I42.1 ICD-10-CM Code: Obstructive hypertrophic cardiomyopathy

Coding instructions

Official Tabular Instructional Notes: the inclusion, exclusion and sequencing notes published for I42.1 in the official ICD-10-CM tabular list, quoted as published.

Source: CMS/CDC — ICD-10-CM Tabular ListRelease: FY2026Effective: October 1, 2025

Notes without a marker are published on I42.1 itself; “inherited from” names the category or block whose note applies here.

Coder workflow for I42.1

MedCoder structured workflow — derived from this code’s own official record

Choose the right path

1. Is the underlying (etiologic) condition the Code First note names documented?
   Yes → Sequence the underlying condition first, then I42.1.
   No → Continue; do not add an underlying condition the record does not document.

   ReviewO99.4

Consider I42.1. Then confirm the code is valid for the date of service in the Verify section.

Documentation check

The provider’s diagnostic statement

Codes are assigned from the provider’s documented diagnosis, not from clinical criteria, test values or a medication list (Guidelines I.A.19).

The underlying (etiologic) condition

Named in the Code First note; sequenced before this code when documented (Guidelines I.A.13).

Official instructions as workflow

• Excludes2 — not part of I42.1(3 notes)

  Coding workflow: The conditions named in this note are not included in I42.1. When the record documents both, both may be reported; the note is a boundary, not a prohibition.

  CompareI25.5, O90.3, I51.7

  See the official tabular notes · Guidelines I.A.12.b

• Code First — sequencing check(1 note)

  Coding workflow: Check whether the underlying or etiologic condition the note names is documented. When it is, sequence it before I42.1. Do not add an underlying condition the record does not document.

  ReviewO99.4

  See the official tabular notes · Guidelines I.A.13

Every row is derived from this code’s own record — its title, tabular notes, 7th-character family and same-category siblings — with fixed MedCoder wording; nothing is inferred about a patient. The official notes and guideline text are in the sections each row links to, and they control.

Indexed Clinical Terms (7)

Official source data — entries quoted as published, in the Index’s own lookup phrasing

Clinical term phrases from the official ICD-10-CM Index to Diseases and Injuries that map to this code. These are alphabetic-index entries shown as the Index writes them — lookup phrasing, not necessarily the wording of a final diagnosis.

• Cardiomyopathy (familial) (idiopathic), hypertrophic (nonobstructive), obstructive
• Myocardiopathy (congestive) (constrictive) (familial) (hypertrophic nonobstructive) (idiopathic) (infiltrative) (obstructive) (primary) (restrictive) (sporadic), hypertrophic obstructive
• Stenosis, stenotic (cicatricial), hypertrophic subaortic (idiopathic)
• Stenosis, stenotic (cicatricial), myocardium, myocardial, hypertrophic subaortic (idiopathic)
• Stenosis, stenotic (cicatricial), subaortic (congenital), hypertrophic (idiopathic)
• Stricture, myocardium, myocardial, hypertrophic subaortic (idiopathic)
• Stricture, subaortic, hypertrophic (acquired) (idiopathic)

Coding Notes

MedCoder editorial — not official ICD-10-CM text; reviewed against the cited source

What this code means

I42.1 reports hypertrophic cardiomyopathy in which outflow obstruction is documented — the classification’s split point for this condition. Its inclusion term is the older clinical name for the same entity, hypertrophic subaortic stenosis (idiopathic). The neighbouring code I42.2, Other hypertrophic cardiomyopathy, carries nonobstructive hypertrophic cardiomyopathy, so the two codes divide one disease along a single documented finding: whether obstruction is present.

Coding guidance

• Obstruction is what selects this code over I42.2. The classification does not treat the obstructive form as a default: when hypertrophic cardiomyopathy is documented without obstruction, I42.2 carries the nonobstructive form as its own inclusion term.
• The Code First note applies to pregnancy only: a pre-existing cardiomyopathy complicating pregnancy and the puerperium is sequenced after O99.4. Outside pregnancy the code has no sequencing instruction.
• Three conditions are Excludes2, meaning they are separate conditions that may be reported alongside this code when both are documented: ischemic cardiomyopathy (I25.5), peripartum cardiomyopathy (O90.3), and ventricular hypertrophy (I51.7).
• Ventricular hypertrophy (I51.7) as an Excludes2 is the one worth pausing on: thickened myocardium described only as hypertrophy, without a cardiomyopathy diagnosis, is not this code.

Decision-rule scenarios

Scenarios state the decision rule, not a code assignment — code selection always follows the provider's documentation and the ICD-10-CM Official Guidelines.

