Alphabetic Index FY2027

I21.A1

Myocardial infarction type 2

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.A1 (also written I21A1) is a billable ICD-10-CM diagnosis code for myocardial infarction type 2. 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 Code first instruction, 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

I22 is only for a subsequent MI when both the initial and the subsequent MI are type 1 or unspecified. Subsequent type 2, 4 or 5 MIs have their own I21.A codes.

Guidelines I.C.9.e.4Subsequent acute myocardial infarctionRead the official text

Type 2 MI is I21.A1 with the underlying cause coded first, even if described as STEMI or NSTEMI. I24.89 is not assigned for demand ischemia. Types 3, 4 and 5 go to I21.A9.

Guidelines I.C.9.e.5Type 2 and other types of myocardial infarctionRead the official text

Code first

If the underlying condition listed here is present and known, sequence it first.

This code

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
  • Myocardial infarction due to demand ischemia
  • Myocardial infarction secondary to ischemic imbalance

Alphabetic Index terms

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