Alphabetic Index FY2027

I21.29

ST elevation (STEMI) myocardial infarction involving other sites

FY2027Chapter 9: Diseases of the circulatory system/I20–I25 Ischemic heart diseases

BillableComplete code, valid to report
Care settingsHome Health Primary · MMTA - Cardiac and CirculatoryHospital MCC

I21.29 (also written I2129) is a billable ICD-10-CM diagnosis code for ST elevation (STEMI) myocardial infarction involving other sites. It is part of the FY2027 code set, effective October 1, 2026. It is one of the 13 codes in category I21 (Acute myocardial infarction). The Alphabetic Index lists it under 8 entries. Its coding notes include a Use additional code instruction, a Code also instruction and 13 Excludes2 entries.

Guidelines tips

From the FY2027 Official Guidelines. ICD10Ref adds no rules of its own.

I21 may continue to be reported while the MI is four weeks old or less. After that, continuing care is coded as aftercare, and a healed MI may be coded I25.2.

Guidelines I.C.9.e.1Acute myocardial infarction: the four-week time frameRead the official text

A type 1 NSTEMI that evolves to STEMI is coded as STEMI. I21.9 is the default for an unspecified AMI. A nontransmural or subendocardial AMI is still coded as subendocardial when the site is given.

Guidelines I.C.9.e.1; I.C.9.e.2; I.C.9.e.3STEMI and NSTEMIRead the official text

Use additional code

Add the listed code as a secondary code, if known.

Inherited from I21 Acute myocardial infarction
  • code, if applicable, to identify:
    • exposure to environmental tobacco smoke (Z77.22)
    • history of tobacco dependence (Z87.891)
    • occupational exposure to environmental tobacco smoke (Z57.31)
    • status post administration of tPA (rtPA) in a different facility within the last 24 hours prior to admission to current facility (Z92.82)
    • tobacco dependence (F17.-)
    • tobacco use (Z72.0)

Code also

Two codes may be needed to describe the condition. This note gives no sequencing order.

Also from chapter/block level (1 note)
Inherited from I20–I25 Ischemic heart diseases
  • the presence of hypertension (I10-I1A)

Excludes2

Not included here. Both codes may be reported if the patient has both conditions.

Inherited from I21 Acute myocardial infarction
  • old myocardial infarction (I25.2)
  • postmyocardial infarction syndrome (I24.1)
  • subsequent type 1 myocardial infarction (I22.-)
Also from chapter/block level (10 notes)
Inherited from Chapter 9 Diseases of the circulatory system
  • certain conditions originating in the perinatal period (P04-P96)
  • certain infectious and parasitic diseases (A00-B99)
  • complications of pregnancy, childbirth and the puerperium (O00-O9A)
  • congenital malformations, deformations, and chromosomal abnormalities (Q00-Q99)
  • endocrine, nutritional and metabolic diseases (E00-E88)
  • injury, poisoning and certain other consequences of external causes (S00-T88)
  • neoplasms (C00-D49)
  • symptoms, signs and abnormal clinical and laboratory findings, not elsewhere classified (R00-R94)
  • systemic connective tissue disorders (M30-M36)
  • transient cerebral ischemic attacks and related syndromes (G45.-)

Care settings

How CMS programs treat this code. These are CMS claim edits and list entries, not coverage or eligibility rules.

Home Health Primary

Accepted as the primary (principal) diagnosis on a home health claim. The Home Health Grouper assigns it to the MMTA - Cardiac and Circulatory clinical group.

Source: CMS Home Health Grouper Software v07.2.26, effective October 1, 2026

Hospital MCC

Counts as a major complication or comorbidity (MCC) when reported as a secondary diagnosis on an inpatient claim. It does not count when the principal diagnosis is one of the 25 codes in its exclusion list. Some MS-DRGs also ignore a CC or MCC that defines the DRG.

Source: CMS MS-DRG Definitions Manual v44, effective October 1, 2026, Appendix C

Includes

Further defines or gives examples of the content of the category.

Inherited from I21 Acute myocardial infarction
  • cardiac infarction
  • coronary (artery) embolism
  • coronary (artery) occlusion
  • coronary (artery) rupture
  • coronary (artery) thrombosis
  • infarction of heart, myocardium, or ventricle
  • myocardial infarction specified as acute or with a stated duration of 4 weeks (28 days) or less from onset

Inclusion terms

Conditions classified to this code.

This code
  • 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)
  • Posterior (true) transmural (Q wave) infarction (acute)
  • Posterobasal transmural (Q wave) infarction (acute)
  • Posterolateral transmural (Q wave) infarction (acute)
  • Posteroseptal transmural (Q wave) infarction (acute)
  • Septal transmural (Q wave) infarction (acute) NOS

Alphabetic Index terms

Terms the official Alphabetic Index lists for this code. 8 entries