What are the characteristics of Non-ST Elevation Myocardial Infarction (NSTEMI)?

Characteristics of Non-ST Elevation Myocardial Infarction (NSTEMI)

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

Pathophysiology and Definition

• NSTEMI is part of the acute coronary syndrome (ACS) spectrum, which includes unstable angina (UA), NSTEMI, and STEMI, differing primarily in severity and presence of myocardial damage 1
• NSTEMI typically results from an imbalance between myocardial oxygen supply and demand, most commonly due to coronary artery narrowing caused by a thrombus that developed on a disrupted atherosclerotic plaque 1
• The key distinction between unstable angina and NSTEMI is the presence of detectable cardiac biomarkers of necrosis (primarily troponin) in NSTEMI, indicating myocardial damage 1
• With increasingly sensitive troponin assays, biomarker-negative ACS (i.e., unstable angina) is becoming rarer 1

Clinical Presentation

• Chest pain or discomfort is the most common presenting symptom (pressure, tightness, heaviness, or pain that may radiate to the neck, jaw, shoulders, back, or arms) 1
• Associated symptoms may include dyspnea, nausea, vomiting, diaphoresis, weakness, dizziness, or lightheadedness 1
• Atypical presentations are more common in women, elderly patients, and those with diabetes 2
• Up to 25% of patients with NSTEMI may develop Q-wave MI during hospitalization 2

ECG Findings

• ST-segment depression (≥0.5 mm or 0.05 mV) is a hallmark finding, particularly when present in multiple leads 2
• T-wave inversion (≥2 mm or 0.2 mV), especially when symmetrical and deep in precordial leads 2
• Nonspecific ST-segment and T-wave changes may be present but are not required for diagnosis 2
• Transient ST-segment changes (≥0.05 mV) during symptoms that resolve when the patient becomes asymptomatic strongly suggest acute ischemia 2
• A completely normal ECG does not exclude NSTEMI, as 1-6% of patients with normal ECGs will have myocardial infarction 2

Diagnostic Criteria

• Elevated cardiac troponin (rise and/or fall) with at least one value above the 99th percentile upper reference limit 2
• Appropriate clinical context of myocardial ischemia 1
• Absence of persistent ST-segment elevation on ECG 1
• Serial cardiac troponin measurements (at presentation and 3-6 hours after symptom onset) are essential for diagnosis 2

Risk Stratification

• Risk assessment tools like TIMI, PURSUIT, GRACE, and NCDR-ACTION scores help identify patients at increased risk of adverse outcomes 1
• The magnitude and duration of ST-segment depression correlates with the extent of coronary artery disease and risk of adverse outcomes 2
• One-year incidence of death or new MI is 16.3% with ≥0.5 mm ST-segment deviation compared to 6.8% for isolated T-wave changes and 8.2% for no ECG changes 2
• Continuous ST-segment monitoring during the first 24 hours provides valuable prognostic information 2

Differential Diagnosis

• Non-ACS cardiovascular causes: aortic dissection, expanding aortic aneurysm, pericarditis, pulmonary embolism 1
• Non-cardiac causes: esophageal spasm, musculoskeletal pain, pneumonia, pleurisy, pneumothorax 1
• Alternative causes of ST-segment and T-wave changes: LV aneurysm, pericarditis, myocarditis, Prinzmetal's angina, early repolarization, Takotsubo cardiomyopathy, and Wolff-Parkinson-White syndrome 2
• Central nervous system events and certain medications (tricyclic antidepressants, phenothiazines) can cause deep T-wave inversion 2

Common Pitfalls in Diagnosis

• Relying solely on elevated troponin levels without considering clinical context can lead to misdiagnosis, as troponins can be elevated in various non-ACS conditions 3
• Missing NSTEMI in patients with atypical presentations, particularly in women, elderly, and diabetic patients 2
• Failure to obtain serial ECGs, which can significantly improve diagnostic accuracy 2
• Overlooking posterior MI, which may present with ST-segment depression in anterior precordial leads (V1-V3) 2
• Left circumflex occlusion can present with a non-diagnostic 12-lead ECG 2

Comorbidities and Outcomes

• NSTEMI patients often have multiple comorbidities that can worsen prognosis and complicate treatment 4
• Common comorbidities include hypertension, diabetes mellitus, chronic obstructive pulmonary disease, obesity, dyslipidemia, and smoking 4
• NSTEMI typically carries a poorer long-term prognosis than STEMI, largely due to the older age and higher comorbidity burden of NSTEMI patients 5, 6

Understanding these characteristics of NSTEMI is crucial for prompt diagnosis and appropriate management to improve patient outcomes and reduce morbidity and mortality.