEMI) myocardial infarction involving other sites

A heart attack with ST segment elevation affecting other locations of the heart not specifically categorized, such as the right ventricle or posterior wall.

Buddy Insight

This code captures STEMI in atypical locations such as the right ventricle, posterior wall, or other sites not classified under anterior, inferior, or left circumflex categories.

CMS-HCC V28

Mapped

HCC 228

Acute Myocardial Infarction

RAF 0.0

CMS-HCC V24

Mapped

HCC 86

Acute Myocardial Infarction

RAF 0.274

ACA/HHS

Not risk-adjusted for this model/period.

RAF 0

ESRD/PACE

Mapped

HCC 86

Acute Myocardial Infarction

RAF 0.0

RXHCC

Mapped

HCC 188

Coronary Artery Disease and A...

RAF 0.0

Code Trumping

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Code Book Path

Official

I21Acute myocardial infarction

I21.2ST elevation (STEMI) myocardial infarction of other sites

I21.29ST elevation (STEMI) myocardial infarction involving other sites

Inclusion Terms

Official

Acute transmural myocardial infarction of other sites
Apical-lateral transmural (Q wave) infarction (acute)
Basal-lateral transmural (Q wave) infarction (acute)
High lateral transmural (Q wave) infarction (acute)
Lateral (wall) NOS transmural (Q wave) infarction (acute)

Excludes 2

Official

ICD-10-CM does not list Excludes 2 notes for I21.29 in this effective period.

Related Child Codes

Official

I21.21ST elevation (STEMI) myocardial infarction involving left circumflex coronary artery

Includes

Official

ICD-10-CM does not list Includes notes for I21.29 in this effective period.

Excludes 1

Official

ICD-10-CM does not list Excludes 1 notes for I21.29 in this effective period.

Code First

Official

ICD-10-CM does not list Code First sequencing instructions for I21.29 in this effective period.

Use Additional

Official

ICD-10-CM does not list Use Additional Code instructions for I21.29 in this effective period.

Code Also

Official

ICD-10-CM does not list Code Also instructions for I21.29 in this effective period.

Buddy Documentation Tip

HCC Buddy guidance

Documentation of STEMI with specification of the involved site

Identification of the culprit vessel if known

Electrocardiogram findings supporting the specific location

Cardiac biomarker elevation

MEAT Support

HCC Buddy guidance

Documentation of STEMI with specification of the involved site

Identification of the culprit vessel if known

Electrocardiogram findings supporting the specific location

Cardiac biomarker elevation

Audit Caution

HCC Buddy guidance

Using this code as a default when documentation supports a more specific site code

Confusing this code with I21.3 (unspecified) — I21.29 is for documented but atypical locations

Failing to capture posterior wall STEMI which may present with reciprocal anterior ST depression

Not assigning this code for documented right ventricular infarction

Common Mistakes

HCC Buddy guidance

I21.01-I21.21 — Site-specific STEMI codes: use these when anterior wall, inferior wall, or left circumflex involvement is documented

I21.3 — STEMI of unspecified site: use when the site is truly unknown, not when an atypical site is documented

I22.8 — Subsequent STEMI of other sites: for recurrent events in this territory within 4 weeks

I21.4 — Non-ST elevation myocardial infarction: different electrocardiogram pattern

Last updated: FY2026 ICD-10-CM Apr update, Apr 1, 2026 through Sep 30, 2026. CMS-HCC V28 is 100% phased in for payment year 2026.

Is I21.29 an HCC code?

Yes. I21.29 maps to Acute Myocardial Infarction under the CMS-HCC V28 risk adjustment model (and Acute Myocardial Infarction under V24).

HCC Category Mapping

V28HCC 228, Acute Myocardial Infarction

0.000

V24HCC 86, Acute Myocardial Infarction

0.274

ESRDHCC 86, Acute Myocardial Infarction

0.000

RxHCCHCC 188, Coronary Artery Disease and Angina

0.000

RAF weights shown are the community, non-dual, aged base weights from the CMS risk adjustment model file. Actual per-patient RAF contribution depends on member segment, interactions, and the model year used by the payer. V28 is the CMS-HCC model phased in over payment years 2024–2026; V24 remains in use during the transition and for historical data.

MEAT Criteria for I21.29

For I21.29to count as a valid HCC diagnosis in a given encounter, the provider's documentation must show MEAT: Monitor, Evaluate, Assess, or Treat. A diagnosis from a prior year does not carry forward automatically, it has to be re-documented and supported each calendar year.

MMonitor: signs, symptoms, disease progression, or lab trending documented in the note
EEvaluate: test results, medication response, or physical findings reviewed by the provider
AAssess: explicit mention in the assessment or plan with acknowledgment of status
TTreat: medication, referral, procedure, therapy, or counseling tied to the diagnosis

Only one of M/E/A/T is required to support the code, but the documentation must be specific enough to show that the provider actually addressed I21.29 during that encounter, not just copy-forwarded from a problem list.

What This Code Means

I21.29 is the ICD-10-CM diagnosis code for st elevation (stemi) myocardial infarction involving other sites. A heart attack with ST segment elevation affecting other locations of the heart not specifically categorized, such as the right ventricle or posterior wall. I21.29 sits in the ICD-10-CM chapter for diseases of the circulatory system (i00-i99), within the section covering ischemic heart diseases (i20-i25).

Under the CMS-HCC V28 risk adjustment model, I21.29 maps to Acute Myocardial Infarction (HCC 228) with a community, non-dual, aged base RAF weight of 0.000. Under the older V24 model, I21.29 mapped to the same category but with a base RAF weight of 0.274, V28 recalibrated weights across the entire model. V28 is the CMS-HCC risk adjustment model that reached 100% phase-in for payment year 2026, replacing V24 which was used during the PY2024–PY2025 transition.

This code is used for STEMI in atypical locations or when the specific site is documented but doesn't fit other categories. Because I21.29 maps to a payment HCC, the provider's documentation must satisfy MEAT criteria (Monitor, Evaluate, Assess, or Treat) for the encounter to count toward the patient's Medicare Advantage risk adjustment score. When documentation is ambiguous, coders should issue a provider query rather than assume the highest-specificity variant.

HCC Buddy maintains structured V28 and V24 mapping, RAF weights, and MEAT documentation criteria for I21.29 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.

Coding Tips

•This code is used for STEMI in atypical locations or when the specific site is documented but doesn't fit other categories
•Ensure the documentation clearly identifies the location and culprit vessel before assigning this code

Clinical Significance

This code captures STEMI in atypical locations such as the right ventricle, posterior wall, or other sites not classified under anterior, inferior, or left circumflex categories. These atypical presentations may carry unique treatment considerations, such as avoiding preload reduction in right ventricular infarction. Accurate coding ensures appropriate risk stratification for these less common but clinically significant presentations.

Documentation Requirements

✓Documentation of STEMI with specification of the involved site
✓Identification of the culprit vessel if known
✓Electrocardiogram findings supporting the specific location
✓Cardiac biomarker elevation
✓Cardiac catheterization or imaging results
✓Timing within the 4-week acute window

Commonly Confused Codes

•I21.01-I21.21: Site-specific STEMI codes: use these when anterior wall, inferior wall, or left circumflex involvement is documented
•I21.3: STEMI of unspecified site: use when the site is truly unknown, not when an atypical site is documented
•I22.8: Subsequent STEMI of other sites: for recurrent events in this territory within 4 weeks
•I21.4: Non-ST elevation myocardial infarction: different electrocardiogram pattern