Myocardial Infarction Coding Made Simple: Types 1-5, STEMI, NSTEMI & Old MI
Updated: Aug 29
Myocardial infarction coding can feel overwhelming because finding the word “heart attack” in the medical record is only the beginning.
Before choosing an ICD-10-CM code, a medical coder must answer several questions:
Did the provider diagnose an MI, or is there only an elevated laboratory result?
Is the MI acute, subsequent, or old?
What type of MI is documented?
Is it a STEMI or NSTEMI?
Is the affected coronary artery or heart wall documented?
Is there an underlying cause, complication, or additional coding instruction?
Let’s organize these rules into a coding path that is easier to understand and remember.
Medical Coding by Jen Memory Line: Diagnosis → Timing → Type → Site → Additional Codes
What Is a Myocardial Infarction?
A myocardial infarction, commonly called an MI or heart attack, occurs when coronary blood flow suddenly decreases and part of the heart muscle is damaged or dies.
When an MI is suspected, the provider may order laboratory tests such as:
Creatine phosphokinase, or CPK
Cardiac troponin
Elevated CPK or troponin can indicate heart-muscle damage, but an elevated laboratory result does not automatically allow the coder to report an MI.
If the provider documents only elevated CPK or elevated troponin, select the appropriate abnormal-finding code by following the ICD-10-CM Alphabetic Index and Tabular List.
Beginner Mistake #1
Do not diagnose an MI from laboratory results yourself. Code the condition documented by the provider.
The Five Types of Myocardial Infarction
The type of MI tells us what caused the myocardial injury and determines the appropriate coding path.
Type 1 Myocardial Infarction
A Type 1 MI is the classic spontaneous heart attack caused by a primary coronary event, such as plaque rupture, erosion, fissuring, or dissection.
Both STEMI and NSTEMI can represent Type 1 myocardial infarctions.
STEMI
STEMI means ST-elevation myocardial infarction.
In a STEMI, the coronary artery is generally completely blocked, placing the heart muscle supplied by that artery at risk.
ICD-10-CM uses the following codes for Type 1 STEMI:
I21.0-I21.2
I21.3
The STEMI code should identify the affected heart wall, coronary artery, or site when the documentation provides that information.
Examples include:
I21.01: STEMI involving the left main coronary artery
I21.02: STEMI involving the left anterior descending coronary artery
I21.09: STEMI involving another coronary artery of the anterior wall
I21.3: STEMI of an unspecified site
Code I21.3 may be reported when the documentation states only Type 1 STEMI or transmural MI without identifying the site.
NSTEMI
NSTEMI means non-ST-elevation myocardial infarction.
An NSTEMI generally involves a partial coronary blockage. ICD-10-CM code I21.4 is used for a Type 1 NSTEMI, nontransmural MI, or acute subendocardial MI.
If a Type 1 NSTEMI evolves into a STEMI, report the STEMI code. If a Type 1 STEMI converts to an NSTEMI because of thrombolytic therapy, it is still coded as a STEMI.
The FY 2026 ICD-10-CM Official Guidelines explain the Type 1 STEMI and NSTEMI requirements in Section I.C.9.e.
Type 2 Myocardial Infarction
A Type 2 MI is caused by an imbalance between the oxygen supply available to the heart and the heart’s oxygen demand.
Possible causes include:
Anemia
Arrhythmia
Coronary artery spasm
Coronary embolism
Hypertension
Hypotension
Other conditions that decrease supply or increase demand
Assign I21.A1, Myocardial infarction type 2.
The underlying cause must be coded first when applicable.
Type 2 MI Example
The provider documents:
Type 2 myocardial infarction caused by anemia.
The coding path is:
Report the documented anemia code first when applicable.
Report I21.A1 for the Type 2 MI.
If a Type 2 MI is described as a STEMI or NSTEMI, the official guidelines direct the coder to assign only I21.A1 for the MI. Do not also assign the Type 1 STEMI or NSTEMI codes.
Beginner Mistake #2
Do not report I21.4 simply because the provider uses the word NSTEMI if the provider clearly identifies the condition as a Type 2 MI.
The documented MI type controls the coding path.
Type 3 Myocardial Infarction
Type 3 MI involves sudden, unexpected cardiac death with findings suggesting myocardial ischemia.
The patient may die before blood samples can be collected or before cardiac markers appear in the blood.
Type 4 Myocardial Infarction
Type 4a, Type 4b, and Type 4c myocardial infarctions are associated with coronary revascularization procedures.
The documentation and Tabular List instructions must be carefully reviewed for procedure-related conditions and complications.
Type 5 Myocardial Infarction
Type 5 MI is associated with coronary artery bypass graft surgery, commonly called CABG.
