Cracking the Code

Coding Myocardial Infarctions (MI)

According to the Mayo Clinic a myocardial infarction is when “the flow of blood to the heart is severely reduced or blocked. The blockage is usually due to a buildup of fat, cholesterol, and other substances in the heart (coronary) arteries. The fatty, cholesterol-containing deposits are called plaques. The process of plaque buildup is called atherosclerosis.” Myocardial infarctions or MI are also known as heart attacks. There are five classifications of MIs. Type 1 MIs are assigned to codes I21.0-I21.4. Type 2 MIs are assigned to I21.A1 with a code for the underlying cause. Acute MIs type 3-5 are assigned to code I21.A9. MIs are classified as acute if the duration is four weeks or less. When a provider documents Acute MI with no further specification, I21.9 is reported. Type 1 MIs also can be classified by whether there is an ST-segment elevation. STEMI (ST-elevation myocardial infarction) is coded with subcategories I21.0-I21.3, I22.0, I22.1, and I22.8 based on location. In this case, the coronary artery is completely blocked, and nearly all the heart muscle being supplied by the affected artery starts to die. The ST refers to the S-T segment or wave on an ECG. If the specific location is not documented, the proper code is I21.3 ST elevation (STEMI) myocardial infarction unspecified site.


NSTEMI describes when the blood clot only partly occludes the artery, and only a portion of the heart muscle being supplied by the affected artery dies. This would be coded using subcategory I21.4. A subsequent NSTEMI is reported with I22.2.


If during the encounter STEMI is converted to NSTEMI, select a code for STEMI. If NSTEMI evolves to STEMI, select a code for STEMI.


A code from category I22 Subsequent ST elevation (STEMI) and non-ST elevation (NSTEMI) myocardial infarction is reported when a patient has suffered an AMI and then has a new AMI within four weeks from the initial AMI. A code from category I22 is to be used with a code from category I21. The sequencing of these codes depends on the circumstances of the encounter.


Codes from category I22 are not to be used to report MIs other than type 1 or unspecified. For subsequent type 2 AMI assign only I21.A1. For subsequent type 4 or type 5 AMI, assign only code I21.A9. If a subsequent MI of one type occurs within four weeks of an MI of a different type, a code from category I21 is reported instead of a code from I22.

Q: If an ST-elevation myocardial infarction converts to a non-ST elevation myocardial infarction during thrombolytic therapy, how is it coded?
A: Code only STEMI. Rationale: According to ICD-10-CM Official Coding guideline I.C.9.e.1, state that if STEMI converts to NSTEMI due to thrombolytic therapy, it is still coded as STEMI.
Reference: FY 2024 ICD-10-CM code book, guidelines, and Mayo Clinic
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