What are the diagnostic criteria for a patient suspected of having a non-ST-elevation myocardial infarction (NSTEMI)?

NSTEMI Diagnostic Criteria

NSTEMI is diagnosed when cardiac troponin rises and/or falls with at least one value above the 99th percentile upper reference limit, combined with clinical evidence of acute myocardial ischemia, in the absence of persistent ST-segment elevation on ECG. 1

Core Diagnostic Requirements

The diagnosis requires all three components to be present simultaneously: 1, 2

• Elevated cardiac biomarkers: High-sensitivity cardiac troponin (hs-cTn) T or I above the 99th percentile with a rising and/or falling pattern 1
• Clinical evidence of myocardial ischemia including at least one of: 1
  • Symptoms of myocardial ischemia (chest pain, pressure, tightness, burning, or anginal equivalents like dyspnoea or epigastric pain)
  • New ischemic ECG changes
  • Development of pathological Q waves
  • Imaging evidence of new loss of viable myocardium or regional wall motion abnormality
  • Intracoronary thrombus on angiography
• Absence of persistent ST-segment elevation (>20 minutes) on 12-lead ECG 1

ECG Findings in NSTEMI

While persistent ST-elevation is absent, the ECG is rarely completely normal: 3

• ST-segment depression ≥0.5 mm (0.05 mV) in multiple leads is the hallmark finding and correlates with increased mortality 3
• Deep T-wave inversion ≥2 mm (0.2 mV), especially symmetrical inversions in precordial leads, strongly suggests critical LAD stenosis 3
• Transient ST-segment changes (≥0.05 mV) during symptoms that resolve when asymptomatic indicate severe underlying CAD 3
• Nonspecific changes (ST deviation <0.5 mm or T-wave inversion ≤2 mm) are less diagnostically helpful but may still indicate ischemia 3
• Posterior MI patterns: ST-depression in V1-V3 may represent posterior infarction; obtain posterior leads V7-V9 3

Critical Timing of Troponin Measurements

Serial troponin testing is mandatory when initial values are normal or equivocal: 1

• Obtain initial troponin at presentation (time 0) 1, 2
• Repeat troponin using validated algorithms: 1, 2
  • 0h/1h protocol if high-sensitivity assay with validated algorithm available
  • 0h/2h protocol as alternative with validated hs-cTn test
  • Traditional 0h/3-6h protocol acceptable
• Additional troponin beyond 6 hours after symptom onset when initial values are normal but clinical suspicion remains intermediate-to-high 1
• Troponin results must be available within 60 minutes of blood draw 3

Immediate Evaluation Protocol

Execute this sequence within the first 10 minutes: 3, 2

• 12-lead ECG obtained and interpreted within 10 minutes of presentation 3, 2
• Continuous ECG monitoring with defibrillation capability immediately 3
• Serial ECGs at 15-30 minute intervals during first hour if initial ECG non-diagnostic but suspicion high 3
• Initial blood work: troponin, creatinine, hemoglobin, hematocrit, platelets, glucose, INR (if anticoagulated) 3

Risk Stratification Requirements

Calculate objective risk scores for all patients with confirmed NSTEMI: 1, 2

• GRACE risk score >140 or TIMI risk score >4 defines high-risk patients requiring early invasive strategy within 24 hours 1, 2
• Risk stratification guides timing of angiography: 1
  • High-risk: within 12-24 hours
  • Intermediate-risk: within 24-72 hours

Critical Pitfalls to Avoid

• Normal ECG does NOT exclude NSTEMI: 1-6% of patients with completely normal ECG and chest pain will have MI, and at least 4% will have unstable angina 3
• Certain populations present atypically: elderly, diabetics, and women more likely to have non-diagnostic ECGs and atypical symptoms 3
• Do not delay treatment waiting for troponin if clinical presentation strongly suggests ACS; initiate monitoring and antiplatelet therapy immediately 3
• 22.8% of NSTEMI patients can present with completely normal ECG within 24 hours of symptom onset 4
• Left circumflex occlusion can present with non-diagnostic 12-lead ECG; maintain high clinical suspicion 3
• Alternative causes of troponin elevation (Type 2 MI) must be considered: severe hypertension, tachyarrhythmias, severe anemia, hypotension, respiratory failure, coronary spasm 1, 5

Prognostic Significance of ECG Patterns

The magnitude of ECG abnormalities provides independent prognostic information: 3

• ST depression in ≥3 leads with maximal depression ≥0.2 mV increases likelihood of acute non-Q-wave MI by 3-4 times 3
• One-year mortality: 16.3% with ≥0.5 mm ST-deviation vs 6.8% with isolated T-wave changes vs 8.2% with no ECG changes 3
• Up to 25% of NSTEMI patients with elevated biomarkers develop Q-wave MI during hospitalization 3