How do you approach reading an electrocardiogram (ECG) in an emergency unit for a patient with a history of coronary artery disease (CAD) and presenting with symptoms of chest pain, considering potential signs of ischemia, infarction, or arrhythmias?

Systematic Approach to ECG Interpretation in Emergency Department Chest Pain

In patients with chest pain and known coronary artery disease presenting to the emergency department, obtain and interpret a 12-lead ECG within 5 minutes of arrival, focusing first on ST-segment elevation as the most critical finding that determines immediate reperfusion therapy, while recognizing that a normal ECG does not exclude acute myocardial infarction. 1

Immediate ECG Acquisition and Timing

• Obtain a 12-lead ECG within 5 minutes of patient arrival in any patient presenting with chest pain or suspected acute coronary syndrome 1
• The ECG should be interpreted immediately to identify patients requiring fast-track management and emergent reperfusion therapy 1
• If prehospital ECG capability exists, acquire and interpret the ECG before hospital arrival to expedite catheterization laboratory activation 1

Systematic ECG Interpretation Priority

Step 1: Identify ST-Segment Elevation (Highest Priority)

• ST-segment elevation is the most sensitive and specific ECG marker for acute myocardial infarction, appearing within minutes of symptom onset 1
• New localized ST-elevations indicate acute myocardial infarction in 80-90% of cases 1
• Patients with ST-elevation require thrombolytic therapy within 30 minutes or immediate catheterization laboratory activation 1
• ST-elevations may be more pronounced in men than women with acute myocardial infarction 1

Step 2: Evaluate for ST-Segment Depression

• ST-segment depression indicates myocardial ischemia but has poor predictive power for ongoing infarction—only 50% of patients with ST-depression will develop acute myocardial infarction 1
• ST-depression (except in leads V1-V3) typically indicates subendocardial ischemia from arterial subocclusion, distal embolization, or supply-demand mismatch 2
• ST-depression in leads V1-V3 may represent posterior wall ST-elevation myocardial infarction 2

Step 3: Assess for Arrhythmias and Conduction Abnormalities

• Identify life-threatening arrhythmias requiring immediate intervention 1
• Look for conduction abnormalities (atrioventricular block, bundle branch blocks) that may progress to complete heart block 3
• Evaluate QT interval prolongation suggesting risk for torsades de pointes 3
• Apply modified Sgarbossa criteria when left bundle branch block or ventricular pacing is present to identify acute myocardial infarction 4

Step 4: Look for Signs of Structural Heart Disease

• Identify left ventricular hypertrophy, which may cause ST-segment changes mimicking ischemia 1, 2
• Look for pathological Q waves suggesting prior myocardial infarction 1, 3
• Assess for right ventricular strain patterns suggesting pulmonary embolism 1

Step 5: Evaluate T-Wave Changes

• T-wave inversions have the lowest early case fatality rate among ECG abnormalities but still indicate cardiac pathology 1
• Isolated T-wave changes are less specific but may represent evolving ischemia 2

Critical Pitfalls and Limitations

The Normal ECG Does Not Exclude Acute Myocardial Infarction

• Approximately one-third of patients with acute chest pain have a normal ECG, yet 5-40% of these patients have evolving acute myocardial infarction 1
• Only 30-40% of patients who develop acute myocardial infarction initially have ST-elevations on hospital admission 1
• The ECG sensitivity for identifying ischemia is as low as 50%, and 2-4% of patients with evolving myocardial infarction are inappropriately discharged due to normal ECG findings 1
• Women are more often affected by false-negative ECGs than men 1

Patients with History of Coronary Artery Disease

• Among patients with acute chest pain and no ECG signs of ischemia, 4% of patients with history of coronary artery disease will develop acute myocardial infarction (compared to 2% without such history) 1
• These patients require serial ECGs and cardiac biomarker measurements at 10-12 hours after symptom onset 1

Risk Stratification Based on ECG Findings

High-Risk ECG Patterns (Fast-Track Management)

• ST-segment elevation: highest early case fatality rate 1
• New ST-segment depression: intermediate case fatality rate 1
• Suspected myocardial infarction on initial ECG (ST-elevation ≥1mm or pathological Q waves in 2 leads) 1
• Suspected ischemia on initial ECG (ST-depression ≥1mm or T-wave inversion in 2 leads) 1

Prognostic Implications

• Patients with normal ECG have relatively low mortality rate and risk of complications 1
• Both short- and long-term prognosis are clearly related to the admission ECG 1
• During long-term follow-up, mortality is similar among patients with pathological ECG regardless of whether ischemic changes were present 1

Integration with Clinical Assessment

• The ECG must be interpreted in conjunction with clinical presentation—no single ECG finding has sufficient sensitivity and specificity to rule in or rule out acute coronary syndrome independently 1, 2
• Use evidence-based risk stratification tools (Goldman criteria) incorporating ECG findings, systolic blood pressure <110 mmHg, bilateral pulmonary rales, and history of unstable ischemic heart disease 1
• Repeat ECG if symptoms change in quality or severity 2
• Compare to previous ECGs when available to identify new changes 2

Monitoring Duration and Follow-Up

• Continue telemetry monitoring for 12-24 hours until acute myocardial infarction is ruled out by negative biomarkers 1
• Patients with transient myocardial ischemia detected by continuous ST-segment monitoring are 8.5 times more likely to have in-hospital complications 1
• Extend monitoring beyond 48 hours if hemodynamic instability, ongoing ischemic symptoms, or ST-segment changes persist 5

Non-Physician ECG Interpretation

• Paramedics and nurses can accurately diagnose ST-elevation myocardial infarction independently when trained with mandatory initial education and ongoing concurrent medical oversight 1, 6
• Computer-assisted ECG interpretation can be applied if adequate on-site interpretation is unavailable 1
• Field-transmission of ECG for expert interpretation is reasonable when prehospital interpretation is not available 1