Documentation checklist

• An explicit statement of outflow obstruction (or its absence) — the single element separating I42.1 from I42.2.
• That the diagnosis is a cardiomyopathy, not ventricular hypertrophy alone (Excludes2, I51.7).
• Whether the condition pre-dates a documented pregnancy, when one is in the record.

Codes are assigned from provider documentation; a coder never infers a diagnosis.

Common mistakes

• Assigning I42.1 for hypertrophic cardiomyopathy where obstruction is not documented — nonobstructive disease is I42.2.
• Coding ventricular hypertrophy as a cardiomyopathy; I51.7 is an Excludes2 condition with its own code.
• Treating the Code First note as general sequencing guidance rather than the pregnancy instruction it is.

Clinical context (coding perspective)

The obstruction in this condition is dynamic outflow-tract obstruction, which is why reports may describe it with a gradient. "HOCM" and the older "idiopathic hypertrophic subaortic stenosis" both name this code’s entity; the latter is its literal inclusion term.

Written and reviewed by MedCoder against the official ICD-10-CM files; educational, not medical or billing advice. The official tabular notes and guidelines above take precedence in every case.

Don’t confuse I42.1 with…

MedCoder analysis — pairs from this code’s official notes, distinctions written by MedCoder

The codes most likely to be picked instead of I42.1 — named in its own official notes, or sharing its laterality or encounter family — with what separates them and which documentation moves the case. Each links to the full comparison.

• I42.9— Specificity relationship (Review)

  Overlapping diagnoses in the same category.

  Overlapping codes in the same category — the documentation decides which is the more specific fit.

  What moves you there: A record that documents no more than “Cardiomyopathy, unspecified” takes I42.9; specificity in the record moves it back here.

  Full comparison: I42.1 vs I42.9 →

• I25.5— Excludes2 relationship (Informational)

  Distinct conditions — both may be reported.

  A distinct condition — both may be reported when both are documented.

  What moves you there: Documentation of ischemic cardiomyopathy adds I25.5; it is not part of this code and both may be reported when both are documented.

  Full comparison: I42.1 vs I25.5 →

• O90.3— Excludes2 relationship (Informational)

  Distinct conditions — both may be reported.

  A distinct condition — both may be reported when both are documented.

  What moves you there: Documentation of peripartum cardiomyopathy adds O90.3; it is not part of this code and both may be reported when both are documented.

  Full comparison: I42.1 vs O90.3 →

• I51.7— Excludes2 relationship (Informational)

  Distinct conditions — both may be reported.

  A distinct condition — both may be reported when both are documented.

  What moves you there: Documentation of ventricular hypertrophy adds I51.7; it is not part of this code and both may be reported when both are documented.

  Full comparison: I42.1 vs I51.7 →

Relationships are read from this code's own official notes, its laterality siblings and its encounter family; the wording of each distinction is MedCoder editorial, and the documentation named in each “what moves you there” line is the official note phrase or sibling title itself. Source: CMS ICD-10-CM tabular instructional notes; ICD-10-CM Official Guidelines, Section I.A.12.a.

Verify Before Coding

• CC as a secondary diagnosis (FY2026). Can raise the stay's MS-DRG severity tier.

From the code registry, the Medicare Code Editor, and the MS-DRG Definitions Manual. Check it against a full claim in Claim Check.

Relationships & Classification

MedCoder structured relationships — computed from published CMS and AHRQ datasets

Other codes that name I42.1 or its code family, from the CMS ICD-10-CM tabular instructional notes. Tabular-note edges are stored at the code family level that carries each note.

Referenced by 1 Excludes2 note: I35 — Nonrheumatic aortic valve disorders.

These codes’ tabular lists name this diagnosis as distinct — both may be reported when both are documented.

Referenced by 1 Code First instruction: I5A — Non-ischemic myocardial injury (non-traumatic) (via I42.-).

Each of these codes carries a Code First note naming this condition — when that code is reported, THIS code is sequenced first, ahead of it.

Referenced by 1 Code Also instruction: E74.05 — Lysosome-associated membrane protein 2 [LAMP2] deficiency.

These codes suggest coding this condition alongside when both are present.

MS-DRG Grouper Relationships (FY2026)

Potential MS-DRG participation — not a DRG assignment.

FY2026 MS-DRG: CC — Complication or Comorbidity. Reported as a secondary diagnosis, this code raises the stay's MS-DRG severity tier — except when the principal diagnosis is one of 112 clinically related codes on its CMS exclusion list.

