What is the definition and management of Non-ST-Elevation Myocardial Infarction (NSTEMI)?

Definition of Non-ST-Elevation Myocardial Infarction (NSTEMI)

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

Diagnostic Criteria

NSTEMI is part of the acute coronary syndrome (ACS) spectrum and is distinguished by:

1. Biomarkers: Elevated cardiac troponin levels indicating myocardial necrosis

2. ECG Changes: Absence of persistent ST-segment elevation; may show:

   • ST-segment depression
   • T-wave inversions
   • Transient ST-segment elevation
   • Or even normal ECG findings

3. Clinical Context: Symptoms consistent with myocardial ischemia (chest discomfort, anginal equivalent)

Key Distinguishing Features

• NSTEMI vs. Unstable Angina: Both present similarly, but NSTEMI has elevated cardiac biomarkers while unstable angina does not 1, 2
• NSTEMI vs. STEMI: NSTEMI lacks persistent ST-segment elevation and typically results from subtotal coronary occlusion rather than complete occlusion 1

Pathophysiology

NSTEMI typically results from one of two mechanisms:

1. Type 1 MI (65-90% of cases): Spontaneous myocardial infarction related to atherosclerotic plaque disruption, erosion, or fissuring with resulting intraluminal thrombus formation leading to decreased myocardial blood flow 1, 3

2. Type 2 MI: Myocardial oxygen supply-demand imbalance not caused by acute coronary atherothrombosis, but by conditions such as:

   • Coronary spasm
   • Coronary embolism
   • Tachyarrhythmias/bradyarrhythmias
   • Severe anemia
   • Respiratory failure
   • Hypotension/hypertension 1, 2

Diagnostic Approach

1. ECG: Should be obtained and interpreted within 10 minutes of presentation

   • Look for ST-segment depression, T-wave inversions, or transient ST-elevation
   • Some patients may have normal ECGs 1, 2

2. Cardiac Biomarkers:

   • Cardiac troponin is the preferred biomarker
   • Values above the 99th percentile upper reference limit are considered elevated
   • Serial measurements may be needed to detect a rising or falling pattern 1

3. Clinical Assessment:

   • Chest discomfort or anginal equivalent (dyspnea, fatigue, etc.)
   • Risk factors for coronary artery disease
   • Evaluation for alternative causes of troponin elevation 1

Management

Management should be tailored based on risk stratification:

Immediate Management

1. Antiplatelet Therapy:

   • Aspirin (162-325 mg loading dose, then 81-325 mg daily)
   • P2Y12 inhibitor (ticagrelor or clopidogrel) for patients with intermediate to high-risk features 1, 2

2. Anticoagulation:

   • Unfractionated heparin, enoxaparin, or bivalirudin 4, 5

3. Anti-ischemic Therapy:

   • Nitrates for ongoing chest pain
   • Beta-blockers (if no contraindications)
   • Oxygen therapy if hypoxemic 2

Invasive Strategy Decision

Risk stratification guides timing of invasive strategy:

1. Very High-Risk (hemodynamic instability, recurrent/ongoing chest pain, life-threatening arrhythmias): Immediate invasive strategy (<2 hours)

2. High-Risk (dynamic ECG changes, GRACE score >140): Early invasive strategy (<24 hours)

3. Intermediate-Risk: Invasive strategy within 72 hours

4. Low-Risk: Non-invasive testing may be appropriate before deciding on invasive strategy 1, 2

Long-term Management

1. Secondary Prevention:
   • Dual antiplatelet therapy (duration based on risk and stent type if PCI performed)
   • Statin therapy (high-intensity)
   • Beta-blockers
   • ACE inhibitors/ARBs (especially if reduced ejection fraction or diabetes)
   • Lifestyle modifications (smoking cessation, diet, exercise) 1

Special Considerations

1. Type 2 NSTEMI: Management should focus on treating the underlying cause (e.g., correcting anemia, controlling heart rate) in addition to standard ACS therapy 2, 3

2. Atypical Presentations: More common in women, elderly, and patients with diabetes or chronic kidney disease 2

3. STEMI-Equivalents: Some NSTEMI patients may have complete coronary occlusion despite lacking ST-elevation and may benefit from urgent angiography 6

Prognosis

NSTEMI patients often have worse long-term outcomes compared to STEMI patients due to:

• Higher burden of comorbidities
• Older age
• More extensive coronary artery disease
• Higher rates of recurrent events 2

Risk stratification tools (TIMI, GRACE scores) help predict short and long-term outcomes and guide management decisions.