Last month's column discussed the definitions and distinctions of the six types of myocardial infarction (MI) identified by the Third Universal Definition of Myocardial Infarction, published in Circulation on Oct. 16, 2012. This month we will explore the new codes and coding rules for types of MI (see Table) provided by the 2018 International Classification of Diseases, 10th Revision, Clinical Modification (ICD-10-CM), which became effective on Oct. 1, 2017. Having codes for the various types of MI is new to the 2018 ICD-10-CM.
An acute type 1 MI is now identified by a group of codes in category I21 for ST-elevation MI (STEMI), including Q-wave MI, and non-ST-elevation MI (NSTEMI). An MI is coded as acute for a period of four weeks following onset; after that it is assigned code I25.2 (old MI).
Codes in category I22 are also provided for a subsequent type 1 MI (STEMI or NSTEMI), defined as another MI occurring within four weeks of a previous (initial) MI. In this situation, a code from I21 is also assigned for the initial MI.
With the 2018 ICD-10-CM, we finally have codes to identify type 2 MI and make the important distinction between it and type 1. In the past, type 2 was coded as NSTEMI, creating many practical problems, especially since these two types of MI have completely different causes, pathophysiology, implications, outcomes, and management. Furthermore, this situation improperly labeled patients who had supply/demand mismatch (type 2) as having acute coronary thrombosis primarily due to coronary artery disease, causing significant consequences for patients, clinicians, and the health care database.
Type 2 MI (whether a new initial or subsequent) is assigned to one code (I21.A1). The code also includes any description of MI due to “demand ischemia” or “ischemic imbalance.” The diagnosis of type 2 MI may have major impact on severity classification, affecting diagnosis-related group (DRG) assignment, quality reporting, and reimbursement, just as type 1 MI does.
A diagnosis of “demand ischemia” has always been problematic. It is still assigned to code I24.8 (other forms of acute ischemic heart disease). Demand ischemia is supposed to be reserved for supply/demand mismatch causing ischemia without necrosis where biomarkers remain below the 99th upper reference limit, but instead it is often used by clinicians to describe what is technically a type 2 MI. A clinically correct distinction between demand ischemia and type 2 MI is crucial because demand ischemia has far less impact on severity classification.
Types 3, 4a, 4b, and 5 are all lumped together in a single code (I21.A9). I find this a little disappointing since distinction among these types would be really useful for the national health care database, allowing precise, meaningful analysis of clinical outcomes, resource utilization, and other important data on these different types of MI.
In summary, acute type 1 MI (Q-wave, STEMI, and NSTEMI) is assigned to codes in category I21 and, if occurring as a subsequent MI, in category I22. Type 2 MI (supply/demand mismatch) is coded as I21.A1 whether initial or subsequent. All other MI types are assigned code I21.A9. The code for demand ischemia is still I24.8. A clinically correct distinction between demand ischemia and type 2 MI is crucial.
Correct documentation and coding of the specific types of MI are clinically important and essential for billing, compliant reimbursement, and reporting for the national health care database.