Cardiology Coding Alert

Know the timing to choose correct category of MI.

The ICD-10-CM Official Guidelines for Coding and Reporting share a lot of info on how to handle coding for myocardial infarctions (MIs) that you might miss if you don't know where to look. Brush up on your basic knowledge of MIs and see what the Official Guidelines say.

Learn the ABCs of Myocardial Infarction

As a cardiology coder, a common diagnosis you will see in documentation is MI or acute MI (AMI) - commonly known as a heart attack.

Myocardial refers to the muscular tissue of the heart (the myocardium), and an infarction occurs when the blood supply to an organ or region of tissue is cut off, resulting in tissue death. A patient may have an MI because of plaque buildup or a clot blocking blood flow through the heart muscle.

ICD-10-CM classifies MIs as ST elevation myocardial infarction (STEMI) or non-ST elevation myocardial infarction (NSTEMI).

STEMI and NSTEMI "reflect differences in the ECG [electrocardiogram] tracings - specifically whether a portion of the ECG called the ST segment is elevated above baseline, or not," says Cardio Smart's "Heart Attack Treatment Guidelines" from the American College of Cardiology (ACC). You can read the article here: www.cardiosmart.org/Heart-Conditions/Guidelines/Heart-Attack-Guidelines.

Count to 28: ICD-10 includes the codes for AMI under I21.- (ST elevation [STEMI] and non-ST elevation [NSTEMI] myocardial infarction). There is a note with that code that states the category includes "myocardial infarction specified as acute or with a stated duration of four weeks (28 days) or less from onset." Check the "Know That Timing Determines Category" section below for more info on how timelines connect to code choice.

Pinpoint Proper Codes for STEMI

The ICD-10 code set gives you the following codes for STEMI:

• I21.0-, ST elevation (STEMI) myocardial infarction of anterior wall

                o I21.01, ST elevation (STEMI) myocardial infarction involving left main coronary artery
                o I21.02, ST elevation (STEMI) myocardial infarction involving left anterior descending coronary artery
                o I21.09, ST elevation (STEMI) myocardial infarction involving other coronary artery of anterior wall

• I21.1-, ST elevation (STEMI) myocardial infarction of inferior wall

​                o I21.11, ST elevation (STEMI) myocardial infarction involving right coronary artery
                o I21.19, ST elevation (STEMI) myocardial infarction involving other coronary artery of inferior wall

• I21.2-, ST elevation (STEMI) myocardial infarction of other sites

                o I21.21, ST elevation (STEMI) myocardial infarction involving left circumflex coronary artery
                o I21.29, ST elevation (STEMI) myocardial infarction involving other sites

• I21.3, ST elevation (STEMI) myocardial infarction of unspecified site.

Important: If the site of the STEMI is unspecified, you would choose I21.3. Also, if the documentation shows only STEMI or transmural MI without the site, use code I21.3, according to section I.C.9.e.2 of the 2017 ICD-10 Official Guidelines, available at www.cdc.gov/nchs/data/icd/10cmguidelines_2017_final.pdf.

Understand What Makes MI NSTEMI

As mentioned above, NSTEMI is a second type of MI. Although STEMI and NSTEMI appear differently on an ECG, the ACC also notes other distinctions between the two.

"With NSTEMI, the heart attack damage usually doesn't extend through the full depth of the heart muscle," says the ACC. "Equally important, NSTEMI heart attacks are caused by different types of blood clots than STEMI heart attacks, with differing amounts of clotting proteins and platelet blood cells."

To report NSTEMI and nontransmural MIs, you would choose I21.4 (Non-ST elevation [NSTEMI] myocardial infarction), says section I.C.9.e.1 of the Official Guidelines.

Grasp Other MI Coding Rules from Official Guidelines

Getting to know the Official Guidelines for MIs pays off with some helpful hints on how to code when you encounter certain situations.

One rule to note in particular is that "if NSTEMI evolves to STEMI, assign the STEMI code," according to section I.C.9.e.1 of the Official Guidelines. "If STEMI converts to NSTEMI due to thrombolytic therapy, it is still coded as STEMI."

Additionally, according to section I.C.9.e.3 of the Official Guidelines, "If an AMI is documented as nontransmural or subendocardial, but the site is provided, it is still coded as a subendocardial AMI."

Term tip: To better understand the meaning of the terms "nontransmural or subendocardial," look to direction from Theresa Dix, CCS-P, CPMA, CCC, ICDCT-CM, coder/auditor of East Tennessee Heart Consultants in Knoxville, Tenn.: "When an AMI is documented as nontransmural or subendocardial, this means that the AMI does not extend through the whole thickness of the heart muscle," says Dix.

Determining this distinction of "nontransmural or subendocardial" is important because those terms are specific to NSTEMI.

"If coders select a code by site, it will lead them to AMI codes for STEMI," says Dix. "STEMI AMIs extend through the whole thickness of the heart muscle." Choosing a STEMI site-specific code would be incorrect if the documentation specifies NSTEMI.

Know That Timing Determines Category

When coding MIs, time is a vital factor because knowing when the MI occurred determines which category of codes you should choose from. The Official Guidelines provide important information to help you code correctly.

"For encounters occurring while the myocardial infarction is equal to, or less than, four weeks old, including transfers to another acute setting or a postacute setting, and the myocardial infarction meets the definition for 'other diagnoses' ... , codes from [STEMI and NSTEMI] category I21.- may continue to be reported," according to section I.C.9.e.1 of the Official Guidelines.

The Official Guidelines further explain the importance of time in regards to MIs when they state, "For encounters after the four-week time frame and the patient is still receiving care related to the myocardial infarction, the appropriate aftercare code should be assigned, rather than a code from category I21.-. For old or healed myocardial infarctions not requiring further care, code I25.2 (Old myocardial infarction) may be assigned."

Master Subsequent for Some New AMIs

If the patient experiences a new AMI within four weeks of his initial AMI, you should choose a code from category I22.- (Subsequent [STEMI] and non-ST elevation [NSTEMI] myocardial infarction), says section I.C.9.e.4 of the Official Guidelines.

Don't miss: The Official Guidelines also note that you must use a code from I22.- (subsequent) in conjunction with a code from AMI category I21.-. Sequence the codes based on the reason for the encounter.

Your choices for subsequent MIs are as follows:

• I22.0, Subsequent ST elevation (STEMI) myocardial infarction of anterior wall
• I22.1, Subsequent ST elevation (STEMI) myocardial infarction of inferior wall
• I22.2, Subsequent non-ST elevation (NSTEMI) myocardial infarction
• I22.8, Subsequent ST elevation (STEMI) myocardial infarction of other sites
• I22.9, Subsequent ST elevation (STEMI) myocardial infarction of unspecified site

Don't Forget Important Instructional Note

As a final tip, make sure not to overlook the instructional note in ICD-10 with I21.- and I22.- when you choose a code from those categories:

Use additional code, if applicable to identify:

• Exposure to environmental tobacco smoke (Z77.22)
• History of tobacco dependence (Z87.891)
• Occupational exposure to environmental tobacco smoke (Z57.31)
• Status post administration of tPA (rtPA) in a different facility within the last 24 hours prior to admission to current facility (Z92.82)
• Tobacco dependence (F17.-)
• Tobacco use (Z72.0).