I48.4 ICD-10-CM Code: Atypical atrial flutter
I48.4 maps to CMS-HCC V28 238. A source-labeled RAF reference is available. Confirm the documented diagnosis and applicable coding requirements. MEAT criteria · RAF Calculator · HCC Buddy coding tools
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FY 2026 Apr update / Diseases of the circulatory system (I00-I99) / Other forms of heart disease (I30-I5A)
I48.4
Billable / SpecificICD-10-CMOfficial ICD-10-CMCodebook guidanceAtypical atrial flutter
Atypical atrial flutter is an irregular heart rhythm disorder where the heart's upper chambers beat in an abnormal pattern that doesn't follow the typical flutter pattern. This condition causes the heart to beat too fast and irregularly, which can reduce the heart's efficiency in pumping blood.
Buddy Insight
Atypical atrial flutter involves macro-reentrant circuits not dependent on the cavotricuspid isthmus, making it more complex to treat with catheter ablation.
CMS-HCC V28
MappedHCC 238
Coefficient HCC 238: 0.299 (Community Non-Dual Aged (CNA))
CMS-HCC V24
HistoricalHistorical
Not used for CY2026 payment
ACA/HHS
MappedHCC 142
Code-level coefficient reference
ESRD/PACE
MappedHCC 96
Code-level coefficient reference
RXHCC
MappedHCC 193
Code-level coefficient reference
Code Book Path
Inclusion Terms
Official- Type II atrial flutter
Excludes 2
Official- certain conditions originating in the perinatal period (P04-P96)Inherited from I00-I99
- certain infectious and parasitic diseases (A00-B99)Inherited from I00-I99
- complications of pregnancy, childbirth and the puerperium (O00-O9A)Inherited from I00-I99
- congenital malformations, deformations, and chromosomal abnormalities (Q00-Q99)Inherited from I00-I99
- endocrine, nutritional and metabolic diseases (E00-E88)Inherited from I00-I99
- injury, poisoning and certain other consequences of external causes (S00-T88)Inherited from I00-I99
- neoplasms (C00-D49)Inherited from I00-I99
- symptoms, signs and abnormal clinical and laboratory findings, not elsewhere classified (R00-R94)Inherited from I00-I99
- systemic connective tissue disorders (M30-M36)Inherited from I00-I99
- transient cerebral ischemic attacks and related syndromes (G45.-)Inherited from I00-I99
Related Codes
Includes
OfficialNo Includes notes are included in this display for I48.4. Check the code and parent instructions in the Code Book.
Excludes 1
OfficialNo Excludes 1 notes are included in this display for I48.4. Check the code and parent instructions in the Code Book.
Code First
OfficialNo Code First sequencing instructions are included in this display for I48.4. Check the code and parent instructions in the Code Book.
Use Additional
OfficialNo Use Additional Code instructions are included in this display for I48.4. Check the code and parent instructions in the Code Book.
Code Also
OfficialNo Code Also instructions are included in this display for I48.4. Check the code and parent instructions in the Code Book.
Buddy Documentation Tip
MEAT Support
Audit Caution
Common Mistakes
Current with CMS: FY2026 ICD-10-CM Apr 1 update (effective Apr 1 – Sep 30, 2026) · CMS-HCC V28, 100% phased in for payment year 2026. FY2027 code set already staged for October 1, 2026. How HCC Buddy stays current →
Is I48.4 an HCC code?
Yes. I48.4 (Atypical atrial flutter) maps to HCC 238, Specified Heart Arrhythmias under the CMS-HCC V28 risk adjustment model, with a source-labeled community non-dual aged reference coefficient of 0.299. Source-labeled code-level coefficients are references, not member totals. Actual contribution depends on complete member context, hierarchy and cleanup rules, interactions, and model year. HCC Buddy's RAF Calculator supports CMS-HCC V28 PY2026 and shows no score unless every required source and calculation check passes. It is billable for payment year 2026.
Coder answer: I48.4 is billable and maps to V28 HCC 238, Specified Heart Arrhythmias. Open it in the Code Book for the tabular path, RAF, and MEAT checklist.
