2026 ICD-10-CM Diagnosis Code Q21.4Aortopulmonary septal defect

ICD-10-CM Codes›Q00-Q99›Q20-Q28›Q21

ICD-10-CM Q21.4
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

Q21.4 is a billable ICD-10-CM diagnosis code for aortopulmonary septal defect. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026) and groups to MS-DRG 306 through 307. The code is exempt from POA reporting. Coders also document this condition as aortopulmonary window. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Cardiac and circulatory congenital anomalies.

Code Identity

ICD-10-CM Code
Q21.4
Billable Status
Yes — Valid for Submission
Code Describes
Aortopulmonary septal defect
Short Description
Aortopulmonary septal defect
Same as the full description in the CMS dataset.
Parent Code
Q21
Congenital malformations of cardiac septa

Code Classification

ChapterQ00-Q99Congenital malformations, deformations and chromosomal abnormalities
SectionQ20-Q28Congenital malformations of the circulatory system
CategoryQ21Congenital malformations of cardiac septa
This CodeQ21.4Aortopulmonary septal defect

Present on Admission (POA)Billing

Q21.4 is exempt from POA reporting on inpatient claims to general acute care hospitals. Review other POA exempt codes.

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Aortopulmonary window
  • Aortopulmonary window with tubular connection
  • Confluent aortopulmonary window with minimal superior and inferior rim
  • Distal aortopulmonary window with minimal superior rim
  • Endarteritis
  • Infective aortitis
  • Infective endarteritis at site of aortopulmonary window
  • Intermediate aortopulmonary window with adequate superior and inferior rim
  • Proximal aortopulmonary window with minimal inferior rim

Tabular List NotesGuidance

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

Inclusion Terms

  • Aortic septal defect
  • Aortopulmonary window

Index to Diseases and InjuriesGuidance

Alphabetical index entries that point to this code.

External Cause of Injuries IndexGuidance

References for this code in the External Cause of Injuries Index.

    • Absence(of) (organ or part) (complete or partial)
      • septum
        • between aorta and pulmonary artery
    • Agenesis
      • septum
        • between aorta and pulmonary artery
    • Communication
      • between
        • base of aorta and pulmonary artery
    • Defect, defective
      • aorticopulmonary septum
    • Fenestration, fenestrated
      • aortico-pulmonary
    • Imperfect
      • closure (congenital)
        • septum
          • aorticopulmonary
    • Imperfect
      • closure (congenital)
        • septum
          • between aorta and pulmonary artery
    • Window
      • aorticopulmonary

Clinical ClassificationClinical

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

CCSR MAL001
Cardiac and circulatory congenital anomalies
Default principal diagnosis: inpatient Yes · outpatient Yes

Clinical InformationClinical

  • Aortopulmonary Septal Defect

    a developmental abnormality in which the spiral (aortopulmonary) septum failed to completely divide the truncus arteriosus into ascending aorta and pulmonary artery. this abnormal communication between the two major vessels usually lies above their respective valves (aortic valve; pulmonary valve).

  • Endarteritis

    inflammation of the inner endothelial lining (tunica intima) of an artery.

  • Tunica Intima

    the innermost layer of an artery or vein, made up of one layer of endothelial cells and supported by an internal elastic lamina.

  • Endarteritis

    inflammation of the arterial intima.

Patient EducationClinical

Congenital Heart Defects

Congenital heart defects (CHDs) are problems with the structure of the heart. "Congenital" means that that the problems are present at birth. These defects happen when a fetus's heart doesn't develop normally during pregnancy. Congenital heart defects are the most common type of birth defect.

The full article covers:

  • What are congenital heart defects?
  • What causes congenital heart defects?
  • Who is more likely to have a baby with a congenital heart defect?
  • What are the symptoms of congenital heart defects?
  • What other problems do congenital heart defects cause?
  • How are congenital heart defects diagnosed?
  • What are the treatments for congenital heart defects?

Read the full article at MedlinePlus

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

Convert Q21.4 to ICD-9-CMHistory

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

ICD-9-CM
745.8 Septal closure anom NEC
Approximate The match is approximate rather than exact.

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 Q21.4Overview

Is Q21.4 (Congenital malformations of cardiac septa) a billable code?

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

What MS-DRG does Q21.4 group to?

When aortopulmonary septal defect is the principal diagnosis on an inpatient stay, it groups to MS-DRG 306, 307, with relative weights from 0.9132 to 1.5758 depending on complications. Higher weights mean higher Medicare reimbursement.

Is Q21.4 exempt from POA reporting?

Yes. CMS lists this code among those exempt from present on admission reporting, so hospitals do not assign a POA indicator for aortopulmonary septal defect on inpatient claims.

What is the ICD-9 equivalent of Q21.4?

Under the General Equivalence Mappings, aortopulmonary septal defect converts to ICD-9-CM 745.8 (septal closure anom NEC). The mapping is approximate, so confirm the match fits the documentation.

Footnotes

[1] Chronic - a chronic condition code indicates a condition lasting 12 months or longer and its effect on the patient based on one or both of the following criteria:

  • The condition results in the need for ongoing intervention with medical products,treatment, services, and special equipment
  • The condition places limitations on self-care, independent living, and social interactions.