What are the diagnostic criteria for acute coronary syndrome (ACS)?

Diagnostic Criteria for Acute Coronary Syndrome

Acute coronary syndrome (ACS) is diagnosed when a patient presents with symptoms of myocardial ischemia (typically chest pain or anginal equivalents) combined with either ECG changes or elevated cardiac biomarkers, specifically high-sensitivity cardiac troponin above the 99th percentile upper reference limit. 1

Core Diagnostic Components

The diagnosis requires integration of three key elements 1, 2:

1. Clinical Presentation

• Chest discomfort described as pain, pressure, tightness, or burning is the leading symptom, present in approximately 79% of men and 74% of women 1, 3
• Anginal equivalents include dyspnea, epigastric pain, pain radiating to the left arm, or diaphoresis 1, 3
• Atypical presentations occur in approximately 40% of men and 48% of women, particularly in elderly patients, women, and those with diabetes or chronic renal failure 1, 3
• Symptoms exacerbated by physical exertion or relieved by rest/nitrates support the diagnosis 1

2. Electrocardiogram (ECG) Findings

The 12-lead ECG must be obtained within 10 minutes of first medical contact and immediately interpreted 1, 3:

For STEMI (ST-Segment Elevation ACS):

• Persistent ST-segment elevation (>20 minutes) in two contiguous leads 1
• Specific thresholds: ≥0.25 mV in men <40 years, ≥0.2 mV in men ≥40 years, or ≥0.15 mV in women in leads V2-V3; ≥0.1 mV in all other leads 4
• New or presumed new left bundle branch block with clinical suspicion of ongoing ischemia 1, 4

For NSTE-ACS (Non-ST-Segment Elevation ACS):

• Transient ST-segment elevation 1
• Persistent or transient ST-segment depression 1
• T-wave inversion, flat T waves, or pseudo-normalization of T waves 1
• The ECG may be completely normal in up to 41% of NSTE-ACS cases 1, 3

3. Cardiac Biomarkers

High-sensitivity cardiac troponin (hs-cTn) T or I is the preferred biomarker 1, 5:

• Detection of a rise and/or fall in troponin values with at least one value above the 99th percentile upper reference limit 1, 5
• Serial measurements are essential—repeat at 3 hours, 6-9 hours, and 24 hours after presentation 1
• Elevated troponin distinguishes NSTEMI from unstable angina 1, 2

Additional Supporting Criteria

At least one of the following must accompany elevated troponin 1, 5:

• Symptoms of myocardial ischemia 1
• New ischemic ECG changes 1
• Development of pathological Q waves on ECG 1
• Imaging evidence of new loss of viable myocardium or new regional wall motion abnormality in a pattern consistent with ischemic etiology 1
• Intracoronary thrombus detected on angiography or autopsy 1

Classification Within ACS Spectrum

Once ACS is suspected, patients are categorized based on ECG and troponin 1:

STEMI (ST-Segment Elevation Myocardial Infarction)

• Persistent ST elevation (>20 minutes) on ECG 1
• Reflects acute total or subtotal coronary occlusion 1
• Accounts for approximately 30% of ACS cases 3
• Requires immediate reperfusion therapy 1, 3

NSTEMI (Non-ST-Segment Elevation Myocardial Infarction)

• No persistent ST elevation but elevated troponin above 99th percentile 1
• ECG may show ST depression, T-wave changes, or be normal 1
• Reflects cardiomyocyte necrosis from partial or intermittent coronary occlusion 1, 3

Unstable Angina

• Myocardial ischemia without troponin elevation (no cardiomyocyte necrosis) 1, 2
• Increasingly rare diagnosis with high-sensitivity troponin assays 1, 2
• Patients have substantially lower risk of death compared to NSTEMI 1

Critical Diagnostic Pitfalls

Normal ECG does not exclude ACS: Nearly two-thirds of ischemic episodes are clinically silent, and a completely normal ECG occurs in a significant proportion of NSTE-ACS patients 1, 3. Ischemia in the circumflex artery territory or isolated right ventricular ischemia frequently escapes standard 12-lead ECG detection 1.

Consider additional ECG leads: Record leads V7-V9 for suspected posterior MI and leads V3R-V4R for suspected right ventricular involvement in inferior MI 1, 4.

ACS can occur with non-obstructive coronary arteries: Approximately 5-10% of ACS cases occur without obstructive coronary artery disease on angiography, particularly in women, due to coronary vasospasm, spontaneous dissection, embolism, or microvascular dysfunction 2, 5.

Precipitating conditions must be identified: Anemia, infection, fever, thyroid disorders, tachyarrhythmias, or severe hypertension can precipitate or exacerbate ACS and must be addressed 1.

Risk Stratification Tools

Use validated risk scores to guide management decisions 1, 2:

• HEART score (History, ECG, Age, Risk factors, Troponin) is specifically designed for emergency department chest pain evaluation 1, 2
• TIMI score (Thrombolysis in Myocardial Infarction) 1
• GRACE score (Global Registry of Acute Cardiac Events) 1

These scores incorporate multiple variables and are more accurate than any single clinical element for predicting major adverse cardiac events 2.