- Code
- I48.4
- Description
- Atypical atrial flutter
- HCC (V28)
- HCC 238 — Specified Heart Arrhythmias
- RAF reference coefficient
- 0.299
- Billable
- Yes
- Payment year
- 2026
HCC Category Mapping
These are source-labeled model coefficients, not member totals. A category may still be removed by hierarchy or model cleanup rules. Weights are not directly comparable across models: CMS-HCC V28 and V24 use Community, Non-Dual, Aged; ESRD uses the dialysis continuing-enrollee model; RxHCC is the Part D continuing-enrollee, non-low-income, aged weight (a larger scale than CMS-HCC). ACA/HHS has no single weight — it varies by metal level. Actual per-patient RAF contribution depends on member context, hierarchy and cleanup rules, interactions, and the model year used by the payer. V28 is the CMS-HCC model phased in over payment years 2024–2026; V24 remains available for historical review.
Work I48.4 in the Code Book — tabular path, V28 RAF reference, and MEAT checklist →
MEAT review for I48.4
For I48.4, confirm that the documentation supports the diagnosis and meets the applicable coding, encounter, program and payer requirements. MEAT (Monitor, Evaluate, Assess, or Treat) is a review mnemonic, not a universal CMS coding rule.
- 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
Coder workflow notes
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What This Code Means
I48.4 is the ICD-10-CM diagnosis code for atypical atrial flutter. Atypical atrial flutter is an irregular heart rhythm disorder where the heart's upper chambers beat in an abnormal pattern that doesn't follow the typical flutter pattern. This condition causes the heart to beat too fast and irregularly, which can reduce the heart's efficiency in pumping blood. I48.4 sits in the ICD-10-CM chapter for diseases of the circulatory system (i00-i99), within the section covering other forms of heart disease (i30-i5a).
Under the CMS-HCC V28 risk adjustment model, I48.4 maps to Specified Heart Arrhythmias (HCC 238) with a source-labeled community, non-dual, aged reference coefficient of 0.299. For CY2026 non-PACE Medicare Advantage, CMS uses 100% of the 2024 CMS-HCC model (V28). PACE uses a separate model blend. Source-labeled code-level coefficients are references, not member totals. Actual contribution depends on complete member context, hierarchy and cleanup rules, interactions, and model year. HCC Buddy's RAF Calculator supports CMS-HCC V28 PY2026 and shows no score unless every required source and calculation check passes.
Atypical atrial flutter requires ECG or provider documentation of flutter that is not the typical counterclockwise cavotricuspid isthmus-dependent circuit. An EP study finding or cardiology note naming atypical flutter supports the code. For I48.4, confirm that the documentation supports the diagnosis and meets the applicable coding, encounter, program and payer requirements. MEAT (Monitor, Evaluate, Assess, or Treat) is a review mnemonic, not a universal CMS coding rule. 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, source-labeled coefficient references, and MEAT documentation criteria for I48.4 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •Atypical atrial flutter requires ECG or provider documentation of flutter that is not the typical counterclockwise cavotricuspid isthmus-dependent circuit. An EP study finding or cardiology note naming atypical flutter supports the code.
- •Do not confuse I48.4 with I48.3. Atypical flutter is a distinct clinical entity with different ablation considerations; assign based on documented circuit type.
- •Same HCC as other arrhythmia codes.
Clinical Significance
Atypical atrial flutter involves macro-reentrant circuits not dependent on the cavotricuspid isthmus, making it more complex to treat with catheter ablation. It is more commonly seen after cardiac surgery or prior ablation procedures. The atypical nature often requires advanced electrophysiology mapping for successful treatment and carries a higher recurrence rate.
Documentation Requirements
- ✓Provider diagnosis specifically stating 'atypical' atrial flutter
- ✓Electrocardiogram findings distinguishing from typical flutter pattern
- ✓Electrophysiology study results if available (non-isthmus-dependent circuit)
- ✓History of prior cardiac surgery or ablation that may predispose to atypical flutter
- ✓Treatment strategy and anticoagulation plan