When is a troponin elevation an acute myocardial infarction?

Hospitalists encounter troponin elevations daily, but we have to use clinical judgment to determine if the troponin elevation represents either a myocardial infarction (MI), or a non-MI troponin elevation (i.e. a , nonischemic myocardial injury).

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It is important to remember that an MI specifically refers to myocardial injury due to acute myocardial ischemia to the myocardium. This lack of blood supply can be due to an acute absolute or relative deficiency in coronary artery blood flow. However, there are also many mechanisms of myocardial injury unrelated to reduced coronary artery blood flow, and these should be more appropriately termed non-MI troponin elevations.

Dr. Jessica Nave

Historically, when an ischemic mechanism of myocardial injury was suspected, providers would categorize troponin elevations into ST-elevation MI (STEMI) versus non-ST-elevation MI (NSTEMI) based on the electrocardiogram (ECG). We would further classify the NSTEMI into type 1 or type 2, depending on the mechanism of injury. The term “NSTEMI” served as a “catch-all” term to describe both type 1 NSTEMIs and type 2 MIs, but that classification system is no longer valid.

Dr. Abhinav Goyal

As of Oct. 1, 2017, ICD-10 and the Centers for Medicare & Medicaid Services have a new ICD-10 diagnosis code for type 2 MI (I21.A1), distinct from NSTEMI (I21.4) based on updated definitions from the American College of Cardiology, American Heart Association, European Society of Cardiology, and World Heart Federation. The term “NSTEMI” should be used only when referring to a type 1 MI not when referring to a type 2 MI.¹

Classification of MI types

The Fourth Universal Definition of MI published in August 2018 further updated the definitions of MI (summarized in Figure 1).² This review focuses on type 1 and type 2 MIs, which are the most common types encountered by hospitalists. Types 3-5 MI (grouped under a common ICD-10 diagnosis code for “Other MI Types,” or I21.A9) would rarely be diagnosed by hospitalists.

Figure 1: Classification of MI

The diagnosis of a type 1 MIs (STEMI and NSTEMI) is supported by the presence of an acute coronary thrombus or plaque rupture/erosion on coronary angiography or a strong suspicion for these when angiography is unavailable or contraindicated. Type 1 MI (also referred to as spontaneous MI) is generally a primary reason (or “principal” diagnosis) for a patient’s presentation to a hospital.³ Please note that a very high or rising troponin level alone is not diagnostic for a type 1 or type 2 NSTEMI. The lab has to be taken in the context of the patient’s presentation and other supporting findings.

In contrast to a type 1 MI (STEMI and NSTEMI), at type 2 MI results from an imbalance between myocardial oxygen supply and demand unrelated to acute coronary artery thrombosis or plaque rupture. A type 2 MI is a relative (as opposed to an absolute) deficiency in coronary artery blood flow triggered by an abrupt increase in myocardial oxygen demand, drop in myocardial blood supply, or both. In type 2 MI, myocardial injury occurs secondary to an underlying process, and therefore requires correct documentation of the underlying cause as well.

Common examples of underlying causes of type 2 MI include acute blood loss anemia (e.g. GI bleed), acute hypoxia (e.g. COPD exacerbation), shock states (cardiogenic, hypovolemic, hemorrhagic, or septic), coronary vasospasm (e.g. spontaneous), and bradyarrhythmias. Patients with type 2 MI often have a history of fixed obstructive coronary disease, which when coupled with the acute trigger facilitates the type 2 MI; however, underlying CAD is not always present.

Diagnosing a type 2 MI requires evidence of acute myocardial ischemia (Figure 2) with an elevated troponin but must also have at least one of the following:²

• Symptoms of acute myocardial ischemia such as typical chest pain.
• New ischemic ECG changes.
• Development of pathological Q waves.
• Imaging evidence of new loss of viable myocardium, significant reversible perfusion defect on nuclear imaging, or new regional wall motion abnormality in a pattern consistent with an ischemic etiology.

Distinguishing a type 1 NSTEMI from a type 2 MI depends mainly on the clinical context and clinical judgment. A patient whose presenting symptoms include acute chest discomfort, acute ST-T wave changes, and a rise in troponin would be suspected of having a type 1 NSTEMI. However, in a patient presenting with other or vague complaints where an elevated troponin was found amongst a battery of tests, a type 2 MI may be favored, particularly if there is evidence of an underlying trigger for a supply-demand mismatch. In challenging cases, cardiology consultation can help determine the MI type and/or the next diagnostic and treatment considerations.

When there is only elevated troponin levels (or even a rise and fall in troponin) without new symptoms or ECG/imaging evidence of myocardial ischemia, it is most appropriate to document a non-MI troponin elevation due to a nonischemic mechanism of myocardial injury.

Non-MI troponin elevation (nonischemic myocardial injury)

The number of conditions known to cause myocardial injury through mechanisms other than myocardial ischemia (see Figure 2) is growing, especially in the current era of high-sensitivity troponin assays.⁴

Common examples of underlying causes of non-MI troponin elevation include:

• Acute (on chronic) systolic or diastolic heart failure: Usually due to acute ventricular wall stretch/strain. Troponin elevations tend to be mild, with more indolent (or even flat) troponin trajectories.
• Pericarditis and myocarditis: Due to direct injury from myocardial inflammation.
• Cardiopulmonary resuscitation (CPR): Due to physical injury to the heart from mechanical chest compressions and from electrical shocks of external defibrillation.
• Stress-induced (takotsubo) cardiomyopathy: Stress-induced release of neurohormonal factors and catecholamines that cause direct myocyte injury and transient dilatation of the ventricle.
• Acute pulmonary embolism: Result of acute right ventricular wall stretch/strain, not from myocardial ischemia.
• Sepsis without shock: Direct toxicity of circulating cytokines to cardiac myocytes. In the absence of evidence of shock and symptoms/signs of myocardial ischemia, do not document type 2 MI.
• Renal failure (acute kidney injury or chronic kidney disease): Multiple etiologies, but at least partially related to reduced renal clearance of troponin. In general, renal failure in the absence of symptoms/signs of ischemia is best classified as a non-MI troponin elevation. ESRD patients who present with volume overload due to missed dialysis also typically have a non-MI troponin elevation.
• Stroke/intracranial hemorrhage: Mechanisms of myocardial injury and troponin elevation are incompletely understood, but may include catecholamine surges that injure the heart.

Some underlying conditions can cause a type 2 MI or a non-MI troponin elevation depending on the clinical context. For example, hypertensive emergency, severe aortic valve stenosis, hypertrophic cardiomyopathy, and tachyarrhythmias (including atrial fibrillation with rapid ventricular response) may cause increased myocardial oxygen demand, and in patients with underlying CAD, could precipitate a type 2 MI.

However, these same conditions could cause a non-MI troponin elevation in patients without CAD and could also cause myocardial injury and troponin release by causing acute left ventricular stretch/strain. Distinguishing the diagnose of type 2 MI vs. non-MI troponin elevation depends on documenting whether there are ancillary ischemic symptoms, ECG findings, imaging, and/or cath findings of acute myocardial ischemia.