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Coding STEMI VS NSTEMI


Ischemic heart disease continues to be the leading cause of death in the United

States and worldwide (World Health Organization, 2017). It’s no surprise that
myocardial infarctions (MI) are frequently seen as reportable conditions when
coding inpatient encounters. Recent ICD-10-CM changes offer new codes to
further specify the type and cause of MIs.

How and where the myocardial death occurs determines the ICD-10-CM code
assignment.






MIs are categorized in several ways. Historically, MIs were categorized based
on the thickness of the myocardial necrosis. A transmural MI occurs when the
myocardial necrosis is full thickness (extending from the endocardium through
the myocardium to the epicardium), and a non-transmural MI includes necrosis
of the endocardium or the endocardium and myocardium only. 

Electrocardiogram findings are more commonly used to identify the type of MI. This includes ST-elevation MIs (STEMI), non-ST-elevation MIs (NSTEMI), Q-wave MIs, and non-Q-wave MIs.

An important change in the new 2018 ICD-10-CM Official Guidelines for Coding
and Reporting is the addition of a code for an unspecified AMI (I21.9). Previously, the unspecified AMI defaulted to a STEMI (I21.3).

“Management practice guidelines often distinguish between STEMI and non-
STEMI, as do many of the studies on which recommendations are based,”

If the documentation of an unspecified AMI defaults to a STEMI, but it is not treated as a STEMI, this could adversely affect quality measures for clinical performance.

Specifically, based on the 2017 recommendations of the American Hospital
Association and the American College of Cardiology Foundation, the established
goal of treatment for a patient with a diagnosis of an acute STEMI is an elapsed time of 90 minutes or less from first medical contact to primary percutaneous coronary intervention (PCI) when presenting to a facility with PCI capabilities.
AMIs are further identified by site, which corresponds with the coronary artery
involved (e.g., inferior wall MI, anterior wall MI, etc.), physicians should document which coronary artery was affected to capture the most specific code available. Documentation of a transmural MI unspecified by site will code as I21.9, AMI unspecified, but when specified by site or artery will code as a STEMI. Nontransmural MIs code as an NSTEMI.

The cause of an MI further defines the type of MI and the ICD-10-CM code
assignment. As mentioned earlier, the revised definition of a MI in 2007
emphasized the causes of the MI. The classification of MIs was divided into five
types, which is an important piece of the universal definition.
A type 1 MI is often referred to as a “spontaneous” MI and is commonly associated with coronary artery disease (CAD) and myocardial necrosis secondary to plaque rupture, erosion, dissection, ulceration, or erosion that results in a thrombus in one or more of the coronary arteries 

A type 2 MI occurs when myocardial necrosis results from either a reduction in
oxygen supply or decreased blood flow to the heart or an increase in the heart’s
need (demand) for oxygen. According to Sandoval, Smith, Thordsen, & Apple
(2014), anemia, tachyarrythmia, and respiratory failure were the most common
conditions predisposing patients to a type 2 MI, and “it is anticipated that it [type 2 MIs] will be detected more frequently once high sensitivity cardiac troponin assays are approved for clinical use in the United States.”

Some other examples of increased demand include severe aortic valve disease,
hypertension, and shock. One-fourth of the patients observed with AMIs were diagnosed with a type 2 MI, and of those, half had no significant CAD. Please note that it is a common error to call out type 2 MIs as NSTEMI type 2. A type 2 MI is an AMI by definition and not a NSTEMI

It was identified that “distinct demographics, increased prevalence of multiple comorbidities, a high-risk cardiovascular profile and an overall worse outcome” in patients diagnosed with a type 2 MI compared to those diagnosed with a type 1 MI. Specifically, they found patients diagnosed with type 2 MI to be older and female, and to have a higher incidence of prior MIs, PCI or coronary artery bypass graft (CABG), heart failure, chronic renal failure, and diabetes. “It is conceivable,” they stated, “that elderly patients with multiple comorbidities and an underlying coronary disease would be more susceptible to clinical changes that may interfere with the delicate balance of myocardial supply and demand, ensuing in type 2 MI”

Included in the universal definition of MIs (but seen documented less frequently)
are type 3 MI, types 4a and 4b MI, and type 5 MI. Type 3 MI refers to an AMI
when there is evidence of myocardial ischemia based on ECG finding and/or a
new left bundle branch block, but death occurs before cardiac biomarkers can be
obtained. Type 4a MI is an MI occurring after PCI. Type 4b MI is associated with stent thrombosis after PCI. Type 5 MI is associated with an MI after a CABG procedure

A final distinguishing factor for coding purposes is the age of the MI. Subsequent MIs are MIs occurring within four weeks or less of the initial MI. Code I22 is used for subsequent type 1 STEMI, NSTEMI, and AMI, unspecified. A code for the initial MI (I21.-) must be included.

