What are the ECG (electrocardiogram) changes in a heart attack?

ECG Changes in Heart Attack

The key ECG changes in a heart attack include ST-segment elevation, ST-segment depression, T-wave inversion, and Q waves, with the specific pattern depending on the location, extent, and timing of the myocardial infarction. 1

Acute Phase ECG Changes

• ST-segment elevation occurs in two or more contiguous leads with specific cut-points:

  • ≥0.1 mV in all leads except 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

• ST-segment depression and T-wave changes include:

  • 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

• Hyperacute T waves (tall and peaked T waves) may be present during the very early phases of acute myocardial infarction, often preceding ST-segment elevation 1

• Reciprocal ST-segment depression frequently appears in leads opposite to those showing ST-segment elevation, providing additional diagnostic evidence of acute coronary occlusion 1

Evolution of ECG Changes

• The ECG typically follows a progression of abnormality in STEMI:

  • Beginning with hyperacute T waves
  • Followed by ST-segment elevation
  • Eventually developing Q waves
  • Later showing T-wave inversion as the infarction evolves 2

• Serial ECGs are essential as MI findings evolve over time, with recordings recommended at 15-30 minute intervals in symptomatic patients with an initially non-diagnostic ECG 1

ECG Changes Based on Infarct Location

• Anterior MI: ST elevation and Q waves in leads V1-V4 (LAD artery territory) 3

• Inferior MI: ST elevation and Q waves in leads II, III, aVF (usually RCA territory) 1

• Lateral MI: ST elevation and Q waves in leads I, aVL, V5-V6 1

• Posterior MI: ST depression in leads V1-V3 with tall R waves and upright T waves (ST elevation equivalent); best captured using additional posterior leads V7-V9 1, 4

• Right ventricular MI: ST elevation in right precordial leads V3R and V4R (≥0.05 mV, or ≥0.1 mV in men <30 years) 1

Q Wave Development

• Q waves indicating myocardial necrosis typically develop as follows:
  • In leads V2-V3: Q waves ≥0.02 sec or QS complex 1
  • In other leads: Q waves ≥0.03 sec and ≥0.1 mV deep or QS complex in at least two contiguous leads 1

Additional ECG Signs of Ischemia

• Cardiac arrhythmias (particularly ventricular arrhythmias) 1
• Intraventricular and atrioventricular conduction delays 1
• Loss of precordial R wave amplitude 1

Clinical Significance and Interpretation

• More profound ST-segment shift or T-wave inversion involving multiple leads/territories correlates with greater myocardial ischemia and worse prognosis 1

• The ECG allows clinicians to:

  • Time the ischemic event
  • Identify the infarct-related artery
  • Estimate the amount of myocardium at risk
  • Determine prognosis and therapeutic strategy 1

• Prolonged new ST-segment elevation (>20 min), particularly when associated with reciprocal ST-segment depression, usually reflects acute coronary occlusion resulting in myocardial injury with necrosis 1

Important Pitfalls and Caveats

• The ECG by itself is often insufficient to diagnose acute myocardial ischemia or infarction, as ST deviation may be observed in other conditions such as:

  • Acute pericarditis
  • Left ventricular hypertrophy
  • Left bundle branch block
  • Brugada syndrome
  • Stress cardiomyopathy
  • Early repolarization patterns 1

• A QS complex in lead V1 can be normal and should not be automatically interpreted as evidence of septal infarction 1, 3

• Small, non-pathological septal Q waves (≤0.03 sec and ≤25% of R-wave amplitude) may be present in leads I, aVL, aVF, and V4-V6 1

• Approximately 5% of patients with acute coronary syndromes may present with a completely normal ECG, highlighting the importance of serial ECGs and cardiac biomarkers 1