What are the immediate ECG changes and management strategies for myocardial infarction (MI)?

ECG Changes in Myocardial Infarction

The most critical ECG changes in myocardial infarction include ST-segment elevation, ST-segment depression, T-wave inversion, and Q-wave development, with ST-segment elevation being the most urgent finding requiring immediate reperfusion therapy. 1

Immediate ECG Changes in Acute MI

ST-Segment Elevation

• New ST elevation at the J point in two contiguous leads with the following cut-points:
  • ≥0.1 mV in all leads other than V2-V3
  • In leads V2-V3: ≥0.2 mV in men ≥40 years; ≥0.25 mV in men <40 years; ≥0.15 mV in women 1
• Prolonged new ST-segment elevation (>20 min), especially when associated with reciprocal ST-segment depression, typically reflects acute coronary occlusion 1

ST-Segment Depression and T-Wave Changes

• New horizontal or down-sloping ST depression ≥0.05 mV in two contiguous leads
• T-wave inversion ≥0.1 mV in two contiguous leads with prominent R wave or R/S ratio >1 1
• ST-depression in leads V1-V3 with positive terminal T-waves may indicate posterior MI (ST-elevation equivalent) 1

Q-Wave Development

• Q waves may develop during evolution of MI and indicate myocardial necrosis
• Q-wave criteria for prior MI:
  • Any Q wave ≥0.02 sec or QS complex in leads V2-V3
  • Q wave ≥0.03 sec and ≥0.1 mV deep or QS complex in leads I, II, aVL, aVF, or V1-V6 in any two contiguous leads 1

Special ECG Considerations

Non-Diagnostic Initial ECG

• Some patients with acute coronary occlusion may present without ST-segment elevation
• Hyper-acute T-waves may precede ST-segment elevation
• Serial ECGs at 15-30 min intervals are crucial when initial ECG is non-diagnostic 1
• Continuous ECG monitoring is recommended to detect dynamic changes 1

Posterior MI

• ST-segment depression ≥0.05 mV in leads V1-V3 may represent posterior MI
• Additional posterior leads (V7-V9) should be recorded to identify ST-elevation ≥0.05 mV 1

Left Bundle Branch Block (LBBB)

• LBBB can mask typical ST changes
• Concordant ST elevation (in leads with positive QRS) is one of the best indicators of ongoing MI with occluded artery 1
• Patients with clinical suspicion of ongoing ischemia with new or presumed new LBBB should receive prompt reperfusion therapy 1

Prognostic Significance of ECG Changes

• More profound ST-segment shift or T-wave inversion involving multiple leads/territories correlates with greater myocardial ischemia and worse prognosis 1
• Patients with both Q waves and T-wave inversion on admission ECG have the highest 30-day and one-year mortality 2
• ST-depression and T-wave inversion are independent predictors of new-onset heart failure within 30 days 3

Management Strategy Based on ECG Findings

1. Immediate ECG Acquisition and Interpretation:

   • Obtain 12-lead ECG within 10 minutes of clinical presentation 1
   • Compare with previous ECGs when available 1

2. For ST-Segment Elevation MI (STEMI):

   • Immediate reperfusion therapy (primary PCI or fibrinolysis) 1
   • Target door-to-balloon time <90 minutes for PCI or door-to-needle time <30 minutes for fibrinolysis

3. For Non-ST Elevation MI (NSTEMI) or Unstable Angina:

   • Continuous cardiac monitoring with defibrillation capability 1
   • Serial ECGs and cardiac biomarker measurements 1
   • Risk stratification based on ECG changes, clinical presentation, and biomarkers
   • Consider early invasive strategy for high-risk features

4. For Suspected Posterior MI:

   • Additional posterior leads (V7-V9) 1
   • Treat as STEMI if ST-elevation ≥0.05 mV is detected in these leads 1

Common Pitfalls and Caveats

1. ECG Mimics of MI:

   • Early repolarization, pericarditis, LV hypertrophy, LBBB, Brugada syndrome, and stress cardiomyopathy can mimic ST changes of MI 1
   • Always correlate ECG findings with clinical presentation and cardiac biomarkers

2. Non-Diagnostic Initial ECG:

   • A single normal ECG does not exclude ACS
   • Serial ECGs are essential in patients with ongoing symptoms 1

3. Left Circumflex Occlusion:

   • May present without classic ST-elevation
   • Consider posterior leads (V7-V9) to improve detection 1

4. Bundle Branch Blocks:

   • LBBB can mask ischemic changes
   • New or presumed new LBBB with clinical suspicion of MI should be treated as STEMI 1

5. Ventricular Paced Rhythm:

   • Can prevent interpretation of ST changes
   • May require urgent angiography to confirm diagnosis 1

The ECG remains the most critical initial diagnostic tool in MI, allowing for rapid triage, risk stratification, and guidance of reperfusion therapy. Dynamic changes in ST-segments and T-waves provide valuable information about the extent of myocardium at risk and prognosis, making serial ECG monitoring an essential component of management.