The sequencing of these codes depends on the reason for admission. If the
subsequent MI is identified as a type 2 STEMI or NSTEMI, assign only I21.A1 and I21.A9 for type 3, 4, and 5 MIs. MIs that occurred more than four weeks prior but are still being treated have the appropriate aftercare code assigned, and those no longer needing treatment are coded as I25.2, old MI

Querying for specificity of the AMI

Without specific criteria and treatment guidelines, there is room for subjectivity
among clinicians in the diagnosis of a type 2 MI. Until the 2018 ICD-10-CM Official Guidelines for Coding and Reporting, there were no codes to distinguish between a type 1 and a type 2 MI. Clinicians are unable to diagnose a patient with type 2 MI without being penalized by International Classification of Diseases coders for deviating from the accepted guideline driven ACS therapies that are required by Centers for Medicaid and Medicare Services (e.g., aspirin on arrival and discharge, beta-blocker, statin prescribed on discharge, and so on), even though these therapies might not be appropriate for type 2 MI.”

Now that codes are available to distinguish between types of MIs, the subjectivity in diagnosing still remains. This, in turn, lends itself to subjectivity as to when to query for a type 2 MI.


Type 1
Spontaneous MI secondary to a primary coronary event such as plaque erosion and/or rupture, ulceration, fissuring with resulting intraluminal thrombus

Type 2
AMI secondary to supply and demand mismatch (e.g., coronary spasm, anemia, hypotension, respiratory failure, etc.)

Type 3
Sudden cardiac arrest secondary to suspected AMI, but death occurs before cardiac biomarkers in the blood are drawn or have time to appear

Type 4a
MI associated with PCI

Type 4b
MI associated with stent thrombosis

Type 5
MI associated with CABG

Subsequent MI
Occurring <4 weeks (28 days) from initial MI

Old MI
>4 weeks old and no longer needing treatment

The FY 2018 updates for ICD-10-CM include the following information related to coding STEMI and NSTEMI:

ICD-10-CM codes and guidelines for type 1 MI:

-- Type 1 STEMI and transmural MIs identified by site or coronary artery
-- I21.0–I21.2
-- Type 1 STEMI without the site documented
-- I21.3, STEMI of unspecified site
-- Transmural MI, without the specified site or artery
-- I21.9, AMI, unspecified
-- Type 1 NSTEMI and non-transmural/subendocardial MIs
-- I21.4
-- If a type 1 NSTEMI evolves to STEMI, assign the STEMI code
-- If a type 1 STEMI converts to NSTEMI due to thrombolytic, code the STEMI
-- I21.9, AMI, unspecified, is the default for unspecified AMI or unspecified type

 ICD-10-CM codes and guidelines for type 2 MI:


-- Type 2 AMI (STEMI and NSTEMI)
-- Assign to code I21.A1, MI type 2 with a code for the underlying cause
-- Do not assign code I24.8, other forms of acute ischemic heart disease for the
demand ischemia
-- Sequencing of type 2 AMI or the underlying cause is dependent on the circumstances of
admission

ICD-10-CM codes and guidelines for type 3, 4a, 4b, 4c, and 5 MI:

-- Assign code I21.A9, other MI type
-- The “Code also” and “Code first” notes should be followed related to complications, and
for coding of post-procedural MI during or following cardiac surgery

ICD-10-CM codes and guidelines for subsequent MI:

-- A code from category I22, Subsequent STEMI and NSTEMI, is to be used when a patient has suffered a type 1 MI or unspecified AMI and has a new AMI within the four-week time frame of the initial AMI 
-- A code from category I22 must be used in conjunction with a code from
category I21
-- The sequencing of the I22 and I21 codes depends on the circumstances of the
encounter
-- Do not assign code I22 for subsequent MIs other than type 1 or unspecified
-- For a subsequent type 2 MI, assign only code I21.A1
-- For a subsequent type 4 or type 5 MI, assign only code I21.A9

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