Acute Type 3, Type 4a-4c, and Type 5 myocardial infarctions are assigned to:
I21.A9 — Other myocardial infarction type
Follow all applicable Code first and Code also instructions for complications and postprocedural myocardial infarctions. This classification is confirmed in the FY 2026 ICD-10-CM Official Guidelines.
What About an Unspecified Acute MI?
Code I21.9, Acute myocardial infarction, unspecified, is the default when the provider documents an acute MI without identifying its type.
Do not automatically assign I21.3 for every unspecified acute MI.
I21.3 is appropriate when the provider documents a Type 1 STEMI or transmural MI but does not document the site.
The Four-Week Acute MI Rule
Time is extremely important in myocardial infarction coding.
For encounters occurring while the MI is four weeks old or less, including transfers to another acute or post-acute setting, a code from category I21 may continue to be reported when the MI meets reporting requirements.
After the four-week period:
If the patient is still receiving care related to the MI, report the appropriate aftercare code instead of I21.
If the MI is old or healed and no longer requires care, report I25.2, Old myocardial infarction.
Report any current cardiac condition only when it is documented by the provider.
Do not continue reporting I21 simply because the patient has a history of an MI.
When Is Category I22 Reported?
Category I22 is used for a new Type 1 or unspecified acute MI that occurs within four weeks of the initial Type 1 or unspecified MI.
I22 must be reported with a code from I21. The sequencing of I21 and I22 depends on the circumstances of the encounter.
Do not use I22 for every second myocardial infarction.
According to the FY 2026 guidelines:
A subsequent Type 2 MI is reported with I21.A1.
A subsequent Type 4 or Type 5 MI is reported with I21.A9.
If the initial and subsequent MIs are different types, assign the appropriate I21 codes instead of I22.
I22 applies only when both the initial and subsequent MIs are Type 1 or unspecified.
Beginner Mistake #3
Do not select I22 merely because the patient previously experienced an MI. Confirm the timing and the type of both events.
Additional Codes to Check
Categories I21 and I22 contain instructions to use additional codes when applicable.
Review the documentation and Tabular List for:
Tobacco use
Tobacco dependence
Tobacco exposure
History of tobacco use
Administration of tPA or rtPA at another facility within the previous 24 hours
Current complications
Underlying causes
Postprocedural conditions
Never stop coding after finding the first MI code. Read every instructional note associated with the category and final code.
2026 Coding Reminder: I21.B
The current code set also includes I21.B, Myocardial infarction with coronary microvascular dysfunction.
This code is used when the provider documents myocardial infarction with coronary microvascular disease, coronary microvascular dysfunction, or qualifying MINOCA with microvascular disease.
Assign this code only when the documentation supports that specific condition.
The Three Most Common MI Coding Mistakes
1. Coding an MI From Troponin Alone
Elevated troponin may indicate myocardial damage, but it is not automatically a provider diagnosis of MI.
2. Ignoring the Documented MI Type
Type 1, Type 2, and Types 3-5 follow different ICD-10-CM coding paths.
3. Forgetting Timing and Sequencing
The four-week timeline, I21/I22 relationship, underlying cause, and instructional notes can completely change the final code assignment.
The 30-Second MI Coding Checklist
Before choosing your final code, ask:
Is an MI diagnosed, or is only an elevated laboratory result documented?
Is the MI acute, subsequent, old, or receiving aftercare?
What MI type is documented?
For Type 1, is it STEMI or NSTEMI?
Is the site, wall, or coronary artery documented?
Does an underlying cause need to be coded first?
Do I21, I22, tobacco, tPA, complication, or postprocedural instructions apply?
If you cannot answer these questions from the documentation, stop and review the complete medical record and ICD-10-CM Tabular List.
Download the Free MI Coding Study Pack
I turned this lesson into a free eight-page Myocardial Infarction Study Pack for medical coding students.
The study pack includes:
Colorful MI coding sticky notes
A chart-to-code visual map
Jen’s Cardiac Coding Desk memory story
The top three beginner mistakes
A seven-day spaced-repetition plan
Three mental models
A 30-second MI coding checklist
A practical coding example
[DOWNLOAD THE FREE MI CODING STUDY PACK FREE from Jen! CLICK BELOW]
Keep studying, keep practicing, and remember:
Do not code the heart attack until you know its story.
Jen Brewer, CPC, CPMA, CPC-IAAPC Approved InstructorMedicalCodingByJen.com
This article is provided for educational purposes. Always use the ICD-10-CM code set, Official Guidelines, Alphabetic Index, Tabular List, documentation, and payer requirements applicable to the date of service. *Please Like, Share, Comment, Save & Let Me Know What Areas You Struggle With*
A clear and practical explanation of MI coding. The distinctions between different types can easily cause confusion, so accurate documentation really matters. A cursive handwriting translator can also come in handy when reviewing older handwritten notes.