Named in the grouper logic of 3 MS-DRGs: DRG 314 (MDC 05), DRG 315 (MDC 05), DRG 316 (MDC 05).

From the CMS MS-DRG Definitions Manual (Appendices B and C). Actual DRG assignment depends on the complete claim.

Clinical classification (AHRQ CCSR):CIR005 — Myocarditis and cardiomyopathy (default).

Clinical Classifications Software Refined (CCSR) for ICD-10-CM Diagnoses. Healthcare Cost and Utilization Project (HCUP), Agency for Healthcare Research and Quality.

Related Codes

Same clinical process (MS-DRG)

Acts as CC — raises the severity of other admissions. CMS groups these diagnoses into one MS-DRG principal-diagnosis exclusion process — a CC/MCC on this list never raises severity when the principal diagnosis is also on it.

I25.799 — Atherosclerosis of other coronary artery bypass graft(s) with unspecified angina pectoris, I25.811 — Atherosclerosis of native coronary artery of transplanted heart without angina pectoris, I25.812 — Atherosclerosis of bypass graft of coronary artery of transplanted heart without angina pectoris, I25.85 — Chronic coronary microvascular dysfunction, I25.89 — Other forms of chronic ischemic heart disease, I25.9 — Chronic ischemic heart disease, unspecified, I27.841 — Fontan-associated lymphatic dysfunction, I27.848 — Other Fontan-associated condition, I27.849 — Fontan related circulation, unspecified, I42.0 — Dilated cardiomyopathy, I42.2 — Other hypertrophic cardiomyopathy, I42.3 — Endomyocardial (eosinophilic) disease, I42.4 — Endocardial fibroelastosis, I42.5 — Other restrictive cardiomyopathy, I42.6 — Alcoholic cardiomyopathy, I42.7 — Cardiomyopathy due to drug and external agent, I42.8 — Other cardiomyopathies, I42.9 — Cardiomyopathy, unspecified, I44.0 — Atrioventricular block, first degree, I44.1 — Atrioventricular block, second degree, +91 more

Same CMS-HCC risk category (V28)

CMS maps these diagnoses to the same Hierarchical Condition Category (Cardiomyopathy/Myocarditis) for risk-adjusted payment.

A36.81 — Diphtheritic cardiomyopathy, B33.24 — Viral cardiomyopathy, I42.0 — Dilated cardiomyopathy, I42.2 — Other hypertrophic cardiomyopathy, I42.3 — Endomyocardial (eosinophilic) disease, I42.4 — Endocardial fibroelastosis, I42.5 — Other restrictive cardiomyopathy, I42.6 — Alcoholic cardiomyopathy, I42.8 — Other cardiomyopathies, I42.9 — Cardiomyopathy, unspecified, I43 — Cardiomyopathy in diseases classified elsewhere, I51.4 — Myocarditis, unspecified, I51.5 — Myocardial degeneration

Related risk categories

These categories interact through CMS's HCC hierarchy — one can suppress the other's risk-adjustment weight when both are present on a claim.

Heart Failure with Heart Assist Device/Artificial Heart, Acute on Chronic Heart Failure, Acute Heart Failure (Excludes Acute on Chronic), Heart Failure, Except End-Stage and Acute

Same clinical category (CCSR)

AHRQ's Clinical Classifications Software groups these diagnoses under the same clinical category (Myocarditis and cardiomyopathy).

D86.85 — Sarcoid myocarditis, I01.2 — Acute rheumatic myocarditis, I09.0 — Rheumatic myocarditis, I25.5 — Ischemic cardiomyopathy, I40.0 — Infective myocarditis, I40.1 — Isolated myocarditis, I40.8 — Other acute myocarditis, I40.9 — Acute myocarditis, unspecified, I41 — Myocarditis in diseases classified elsewhere, I42.0 — Dilated cardiomyopathy, I42.2 — Other hypertrophic cardiomyopathy, I42.3 — Endomyocardial (eosinophilic) disease, I42.4 — Endocardial fibroelastosis, I42.5 — Other restrictive cardiomyopathy, I42.6 — Alcoholic cardiomyopathy, I42.7 — Cardiomyopathy due to drug and external agent, I42.8 — Other cardiomyopathies, I42.9 — Cardiomyopathy, unspecified, I43 — Cardiomyopathy in diseases classified elsewhere, I51.4 — Myocarditis, unspecified, +12 more

Same Index main term, other category

The ICD-10-CM Index to Diseases and Injuries files this code under the main terms “Stricture”, “Cardiomyopathy”, “Stenosis, stenotic”; these codes share that main term but sit in a different category of the Tabular List.

