ECG in Acute Myocardial Infarction (AMI)

The 12-lead electrocardiogram (ECG) is the fastest and most widely available tool for diagnosing acute myocardial infarction (AMI). Its primary roles are to:

• Identify ST-elevation myocardial infarction (STEMI)
• Identify ischemic changes in Non-ST Elevation MI (NSTEMI)
• Localize the suspected infarct territory
• Guide urgent management and reperfusion strategies

However, ECG interpretation must be combined with clinical symptoms and cardiac biomarkers, because early ischemia or NSTEMI may show subtle or non-diagnostic ECG findings.

Primary References:

• ACLS Medical Training – ECG in AMI
• LITFL ECG Library – MI
• AHA STEMI Guidelines

AMI is diagnosed using a combination of:

• Ischemic symptoms (chest pain, dyspnea, diaphoresis)
• ECG changes consistent with myocardial infarction
• Elevated cardiac biomarkers (troponin rise/fall)

The ECG may show:

• ST-segment elevation (STEMI)
• ST-segment depression
• T-wave inversion
• New pathological Q waves
• Normal ECG (common in early NSTEMI)

ST elevation at the J-point above the baseline in two or more contiguous leads:

• ≥ 1 mm in all leads except V2–V3
• V2–V3 cutoffs:
  • ≥ 2 mm in men ≥40 yrs
  • ≥ 2.5 mm in men <40 yrs
  • ≥ 1.5 mm in women

Early, broad, symmetrical T waves that precede ST elevation.

Defined as:

• ≥ 0.04 sec in duration, AND
• Depth ≥ 25% of the subsequent R wave

(Except in III and aVR)

Indicates established infarction.

ST depression in opposite anatomical leads strengthens the diagnosis of STEMI.

• ST depression
• T-wave inversion
• Pseudonormalization of T waves during pain
• Transient ST elevation
• Often normal ECG despite significant ischemia

NSTEMI requires biomarkers and serial ECGs for diagnosis.

The 12-lead ECG can often localize myocardial injury based on which leads show ST elevation or depression.

Below is the standard localization table used in cardiology.

Contiguous leads refer to leads that examine the same anatomical region:

• V1–V6 (horizontal neighbors)
• I, aVL
• II, III, aVF

Look for at least 1–2 mm of elevation in two contiguous leads.

Supportive for STEMI:

• Inferior MI → reciprocal STD in I, aVL
• Lateral MI → reciprocal STD in II, III, aVF
• Posterior MI → reciprocal STD in V1–V3

Use the localization table above to infer the culprit artery.

Examples:

• Inferior MI + STE in V4R → RV infarction (proximal RCA)
• Inferior MI + STE in V5–V6 → inferolateral MI
• Anterior MI + STD in II, III, aVF → large LAD occlusion

Posterior MI does not produce ST elevation on the standard ECG. Instead, it produces reciprocal findings in anterior leads:

• ST depression V1–V3
• Tall R waves in V1–V3
• Upright T waves
• Wide R/S ratio in V1–V2

Posterior leads V7–V9 should be placed when suspected.

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• ST elevation in two contiguous leads indicates STEMI, but localization is essential for identifying which coronary artery is occluded.
• NSTEMI requires biomarkers and serial ECGs; early ECGs may be normal.
• Reciprocal changes strengthen the diagnosis of STEMI.
• Posterior MI is commonly missed and requires V7–V9 if suspected.
• Hyperacute T waves, ST elevation morphology, and Q-wave evolution help define infarct progression.
• Use the localization table to map ECG changes to specific myocardial regions and arteries.