What is the first step in evaluating a sample electrocardiogram that is suspected to represent an acute coronary syndrome?

First Step in Evaluating an ECG Suspected to Represent Acute Coronary Syndrome

The first step is to obtain and interpret a 12-lead ECG within 10 minutes of first medical contact to distinguish between STEMI and non-ST-segment elevation ACS, as this determination immediately dictates whether urgent reperfusion therapy is required. 1

Immediate ECG Acquisition and Interpretation

• Acquire the 12-lead ECG within 10 minutes of patient arrival in the emergency department or first contact with emergency medical services in the pre-hospital setting 1
• Immediate interpretation by a qualified physician is mandatory—delays in ECG interpretation directly impact mortality outcomes 1
• The ECG must be obtained before any other diagnostic testing, as it is the single most critical tool for initial triage and determines the entire treatment pathway 1

Critical ECG Patterns to Identify

STEMI Criteria (Requires Immediate Reperfusion)

• ST-segment elevation ≥1 mm in ≥2 anatomically contiguous leads (measured at the J-point) in all leads except V2-V3 1
• In leads V2-V3 specifically: ≥2 mm in men ≥40 years, ≥2.5 mm in men <40 years, and ≥1.5 mm in women regardless of age 1
• Persistent ST-elevation lasting >20 minutes distinguishes STEMI from transient ST-elevation seen in NSTE-ACS 1

NSTE-ACS Patterns (Different Management Pathway)

• Horizontal or down-sloping ST-segment depression ≥0.5 mm in ≥2 contiguous leads 1
• T-wave inversion >1 mm in ≥2 contiguous leads with prominent R wave or R/S ratio >1 1
• Transient ST-segment elevation that resolves (high-risk finding requiring aggressive management) 1
• Deep symmetrical T-wave inversions in anterior chest leads suggest proximal left anterior descending artery stenosis 1

Essential Additional ECG Leads

• Obtain posterior leads V7-V9 when you suspect left circumflex occlusion, particularly if there is isolated ST-segment depression ≥0.5 mm in leads V1-V3 1
• Obtain right-sided leads V3R and V4R when you suspect right ventricular ischemia, as this frequently escapes detection on standard 12-lead ECG 1, 2
• Ischemia in the circumflex territory or isolated right ventricular ischemia is missed in up to one-third of cases without these additional leads 1, 2

Critical Pitfalls to Avoid

A Normal ECG Does NOT Exclude ACS

• Approximately 5% of patients with completely normal ECGs who are discharged from emergency departments ultimately have acute MI or unstable angina 1
• Up to one-third of NSTE-ACS patients present with a completely normal initial ECG 2, 3
• A normal ECG during chest pain should prompt consideration of non-cardiac causes, but serial ECGs and troponin measurements remain mandatory 1

Dynamic ECG Changes Require Serial Monitoring

• Repeat ECGs at 15-30 minute intervals during the first hour if the initial ECG is normal or inconclusive but clinical suspicion remains high 2
• Obtain additional ECGs at 3 hours, 6-9 hours, and 24 hours after first presentation, and immediately with any recurrence of chest pain 1
• Almost two-thirds of ischemic episodes during the unstable phase are clinically silent and will be missed without continuous monitoring 1
• Compare with previous ECGs when available, particularly in patients with left ventricular hypertrophy or prior MI, as baseline abnormalities can obscure acute changes 1

Confounding ECG Patterns

• ST-segment changes can occur in non-ACS conditions including acute pericarditis, left ventricular hypertrophy, left bundle branch block, Brugada syndrome, right ventricular pacing, Takotsubo syndrome, and early repolarization 1
• New or presumably new left bundle branch block occurs infrequently and should NOT be considered diagnostic of acute MI in isolation—clinical correlation is required 1
• A new left bundle branch block in an asymptomatic patient does not constitute a STEMI equivalent 1

Simultaneous Actions During ECG Interpretation

While interpreting the ECG, immediately initiate these parallel actions to avoid delays:

• Draw blood for high-sensitivity cardiac troponin measurement (results needed within 60 minutes) 1, 4
• Establish intravenous access and place the patient on continuous cardiac monitoring 4
• Obtain complete blood count, basic metabolic panel (including creatinine), glucose, and coagulation studies 4
• Administer aspirin 162-325 mg (chewed, non-enteric coated) immediately unless contraindicated by active gastrointestinal bleeding or documented aspirin allergy 1, 4

The ECG-Troponin Integration

• The ECG and troponin are complementary, not sequential—both must be obtained within the first 10 minutes 1, 4
• ECG findings determine the urgency of reperfusion (immediate for STEMI vs. early invasive strategy for high-risk NSTE-ACS) 1
• Troponin elevation confirms myocardial necrosis but requires 2-4 hours after symptom onset to become detectable, so serial measurements at 1-2 hours (high-sensitivity assay) or 3-6 hours (conventional assay) are mandatory 1, 3
• Dynamic troponin changes (≥20% rise or fall if initially elevated, or absolute increase >7 ng/L for high-sensitivity troponin T) distinguish acute MI from chronic troponin elevation 3