MI Localization Using 12-Lead EKG

12-Lead EKG: Core Facts

• 12-lead EKG = 10 adhesive electrodes ➜ 12 electrical “views” (leads)
• Each lead inspects a defined anatomic region; ST-segment behavior in those leads localizes the myocardial infarction (MI)
• Gold-standard bedside tool for rapid, non-invasive infarct localization and decision-making (reperfusion, cath lab activation)

Electrode Placement & Landmarks

• Limb electrodes (bipolar / augmented limb leads)
  • RA: right wrist or forearm
  • LA: left wrist or forearm
  • RL: right lower leg (functionally ground)
  • LL: left lower leg
• Precordial (chest) electrodes
  • V1: 4^{th} ICS, R sternal edge
  • V2: 4^{th} ICS, L sternal edge
  • V3: midway between V2 & V4
  • V4: 5^{th} ICS, mid-clavicular line
  • V5: anterior axillary line, level with V4
  • V6: mid-axillary line, level with V4 & V5
• Additional specialty sets
  • V7–V9: posterior axillary to paraspinal line (posterior wall)
  • V1R–V6R: right-sided mirror placement (RV infarction)

Coronary Perfusion Overview

• Coronary arteries deliver O₂ to myocardium; occlusion → ischemia → infarction
• Two dominant systems
  • Right Coronary Artery (RCA)
  • Left Coronary system: Left Main → Left Anterior Descending (LAD) + Left Circumflex (LCx)
• Anatomic supply dictates which leads manifest ST changes

Limb Leads: Electrical Theory

• Bipolar (measure potential between two points)
  • Lead I: RA → LA
  • Lead II: RA → LL
  • Lead III: LA → LL
• Augmented unipolar (reference = average of two limbs; positive exploring electrode)
  • aVR: RA vs (LA + LL)/2
  • aVL: LA vs (RA + LL)/2
  • aVF: LL vs (RA + LA)/2

Precordial Leads

• Lie on horizontal plane, encircle anterior chest wall
• Crucial for diagnosing septal, anterior, lateral, posterior infarctions that may be silent in limb leads

Coronary Territories vs Leads

• Right Coronary Artery (inferior wall)
  • ST elevation: II, III, aVF
• Left Coronary distribution
  • LAD: V1–V4 (septal + anterior)
  • Diagonal branch / High lateral: I, aVL, V5, V6
  • LCx: lateral wall (I, aVL, V5, V6) ± inferior depending on dominance
• “More leads involved = larger myocardial area jeopardized”

Affected vs Reciprocal Leads

• Affected (primary): show direct ST elevation; pinpoint the infarcted segment
• Reciprocal: display mirror-image changes (ST depression, inverted T) opposite the infarcted wall; confirm diagnosis & gauge size
  • Example (text figure): ST ↑ in lead III, ST ↓ in aVL

Location Matrix (Must-Know)

• Anterior (LAD diagonal)
  • Affected: V3, V4
  • Reciprocal: V7–V9
• Anteroseptal (LAD, diagonal + septal)
  • Affected: V1–V4
  • Reciprocal: V7–V9
• Anterolateral (LAD ± LCx)
  • Affected: I, aVL, V3–V6
  • Reciprocal: II, III, aVF, V7–V9
• Inferior (RCA or LCx)
  • Affected: II, III, aVF
  • Reciprocal: I, aVL
• Lateral (high lateral)
  • Affected: I, aVL, V5, V6
  • Reciprocal: II, III, aVF
• Septal (LAD septal branch)
  • Affected: V1, V2
  • Reciprocal: V7–V9
• Posterior (RCA or LCx)
  • Affected: V7–V9
  • Reciprocal: V1–V3 (tall R, ST ↓, upright T)
• Right Ventricle (proximal RCA)
  • Affected: V1R–V6R
  • Reciprocal: I, aVL

Contiguous Lead Groupings (Mnemonic: SALI)

• Septal: V1, V2
• Anterior: V3, V4
  (Antero-septal = V1–V4)
• Lateral: V5, V6 (High lateral = I, aVL)
  (Antero-lateral = I, aVL, V3–V6)
• Inferior: II, III, aVF
• Diagnostic rule: ST elevation must appear in \ge two contiguous leads

EKG Manifestations of Acute MI

• ST-Segment Elevation (STE)
  • Definition (gender-specific):
    • Women: \ge 1.5\ \text{mm}
    • Men: \ge 2.0\ \text{mm}
    measured at the J-point in \ge 2 contiguous leads
• ST-Segment Depression (STD)
  • Often reciprocal to STE elsewhere
• T-Wave changes
  • Early hyperacute (tall, peaked) ➜ later inversion as ischemia evolves/reperfusion occurs
• Q-Wave formation (transmural necrosis)
  • Pathological if width > 0.03\ \text{s} or depth > 0.1\ \text{mV}, or >1/3 of subsequent R amplitude
  • Indicates irreversible cell death; poorer prognosis

ST Elevation Nuances (Figure highlights)

• J-point = junction of QRS termination and ST onset
• ST elevation values in sample tracings:
  a) 2.2\ \text{mm}, b) 1.5\ \text{mm}, c) 1.8\ \text{mm}, d) 2.0\ \text{mm}
• “J-wave” or “fish-hook” may appear, especially in early repolarization vs true MI—clinical context vital

Clinical Pearls & Integration

• Combine contiguous-lead rule with reciprocal confirmation to reduce false positives (pericarditis, early repolarization, LVH strain)
• The greater the number of leads with STE, the larger the ischemic bed—guides urgency (e.g., massive anterior STEMI vs isolated inferior)
• Right-sided and posterior leads must be added in:
  • Hypotensive inferior MI ➜ look for RV infarction (V4R most sensitive)
  • “Silent” posterior MI ➜ obtain V7–V9 if V1–V3 show tall R & ST depression
• Early recognition & re-perfusion (PCI within \le 90\ \text{min}) markedly lowers mortality

Ethical & Practical Considerations

• Rapid ECG interpretation can be life-saving; delays worsen necrosis & outcomes
• Accurate localization dictates additional therapies (e.g., avoid nitrates in RV infarct, prepare for pacer in inferior MI with AV block)
• Over-diagnosis bears risk (unnecessary cath), under-diagnosis fatal—hence reciprocal lead confirmation and serial tracings are ethical imperatives

Key Numbers to Memorize (Flash list)

• 12 leads, 10 electrodes
• STE threshold: Women 1.5\ \text{mm}, Men 2\ \text{mm}
• Pathologic Q: width > 0.03\ \text{s}, depth > 0.1\ \text{mV} or >\frac{1}{3}R
• Door-to-balloon goal: \le 90\ \text{min} (guideline standard)

High-Yield References

• Ahlert B. – “ECGs Made Easy: Pocket Reference”
• Akbar H., Mountfort S. – “Acute ST-Segment Elevation Myocardial Infarction (STEMI).” StatPearls, 2024 update