What are the diagnostic and treatment steps for Acute Coronary Syndrome (ACS) Non-ST-Elevation Myocardial Infarction (NSTEMI)?

NSTEMI Diagnostics and Treatment Algorithm

Immediate Diagnostic Steps (Within 10 Minutes)

All patients with suspected NSTEMI must have a 12-lead ECG obtained and interpreted within 10 minutes of first medical contact, with continuous cardiac monitoring and defibrillation capability immediately available. 1

Initial Clinical Assessment

• Chest pain characteristics: Assess for typical anginal symptoms (pressure, tightness, burning) or atypical presentations including dyspnoea, epigastric pain, or left arm pain 1, 2
• High-risk populations (elderly, diabetics, women) frequently present with atypical symptoms and require heightened suspicion even with non-diagnostic findings 1, 3
• Vital signs: Document systolic blood pressure, heart rate, oxygen saturation, and Killip classification 1
• Physical examination findings: Assess for signs of heart failure, hemodynamic instability, cardiac murmurs (particularly ischemic mitral regurgitation), or alternative diagnoses 1
• Risk factor assessment: Age, gender, diabetes, hyperlipidemia, hypertension, renal insufficiency, known CAD, prior MI, prior revascularization, peripheral or carotid artery disease 1

ECG Interpretation for NSTEMI

The ECG in NSTEMI shows NO persistent ST-segment elevation but may demonstrate: 1, 3, 4

• ST-segment depression ≥0.5 mm (0.05 mV): Most significant when present in multiple leads; correlates with increased mortality and extent of CAD 3
• T-wave inversion ≥2 mm (0.2 mV): Particularly deep, symmetrical inversions in precordial leads suggest critical LAD stenosis 3
• Transient ST-segment changes: ST deviations that resolve when symptoms abate strongly suggest severe underlying CAD 3
• Nonspecific changes: ST deviation <0.5 mm or T-wave inversion ≤2 mm are less diagnostically helpful but don't exclude NSTEMI 3
• Normal ECG: Occurs in up to 41% of NSTE-ACS cases and does NOT exclude the diagnosis 2, 3

Critical pitfall: Posterior MI may present with ST-depression in V1-V3; obtain posterior leads (V7-V9) if suspected. Left circumflex occlusion can present with entirely normal 12-lead ECG. 3

Cardiac Biomarker Testing

Blood work on admission must include high-sensitivity cardiac troponin T or I (preferred), with results available within 60 minutes. 1

Troponin Measurement Protocol

• Initial troponin: Draw at presentation 1
• Repeat troponin timing:
  • At 1-3 hours if high-sensitivity assays are used 1
  • At 6-12 hours if standard assays are used or if high-sensitivity results are equivocal 3
• Diagnostic threshold: Elevation above the 99th percentile upper reference limit with a rise and/or fall pattern in appropriate clinical context 1, 4

NSTEMI is definitively diagnosed when elevated troponin (above 99th percentile with rise/fall pattern) occurs in the clinical context of acute myocardial ischemia, distinguishing it from unstable angina which has no detectable biomarker elevation. 1, 4

Additional Laboratory Tests

• Serum creatinine (for dosing adjustments) 1
• Hemoglobin and hematocrit 1
• Platelet count 1
• Blood glucose 1
• INR if on vitamin K antagonists 1
• Lipid profile once NSTEMI diagnosis is confirmed 1

Risk Stratification and Triage

Patients are categorized into four working diagnoses based on initial assessment: 1

1. STEMI → Immediate reperfusion therapy per STEMI guidelines 1
2. NSTE-ACS with ongoing ischemia or hemodynamic instability → Immediate coronary angiography 1
3. NSTE-ACS without ongoing ischemia or hemodynamic instability → Early invasive strategy within 12-24 hours for high-risk features 3
4. NSTE-ACS unlikely → Assign with extreme caution, especially in elderly and diabetics 1

Immediate Coronary Angiography Indications (Regardless of ECG/Biomarkers)

Proceed directly to catheterization lab if: 1, 3

• Refractory angina despite medical therapy 3
• Hemodynamic instability or cardiogenic shock 1
• Life-threatening ventricular arrhythmias 1, 3
• Mechanical complications of MI 3
• Ongoing myocardial ischemia with echocardiographic regional wall motion abnormality 1

High-Risk Features Requiring Early Invasive Strategy (12-24 Hours)

• GRACE score >140 3
• Elevated troponin levels 3
• Dynamic ST-segment or T-wave changes 3
• LVEF <40% 3
• Diabetes mellitus 3
• Prior PCI or CABG 3

Initial Medical Management

Immediate Interventions

• Oxygen therapy: Only if oxygen saturation <90% or respiratory distress (avoid routine use) 1
• Nitrates: Sublingual or intravenous for persisting chest pain 1
• Morphine: Reserved for severe chest pain refractory to nitrates 1
• Defibrillator patches: Place in case of ongoing ischemia until urgent revascularization 1

Antithrombotic Therapy

Aspirin and parenteral anticoagulation are recommended immediately upon diagnosis. 1

P2Y12 Inhibitor Selection (Based on 2020 ESC Guidelines)

For patients proceeding to PCI, prasugrel should be considered in preference to ticagrelor, but only AFTER coronary angiography is performed. 1

For conservative management (no early invasive strategy): 1

• Ticagrelor is preferred over clopidogrel once NSTEMI diagnosis is established 1
• Avoid routine pre-treatment with P2Y12 inhibitors when coronary anatomy is unknown and early invasive management is planned 1

Clopidogrel considerations: 5

• Loading dose: 300 mg followed by 75 mg daily 5
• In the CURE trial, clopidogrel plus aspirin reduced CV death/MI/stroke from 11.4% to 9.3% (20% relative risk reduction) in NSTE-ACS 5
• Critical caveat: CYP2C19 poor metabolizers (2% of Whites, 4% of Blacks, 14% of Chinese) have diminished antiplatelet response; consider alternative P2Y12 inhibitor if genetic testing identifies poor metabolizer status 5

Anticoagulation Dosing

• Dosing must account for patient age and renal function 1
• Low-molecular-weight heparin is at least as effective and safe as unfractionated heparin 6

Monitoring During Initial Phase

• Continuous ECG monitoring with defibrillation capability 1, 3
• Serial ECGs: Obtain at 15-30 minute intervals during first hour if initial ECG non-diagnostic but clinical suspicion remains high 3
• Vital signs: Assess regularly 1
• Observation setting: Interdisciplinary emergency departments or chest pain units until MI confirmed or ruled out 1

Common Diagnostic Pitfalls

• Normal ECG does not exclude ACS: 1-6% of patients with normal ECG and chest pain will have MI, and at least 4% will have unstable angina 3
• Troponin elevation alone is insufficient: Must interpret within appropriate clinical context; many non-ACS conditions elevate troponin (heart failure, tachyarrhythmias, hypertensive emergency) 1
• Alternative diagnoses to consider: Pulmonary embolism, aortic dissection, pericarditis, myocarditis, Takotsubo cardiomyopathy, esophageal disorders 1, 3
• Misdiagnosis rates: 2-5% of ACS patients are inappropriately discharged from emergency departments 7
• High-risk populations require lower threshold: Elderly, diabetics, and women frequently present atypically 1, 3