What are the diagnostic criteria for non-ST-elevation myocardial infarction (NSTEMI)?

Diagnostic Criteria for NSTEMI

Non-ST-elevation myocardial infarction (NSTEMI) is defined as elevated cardiac biomarkers of necrosis (primarily troponin) in the absence of persistent ST-segment elevation on ECG, occurring in an appropriate clinical context of myocardial ischemia. 1

Essential Diagnostic Components

1. Cardiac Biomarkers

• Troponin elevation is mandatory: Detection of an increase and/or decrease of cardiac troponin (preferably high-sensitivity cardiac troponin) with at least one value above the 99th percentile of the upper reference limit 2
• Serial measurements should be obtained at presentation and 3-6 hours after symptom onset to identify a rising and/or falling pattern 2
• Additional troponin levels should be obtained beyond 6 hours in patients with normal initial values when clinical suspicion remains high 2

2. Clinical Context

At least ONE of the following must be present in addition to elevated troponin 2:

• Symptoms of myocardial ischemia (chest pain, pressure, tightness, burning, or equivalent symptoms)
• New ischemic ECG changes (ST depression, T-wave inversion, etc.)
• Development of pathological Q waves
• Imaging evidence of new loss of viable myocardium or regional wall motion abnormality consistent with ischemia
• Intracoronary thrombus detected on angiography

3. ECG Findings

Common ECG patterns in NSTEMI 2:

• ST-segment depression
• T-wave inversion
• Transient ST-segment elevation
• Flat T waves
• Pseudo-normalization of T waves
• Normal ECG (in some cases)

Classification of NSTEMI by Type

Type 1 NSTEMI (65-90% of cases)

• Results from atherosclerotic plaque rupture, ulceration, fissure, erosion, or dissection
• Leads to intraluminal thrombus formation causing decreased myocardial blood flow and/or distal embolization 2, 1
• Usually requires antiplatelet therapy, anticoagulation, and consideration of invasive management

Type 2 NSTEMI

• Results from an imbalance between myocardial oxygen supply and demand not caused by acute coronary atherothrombosis 2
• Common causes include:
  • Coronary artery spasm
  • Coronary endothelial dysfunction
  • Tachyarrhythmias or bradyarrhythmias
  • Anemia
  • Respiratory failure
  • Hypotension or severe hypertension 1
• Treatment focuses on correcting the underlying cause

Distinguishing NSTEMI from Unstable Angina

Feature	Unstable Angina	NSTEMI
Chest Pain	Ischemic-type discomfort	Ischemic-type discomfort
Cardiac Biomarkers	Normal	Elevated
Myocardial Necrosis	Absent	Present
ECG Changes	May show ischemic changes or be normal	ST depression, T-wave inversion, or normal
Coronary Occlusion	Partial/transient	Partial/subtotal

Diagnostic Algorithm

1. Initial Assessment:

   • Obtain 12-lead ECG within 10 minutes of presentation 2, 1
   • Check for ST-segment elevation (if present, consider STEMI pathway)
   • If no ST-elevation, proceed with NSTE-ACS evaluation

2. Biomarker Testing:

   • Draw high-sensitivity cardiac troponin at presentation 2
   • Repeat troponin 3-6 hours after symptom onset 2
   • Additional troponin may be needed if clinical suspicion remains high despite normal initial values

3. Diagnosis Confirmation:

   • If troponin is elevated + clinical context suggests ischemia = NSTEMI
   • If troponin is normal + clinical context suggests ischemia = Unstable Angina
   • If troponin is elevated but no clinical evidence of ischemia = Consider other causes of troponin elevation

Common Pitfalls and Caveats

1. Normal ECG does not exclude NSTEMI:

   • 1-6% of patients with normal ECGs may still have NSTEMI 2
   • Serial ECGs during symptoms can increase diagnostic yield

2. Troponin timing matters:

   • Early presentation may yield false-negative results
   • Serial measurements are essential to detect a rise and/or fall pattern
   • Using the time of presentation as the time of onset when symptom onset is ambiguous 2

3. Alternative causes of troponin elevation:

   • Renal failure, sepsis, pulmonary embolism, myocarditis, and heart failure can cause troponin elevation without ACS
   • Clinical context is crucial for proper interpretation

4. Multivessel disease in NSTEMI:

   • A significant proportion of NSTEMI patients (approximately 33%) may have multivessel coronary artery disease 3
   • Markedly elevated troponin levels may indicate higher likelihood of multivessel disease

5. Atypical presentations:

   • More common in older adults, women, patients with diabetes, chronic renal failure, and dementia 1
   • May present with dyspnea, epigastric pain, or arm pain rather than classic chest pain

By following this diagnostic approach, clinicians can accurately identify NSTEMI patients and initiate appropriate management strategies to improve morbidity, mortality, and quality of life outcomes.