What ECG changes should not be overlooked?

Critical ECG Changes That Should Not Be Overlooked

ECG changes that should not be overlooked include ST-segment elevation/depression, T-wave abnormalities, pathological Q waves, posterior and right ventricular infarction patterns, and left main coronary obstruction patterns, as these findings can indicate life-threatening conditions requiring immediate intervention. 1

ST-Segment Abnormalities

ST-Segment Elevation

• New ST elevation at the J point in two contiguous leads with specific cut-points:

  • ≥0.1 mV in all leads except V2-V3
  • ≥0.2 mV in men ≥40 years in V2-V3
  • ≥0.25 mV in men <40 years in V2-V3
  • ≥0.15 mV in women in V2-V3 2

• Prolonged ST elevation (>20 min), particularly when associated with reciprocal ST depression, should always be considered significant, even in patients with initially non-diagnostic ECGs 1

ST-Segment Depression

• New horizontal or down-sloping ST depression ≥0.05 mV in two contiguous leads 2
• ST depression in leads V1-V3 may indicate posterior wall MI (inferobasal), especially when the terminal T wave is positive (ST elevation equivalent) 1

T-Wave Abnormalities

• T-wave inversion ≥0.1 mV in two contiguous leads with prominent R wave or R/S ratio >1 2
• Hyperacute T waves (tall, peaked T waves) in the early phase of myocardial infarction 2, 3
• Pseudo-normalization of previously inverted T waves during chest pain may indicate acute myocardial ischemia 1, 2
• T-wave inversion in lead aVL as an early sign of inferior wall ischemia 3

Q Wave Abnormalities

• Any Q wave in leads V2-V3 ≥0.02 sec or QS complex 2
• 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 2
• Distortion of the terminal portion of the QRS complex, which is independently associated with higher hospital mortality in STEMI patients 4

Easily Missed Infarction Patterns

Posterior Myocardial Infarction

• ST depression in leads V1-V3 with upright (positive) terminal T waves 1
• Requires posterior leads (V7-V9) for direct visualization
• Cut-point of 0.05 mV ST elevation in V7-V9 (0.1 mV in men <40 years) 1, 2

Right Ventricular Infarction

• Often accompanies inferior MI
• Requires right-sided leads (V3R, V4R)
• ST elevation ≥0.05 mV in V3R/V4R (≥0.1 mV in men <30 years) 1, 2

Left Main Coronary Obstruction

• ST depression >0.1 mV in eight or more surface leads
• ST elevation in aVR and/or V1
• Often associated with hemodynamic compromise 1

Special Considerations

Bundle Branch Blocks

• Diagnosis of MI is more difficult with LBBB
• Look for concordant ST-segment elevation or comparison with previous ECG 1, 2
• In RBBB, new ST elevation or Q waves should raise suspicion for myocardial ischemia or infarction 1

Dynamic Changes

• Serial ECGs at 15-30 minute intervals in symptomatic patients with initially non-diagnostic ECGs 1, 2
• Multiple ECGs may be needed during evolving ischemia 1
• Compare with prior ECGs when available 1, 2

Other Conditions That May Mimic Ischemia

• Acute pericarditis: diffuse ST elevation with PR depression
• Stress-induced (Takotsubo) cardiomyopathy: ST-T changes disproportionate to cardiac biomarkers 1
• Early repolarization, LV hypertrophy, and conduction delays can cause ST-T abnormalities 1

Prognostic Implications

• More profound ST-segment shift or T-wave inversion involving multiple leads/territories indicates greater myocardial ischemia and worse prognosis 1, 2
• The amount of ST deviation correlates with time to thrombolysis, with larger deviations leading to faster treatment 5
• ST depression on presenting ECG portends highest risk of death at 6 months 2

Clinical Pitfalls to Avoid

• Relying on a single ECG when the condition is evolving; obtain serial ECGs 1, 6
• Overlooking posterior MI by not using posterior leads (V7-V9) when indicated 1
• Missing right ventricular involvement by not recording right precordial leads (V3R, V4R) in inferior MI 1
• Assuming a normal ECG excludes ACS (1-6% of patients with normal ECG later proven to have MI) 2
• Failing to compare with previous ECGs when available 1, 2
• Overlooking subtle early signs of ischemia like T-wave inversion in aVL, terminal QRS distortion, or hyperacute T-waves 3

Remember that ECG changes should be interpreted in the clinical context, and when in doubt, serial ECGs and cardiac biomarkers should be obtained to aid in diagnosis.