What is a Non-ST-Elevation Myocardial Infarction (NSTEMI)?

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

NSTEMI is defined as elevated cardiac biomarkers (primarily troponin) above the 99th percentile of the upper reference limit, in the absence of persistent ST-segment elevation on the electrocardiogram, in an appropriate clinical context of myocardial ischemia. 1

Pathophysiology

NSTEMI typically results from the disruption or erosion of an atherosclerotic plaque with formation of a subtotally occlusive thrombus, leading to:

• Reduction in coronary blood flow
• Imbalance between myocardial oxygen supply and demand
• Myocardial ischemia and subsequent necrosis (cell death) 2, 1

Unlike STEMI (which usually involves complete coronary occlusion), NSTEMI typically involves partial or intermittent occlusion of the coronary artery.

Diagnostic Criteria

The diagnosis of NSTEMI requires:

1. Elevated cardiac biomarkers - specifically troponin above the 99th percentile of the upper reference limit 2, 1
2. Absence of persistent ST-segment elevation on ECG 2
3. Appropriate clinical context - symptoms consistent with myocardial ischemia 2

ECG Findings in NSTEMI

• ST-segment depression
• Transient ST-segment elevation
• T-wave inversion
• Flat T waves
• Pseudo-normalization of T waves
• Or sometimes a normal ECG 2

Clinical Presentation

• Chest discomfort/pain (typically at rest, anginal in character)
• Pain may radiate to arms, back, or jaw
• Associated symptoms may include dyspnea, nausea, or diaphoresis
• Duration often >20 minutes 2, 3

Distinction from Other ACS Types

NSTEMI is part of the acute coronary syndrome (ACS) spectrum, which includes:

1. STEMI - ST-segment elevation MI with complete coronary occlusion, typically leading to Q-wave MI
2. NSTEMI - Elevated cardiac biomarkers without persistent ST elevation
3. Unstable Angina - Similar symptoms to NSTEMI but without biomarker elevation 2

The key distinction between NSTEMI and unstable angina is the presence of myocardial necrosis (cell death) in NSTEMI, evidenced by elevated cardiac biomarkers 2.

Types of NSTEMI

Two major types of NSTEMI are recognized:

1. Type 1 NSTEMI - Results from atherosclerotic plaque rupture, ulceration, fissure, or erosion with resulting intraluminal thrombus 2

2. Type 2 NSTEMI - Results from a condition other than coronary plaque instability causing an imbalance between myocardial oxygen supply and demand (e.g., severe anemia, hypoxemia, tachyarrhythmias, hypotension) 2, 1

Diagnostic Approach

Modern diagnosis of NSTEMI relies on:

• High-sensitivity troponin assays
• Rapid "rule-in" and "rule-out" algorithms (0h/1h or 0h/2h) 1
• Risk stratification tools (TIMI, GRACE, PURSUIT scores) 2

Management Implications

The diagnosis of NSTEMI carries important management implications:

• Dual antiplatelet therapy (aspirin plus P2Y12 inhibitor) 1, 4
• Anticoagulation
• Consideration of early invasive strategy (coronary angiography) 5
• Secondary prevention measures

Prognostic Significance

NSTEMI generally has a worse long-term prognosis than STEMI, often due to:

• Higher burden of comorbidities
• Older patient age
• More extensive coronary disease 1

Type 2 NSTEMI typically has a worse prognosis than Type 1, with mortality often related to non-cardiac causes 1, 6.