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October 18, 2004

Coding for Acute Myocardial Infarction
For The Record
Vol. 16 No. 21 Page 31

Acute myocardial infarction (AMI) is classified to ICD-9-CM category 410. A fourth and fifth digit are also needed to completely code the condition.

Fourth-Digit Subcategory
The fourth digit specifies the site involved. More than one code from category 410 may be assigned to fully describe the patient’s condition. For example, if a patient had an AMI of the inferior wall and anterior wall during the same admission, then both 410.41 and 410.11 may be assigned. (AHA Coding Clinic for ICD-9-CM, 1993, fifth issue, page 14.) However, if the patient had an extension of the MI (reinfarction) of the same site in the same admission, then only the initial infarction will be coded. No additional code for the reinfarction would be assigned. (Coding Clinic, 1993, fifth issue, page 13.)

Subendocardial Infarction
Code 410.7x, Subendocardial infarction or nontransmural infarction, identifies subendocardial infarctions that do not extend through the full thickness of the myocardial wall (ICD-9-CM Coding Handbook, Faye Brown, 2004, page 255). Physicians are starting to document with more frequency non–ST-elevation MI (NSTEMI). According to the American College of Cardiology, the definition of NSTEMI is “an acute process of myocardial ischemia with sufficient severity and duration to result in myocardial necrosis.” The initial electrocardiogram (ECG) in patients with NSTEMI does not show ST-segment elevation. The majority of patients who present with NSTEMI do not develop new Q waves on the ECG and are ultimately diagnosed as having had a non-Q-wave MI. NSTEMI is distinguished from unstable angina by the detection of cardiac markers indicative of myocardial necrosis in NSTEMI and the absence of abnormal elevation of such biomarkers in patients with unstable angina.

There are a minority of patients who might develop a Q-wave MI. It is difficult to definitively state in 100% of the cases that NSTEMIs do not go all the way through the heart wall muscle. Therefore, the final code assignment will depend on the documentation for each specific case, and the physician may need to be queried for clarification. However, in general, it seems that NSTEMI would be classified to code 410.71.

Physicians are also starting to document STEMI, which differs from NSTEMI in that STEMI typically goes all the way through the heart wall muscle. Therefore, it would be appropriate to assign code 410.x1 with the fourth digit identifying the specific location of the MI.

Code 410.9x, Myocardial infarction of unspecified site, should be assigned only when there is no documentation specifying the location. Although not a reliable diagnostic tool, the ECG report is a reliable source of information regarding the site of the AMI. If a diagnostic report (eg, ECG report) provides specificity to a confirmed diagnosis (site of MI), it is appropriate to assign the more specific code (Coding Clinic, 1999, first quarter, page 5).

Fifth-Digit Subclassification
The fifth-digit subclassification for category 410 identifies the episode of care. A fifth digit of 1 indicates the first time the patient was seen and treated for MI. The fifth digit of 1 may be used at the first hospital where the patient received treatment and also at the other acute care hospitals to which the patient is subsequently transferred (without interim discharge). For example, if a patient was admitted to Hospital A for an AMI and then transferred to Hospital B for a cardiac bypass, code 410.x1 would be assigned as principle diagnosis for both acute care hospital stays. If the patient was then readmitted to Hospital A for recovery without ever being discharged home, code 410.x1 could still be assigned as the principal diagnosis.

A fifth digit of 2 is assigned when a patient is admitted for subsequent care of an MI after the initial care but within eight weeks of the initial MI. Assign code 414.8 if the MI is described as chronic or with a duration of eight weeks or more.

If a patient was readmitted with an extension of an MI, it is considered a brand-new MI of the specified site regardless of whether it occurred in the same location as the original infarct or extended to a new location. It is further death of the tissue. Because the patient was released and cardiac enzymes were normal, there will be a new rise in enzymes and the new infarct will be treated with the same treatment plan as before. According to Coding Clinic, 1989, third quarter, pages 3-4, “Any subsequent episode of care for another (repeat) myocardial infarction is also assigned the fifth digit 1.”

Sometimes the MI extends to the same site while in-house for the original MI. In that case, it is not considered an extension since it was at the same site during the same admission. However, if it extended to a different location during the same admission, then assign an additional code from category 410 with a fifth digit of 1 to show the new site to which it extended (Coding Clinic, 1993, fifth issue, pages 13-14).

Coding and sequencing for AMI are dependent upon the physician documentation in the medical record and application of the Official Coding Guidelines for inpatient care. Also, use specific AHA Coding Clinic for ICD-9-CM and American Medical Association CPT Assistant references to ensure complete and accurate coding.

— This information was prepared by Audrey Howard, RHIA, and Vicki Sippel, RHIA, of 3M Health Information Systems (800-367-2447), a leading supplier of coding and classification systems to nearly 4,000 healthcare providers. The company and its representatives do not assume any responsibility for reimbursement decisions or claims denials made by providers or payors as the result of the misuse of this coding information.

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