Table of Contents
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:
Diagnostic Criteria for AMI
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)
ECG Findings in STEMI
1. ST-Segment Elevation
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
2. Hyperacute T Waves
Early, broad, symmetrical T waves that precede ST elevation.
3. Pathological Q Waves
Defined as:
- ≥ 0.04 sec in duration, AND
- Depth ≥ 25% of the subsequent R wave
(Except in III and aVR)
Indicates established infarction.
4. Reciprocal Changes
ST depression in opposite anatomical leads strengthens the diagnosis of STEMI.
ECG Findings in NSTEMI / Unstable Angina
- 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.
Localizing MI on the 12-Lead ECG
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.
| Location of MI | Leads with ST Elevation | Likely Coronary Artery | ECG Features |
|---|---|---|---|
| Anterior | V2–V4 | LAD (diagonal) | Poor R-wave progression, STE, T-wave inversion |
| Anteroseptal | V1–V3 | LAD (septal branch) | Loss of R wave in V1–V2, STE |
| Lateral | I, aVL, V5–V6 | LCx or diagonal LAD | ST elevation; reciprocal ST depression in II, III, aVF |
| Inferior | II, III, aVF | RCA (posterior descending) | STE in inferior leads; reciprocal STD in I, aVL |
| Inferolateral | II, III, aVF + V5–V6 | RCA or LCx | Combined inferior & lateral STE |
| High Lateral | I, aVL | Diagonal LAD or LCx | Subtle STE; reciprocal STD inferiorly |
| Posterior | ST depression V1–V3 (reciprocal) | RCA or LCx | Tall R waves in V1–V3, upright T waves |
| Right Ventricular (RV) | V4R | Proximal RCA | STE in V4R; often accompanies inferior MI |
Contiguous leads refer to leads that examine the same anatomical region:
- V1–V6 (horizontal neighbors)
- I, aVL
- II, III, aVF
How to Systematically Localize an MI
Step 1: Identify ST Elevation
Look for at least 1–2 mm of elevation in two contiguous leads.
Step 2: Identify Reciprocal Changes
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
Step 3: Determine Coronary Territory
Use the localization table above to infer the culprit artery.
Step 4: Evaluate for Extensions
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 (Often Missed on Standard ECG)
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.
Table: ECG Localization Summary (ACLS Format)
| Region | ECG Leads | Culprit Vessel |
|---|---|---|
| Septal | V1–V2 | LAD (septal) |
| Anterior | V2–V4 | LAD (diagonal) |
| Anterolateral | V3–V6, I, aVL | LAD / LCx |
| Lateral | I, aVL, V5–V6 | LCx or diagonal |
| Inferior | II, III, aVF | RCA or LCx |
| Posterior | STD V1–V3, STE V7–V9 | RCA or LCx |
| Right Ventricular | V4R | Proximal RCA |
Sample ECG Images
Anterior STEMI
Image to be added soon
Inferior STEMI
Image to be added soon.
Posterior MI (reciprocal pattern)
Image to be added soon.
Key Takeaways
- 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.