I25.5 — Ischemic cardiomyopathy (ischemic), I28.8 — Other diseases of pulmonary vessels (artery, pulmonary, acquired), I34.2 — Nonrheumatic mitral (valve) stenosis (mitral, specified cause, except rheumatic), I35.0 — Nonrheumatic aortic (valve) stenosis (aortic), I35.2 — Nonrheumatic aortic (valve) stenosis with insufficiency (aortic, with insufficiency), I36.0 — Nonrheumatic tricuspid (valve) stenosis (tricuspid, nonrheumatic), I36.2 — Nonrheumatic tricuspid (valve) stenosis with insufficiency (tricuspid, nonrheumatic, with insufficiency), I37.0 — Nonrheumatic pulmonary valve stenosis (pulmonary, valve), I37.2 — Nonrheumatic pulmonary valve stenosis with insufficiency (pulmonary, valve, with insufficiency), I38 — Endocarditis, valve unspecified (heart, valve), I51.5 — Myocardial degeneration (myocardium, myocardial), I51.81 — Takotsubo syndrome (takotsubo), I70.0 — Atherosclerosis of aorta (aorta, calcified), I70.1 — Atherosclerosis of renal artery (renal artery), I70.8 — Atherosclerosis of other arteries (artery, celiac, atherosclerosis), I77.1 — Stricture of artery (artery), I77.4 — Celiac artery compression syndrome (artery, celiac), I87.1 — Compression of vein (vein), J34.89 — Other specified disorders of nose and nasal sinuses (nose), J38.6 — Stenosis of larynx (larynx), +263 more

Lab tests where this diagnosis supports Medicare coverage (NCD)

Medicare's National Coverage Determination (NCD) program lists this diagnosis as medical justification for these lab tests.

Prothrombin Time Test and INR (PT/INR)

Contextual Map

Every relationship of I42.1 in one view: hierarchy, official tabular instructions in both directions, clinical classification, risk adjustment, MS-DRG participation, index terms and change history — each edge carrying the CMS source it derives from.

Run I42.1 with these 3 related codes in Claim Check

Hierarchy

• I00-I99 — Chapter 9: Diseases of the Circulatory System (I00-I99) (I00-I99)[Hierarchy]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• I30-I5A — Other forms of heart disease[Hierarchy]— CMS ICD-10-CM tabular structure · icd10cm-fy2026

Referenced by Excludes2 notes

• I35 — Nonrheumatic aortic valve disorders[Excludes2]: “hypertrophic subaortic stenosis (I42.1)”— CMS ICD-10-CM tabular instructional notes · icd10cm-fy2026

Referenced by Code First instructions

• I5A — Non-ischemic myocardial injury (non-traumatic)[Code First](via I42.-): “cardiomyopathy (I42.-)”— CMS ICD-10-CM tabular instructional notes · icd10cm-fy2026

Referenced by Code Also instructions

• E74.05 — Lysosome-associated membrane protein 2 [LAMP2] deficiency[Code Also]: “obstructive hypertrophic cardiomyopathy (I42.1)”— CMS ICD-10-CM tabular instructional notes · icd10cm-fy2026

Clinical classification (CCSR)

• CIR005 — Myocarditis and cardiomyopathy[CCSR]— AHRQ CCSR for ICD-10-CM Diagnoses (HCUP)

Risk adjustment (CMS-HCC)

• HCC 227 — Cardiomyopathy/Myocarditis [CMS-HCC]— CMS-HCC V28 · 2026

MS-DRG Grouper

• CC — Complication or Comorbidity [MS-DRG severity]: “As a secondary diagnosis this code can raise the stay's MS-DRG severity tier (CC).”— CMS MS-DRG Definitions Manual (Appendix C) · FY2026
• DRG 314 — OTHER CIRCULATORY SYSTEM DIAGNOSES WITH MCC[MS-DRG]: “OTHER CIRCULATORY SYSTEM DIAGNOSES WITH MCC (MDC 05)”— CMS MS-DRG Definitions Manual (Appendix B) · FY2026
• DRG 315 — OTHER CIRCULATORY SYSTEM DIAGNOSES WITH CC[MS-DRG]: “OTHER CIRCULATORY SYSTEM DIAGNOSES WITH CC (MDC 05)”— CMS MS-DRG Definitions Manual (Appendix B) · FY2026
• DRG 316 — OTHER CIRCULATORY SYSTEM DIAGNOSES WITHOUT CC/MCC[MS-DRG]: “OTHER CIRCULATORY SYSTEM DIAGNOSES WITHOUT CC/MCC (MDC 05)”— CMS MS-DRG Definitions Manual (Appendix B) · FY2026

