Acute coronary syndrome: A cardiac conundrum

“Acute coronary syndrome” (ACS) encompasses a continuum of myocardial ischemia and infarction, which can make the diagnostic and coding criteria for ACS confusing. What's more, coding definitions and requirements don't always match clinical practice and terminology. Physicians should view the ACS diagnosis as provisional and evaluate further for a specific diagnosis.

Narrowing the diagnosis starts with knowing the guidelines. The most recent American Heart Association (AHA)/American College of Cardiology (ACC) Guidelines for the Management of Patients with Unstable Angina/Non-ST-Elevation Myocardial Infarction (NSTEMI) establish ACS as a provisional diagnostic term encompassing “clinical symptoms that are compatible with acute myocardial ischemia.” The term includes myocardial infarction (ST-segment elevation and depression, Q wave and non-Q wave) and unstable angina (UA). The more severe conditions of ST-elevation myocardial infarction (STEMI) and Q-wave infarction are not discussed here as part of the ACS definition.

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Image by Thinkstock.

NSTEMI and UA are a part of the ACS continuum considered to have similar pathogenesis and clinical presentations but different severity, as defined by the release of cardiac biomarker(s) such as troponin I, troponin T or CK-MB (creatine kinase muscle or brain type). According to the guidelines, “NSTEMI is established if a biomarker has been released” at any time to a level above “the 99th percentile reference limit of the normal population.” UA is diagnosed when there is no biomarker release based on 2 samples collected at least 6 hours apart.

Keep in mind the many causes of elevated troponin levels that do not necessarily indicate MI, such as heart failure, renal failure, arrhythmias, myocarditis, pulmonary embolism, and uneventful coronary procedures. Clinical presentation and other diagnostic studies will help to rule out these alternative diagnoses.

The guidelines also say that a provisional diagnosis of ACS should be further classified following evaluation as:

  1. 1. STEMI requiring consideration of immediate reperfusion therapy or percutaneous coronary intervention (PCI),
  2. 2. NSTEMI,
  3. 3. UA (definite, probable, or possible),
  4. 4. Non-ACS cardiovascular condition (for example, pericarditis), or
  5. 5. Non-cardiac condition with a specific cause (for example, gastroesophageal reflux disease) or with unknown cause.

Following this guideline from the AHA and ACC, coding rules classify UA and ACS as the same condition assigned to a low-severity, low-complexity diagnostic code. NSTEMI—even if characterized as mild or “early”—is classified as an acute MI, a more serious and complex diagnosis.

Finally, where does demand ischemia fit into the picture? This term is often used indiscriminately to describe patients with evidence of myocardial ischemia associated with a mismatch between myocardial oxygen demand and supply, but without coronary artery disease (CAD) or at least not primarily due to CAD. The diagnosis of “demand ischemia” is sometimes used when patients experience release of cardiac biomarkers, like troponin.

However, the Third Universal Definition of Myocardial Infarction (Circulation. 2012;126:2020-2035), developed jointly and published by the ACC, AHA, European Society of Cardiology and World Heart Foundation, defines “MI type 2” as “myocardial injury with necrosis [recognized by cardiac biomarker release], where a condition other than CAD contributes to an imbalance between myocardial oxygen supply and/or demand.”

Therefore, “demand ischemia” associated with release of cardiac biomarkers (to a level above the 99th percentile reference limit) actually represents progression to myocardial infarction (such as NSTEMI) as defined by this authoritative professional consensus. This is also consistent with the AHA/ACC guidelines for UA/NSTEMI discussed above.

Examples of conditions involving myocardial oxygen supply/demand imbalance include those listed in Table 1. The circumstances associated with MI as specified by the Universal Definition of MI are classified in Table 2.

In summary, ACS is a provisional description for conditions along a continuum of myocardial ischemia and infarction. A more specific diagnosis should be established based on subsequent evaluation. In the setting of myocardial ischemia, the distinction between NSTEMI and UA is crucial and based on the presence or absence of cardiac biomarker release (troponin or CK-MB). Cardiac biomarker release in the setting of “demand ischemia” actually represents type 2 MI.