MDC crossing

• MDC 05 — Diseases and Disorders of the Circulatory System[MDC crossing]: “Diseases and Disorders of the Circulatory System — the grouper's crossing between diagnoses and procedures: a principal diagnosis sets the MDC, and same-MDC procedures move the stay to its surgical DRGs. 17,209 same-MDC procedures group here; browse them on the MDC page.”— CMS MS-DRG Definitions Manual · FY2026

Index entries

• Cardiomyopathy (familial) (idiopathic), hypertrophic (nonobstructive), obstructive[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026
• Myocardiopathy (congestive) (constrictive) (familial) (hypertrophic nonobstructive) (idiopathic) (infiltrative) (obstructive) (primary) (restrictive) (sporadic), hypertrophic obstructive[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026
• Stenosis, stenotic (cicatricial), hypertrophic subaortic (idiopathic)[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026
• Stenosis, stenotic (cicatricial), myocardium, myocardial, hypertrophic subaortic (idiopathic)[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026
• Stenosis, stenotic (cicatricial), subaortic (congenital), hypertrophic (idiopathic)[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026
• Stricture, myocardium, myocardial, hypertrophic subaortic (idiopathic)[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026
• Stricture, subaortic, hypertrophic (acquired) (idiopathic)[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026

Nearest codes (15)

• I42 — Cardiomyopathy[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• I42.0 — Dilated cardiomyopathy[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• I42.00 — Dilated cardiomyopathy, unspecified[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• I42.01 — Familial-genetic dilated cardiomyopathy[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• I42.09 — Other dilated cardiomyopathy[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• I42.2 — Other hypertrophic cardiomyopathy[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• I42.3 — Endomyocardial (eosinophilic) disease[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• I42.4 — Endocardial fibroelastosis[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• and 7 more

Change history

• FY2016 — In the code set at ICD-10-CM adoption [Change history]— CMS release files (code change ledger) · icd10cm-fy2016

Common coding questions

Sources for this page

Codes, titles, notes, index terms and mappings on this page are transcribed from the datasets below. Relationships MedCoder computed and text MedCoder wrote are labelled where they appear.

Code, title, tabular notes and index terms Official source data

CMS/CDC ICD-10-CM FY2026 tabular list, index and tables, effective October 1, 2025 Release, file and checksum · Publisher’s page

Claim edits Official source data

CMS Definitions of Medicare Code Edits — v43.1 (April 2026) Release, file and checksum · Publisher’s page

Inpatient payment groups Official source data

CMS MS-DRG Definitions Manual (incl. Appendix B diagnosis index, Appendix C CC/MCC list, Appendix E procedure index) and IPPS Final Rule tables — v43 Release, file and checksum · Publisher’s page

Risk adjustment Official source data

2026 Mid-Year Final ICD-10 Mappings + Model Software (cms.gov/files/zip/2026-midyear-final-icd-10-mappings.zip, 2026-midyear-final-model-software.zip) — CMS-HCC V28, PY2026 mid-year final release Release, file and checksum · Publisher’s page

Change history and date-of-service validity Official source data

CMS ICD-10-CM release addenda, ingested release by release into the change ledger Release, file and checksum · Publisher’s page

Comparisons, relationships and the contextual map MedCoder structured relationship

Computed by MedCoder from the tabular notes and tables above; every derived relationship is marked as derived where it appears

Summary, coding notes, decision checklist and FAQ answers MedCoder explanation

Written by MedCoder to explain the sources above: drafted with AI assistance, checked by a person against the release files, and labelled as MedCoder text where it appears. Not official text.

Labels on this page: Official source data · MedCoder structured relationship · MedCoder explanation. How to read the labels · All data sources and release dates

Nearest Codes in This Family

Official ICD-10-CM classifications closest to I42.1 in its code family, with their registry titles.

• I42 — Cardiomyopathy
• I42.0 — Dilated cardiomyopathy
• I42.00 — Dilated cardiomyopathy, unspecified
• I42.01 — Familial-genetic dilated cardiomyopathy
• I42.09 — Other dilated cardiomyopathy
• I42.2 — Other hypertrophic cardiomyopathy
• I42.3 — Endomyocardial (eosinophilic) disease
• I42.4 — Endocardial fibroelastosis
• I42.5 — Other restrictive cardiomyopathy
• I42.6 — Alcoholic cardiomyopathy

View all codes in the I42 family