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Blood Supply of the Heart, ECG Leads & MI Localisation

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PART 1: BLOOD SUPPLY OF THE HEART

Sternocostal (Anterior) View

Diaphragmatic (Posterior/Inferior) View

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Left Coronary Artery (LCA)

• Runs in the anterior interventricular groove
• Supplies:
  • Anterior wall of the left ventricle
  • Anterior 2/3 of the interventricular septum (via septal perforators)
  • Anterior right ventricular wall (small branches)
  • Apex of the heart
• Key branches: Diagonal branches (D1, D2) - supply the anterolateral LV wall; Septal perforators
• The "widow maker" - largest MI territory if occluded proximally

• Runs in the left atrioventricular groove
• Supplies:
  • Left atrium
  • Lateral and posterior wall of the left ventricle
  • SA node (in ~40% of people)
• Gives off: Obtuse marginal (OM) branches - numbered OM1, OM2

• Supplies the high lateral wall

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Right Coronary Artery (RCA)

• Supplies:
  • Right atrium and right ventricle
  • SA node (in ~60% of people via the SA nodal branch, the first branch)
  • AV node (in ~85-90% of people)
  • Posterior 1/3 of interventricular septum
• Key branches:
  • Acute marginal branch - supplies the free wall of the RV
  • Posterior Descending Artery (PDA) - in right dominant systems, runs in the posterior interventricular groove, supplies the inferior LV wall and posterior septum
  • Posterolateral branches - supply the inferior-posterior LV

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Coronary Dominance

• Harrison's Principles of Internal Medicine 22E

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Venous Drainage

• Great cardiac vein (alongside the LAD) - drains the anterior LV
• Middle cardiac vein (posterior interventricular groove) - drains the inferior heart
• Small cardiac vein (right atrioventricular groove) - drains the right heart
• Posterior vein of the LV - drains the lateral LV

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Conduction System Supply

• Tintinalli's Emergency Medicine

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PART 2: ECG LEADS - AN OVERVIEW

Limb Leads (Frontal Plane)

Precordial Leads (Horizontal/Transverse Plane)

• Inferior: II, III, aVF
• Anterior/Septal: V1-V4
• Lateral: I, aVL, V5, V6
• Right ventricular: V3R-V6R (right-sided leads, placed mirror-image)
• Posterior: V7-V9 (posterior axillary line, paraspinal) - or reciprocal changes in V1-V3
• Goldman-Cecil Medicine, ELECTROCARDIOGRAPHIC LEADS section

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PART 3: ECG CHANGES IN MI AND LOCALISATION

Mechanism of ECG Changes in MI

1. Rapid repolarisation (K+ channels open) - current flows out of infarct - ST elevation
2. Decreased resting membrane potential (K+ loss from cells) - current flows into infarct during diastole - manifests as TQ depression, recorded as ST elevation due to the AC-coupled ECG
3. Delayed depolarisation - infarcted area stays positive during repolarisation - current flows out of infarct - ST elevation

• Ganong's Review of Medical Physiology, 26th Edition

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ECG Sequence in STEMI

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MI Localisation Table (The Core of Your Exam)

• Rosen's Emergency Medicine, Table 64.4

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Detailed Breakdown by Territory

Anterior STEMI (V1-V4)

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Inferior STEMI (II, III, aVF)

• ST elevation in II, III, aVF
• Usually RCA (right dominant, ~85%), occasionally LCx
• Key tip: If III > II in elevation → RCA more likely; If II > III → LCx more likely
• Reciprocal ST depression in I and aVL (very sensitive - if absent, reconsider diagnosis)
• Always check right-sided leads (V3R-V4R) - ST elevation in V3R/V4R indicates right ventricular involvement (important because these patients should NOT receive nitrates - preload dependent)
• AV nodal artery is often involved → watch for bradycardia, heart blocks

Right Ventricular MI

• Occurs with proximal RCA occlusion (before the RV marginal branch)
• ST elevation in right-sided leads V3R-V4R (V4R is most sensitive/specific)
• Clinical triad: hypotension + elevated JVP + clear lungs (Kussmaul's sign may be present)
• Avoid nitrates and diuretics (both reduce preload)

High Lateral STEMI (I, aVL)

• Obstruction of the left circumflex or first diagonal (D1) branch of LAD
• Reciprocal changes in inferior leads (III, aVF)
• Can be "silent" - small territory, may produce subtle changes

Posterior MI

• No standard ECG lead looks directly at the posterior wall
• Reciprocal changes in V1-V3: tall broad R wave + horizontal ST depression + upright T wave
• Confirm with posterior leads V7-V9: direct ST elevation
• Associated with inferior STEMI (infero-posterior pattern)
• Artery: RCA (right dominant) or LCx (left dominant)

Left Main / Proximal LAD (Massive Anterior)

• ST elevation in aVR (>1 mm) + widespread ST depression in multiple leads
• This represents diffuse subendocardial ischemia or very proximal LAD/LM occlusion
• If aVR elevation > V1 elevation → left main more likely
• If V1 elevation > aVR → proximal LAD more likely
• Also consider de Winter pattern: J-point depression with tall hyperacute T waves in V1-V6 + aVR elevation (proximal LAD equivalent STEMI)

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Special Patterns and Tips

• Type A: Biphasic T waves in V2-V3
• Type B: Deep symmetric T wave inversions in V2-V3
• Does NOT show ST elevation - often asymptomatic on the ECG - DO NOT stress test

• Two contiguous leads with J-point ST elevation:
  • V2-V3: ≥2 mm in men, ≥1.5 mm in women
  • All other leads: ≥1 mm

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Quick-Reference: Artery → Territory → Leads

RCA (proximal) ─── Inferior + RV + AV node
                   → II, III, aVF + V3R-V4R
                   → Heart blocks, bradycardia

RCA (mid-distal) ── Inferior + posterior
                    → II, III, aVF + reciprocal V1-V3

LAD (proximal) ─── Anteroseptal + lateral + septal
                   → V1-V6, I, aVL

LAD (mid) ────── Anterior
                 → V1-V4

LAD (diagonal) ── High lateral
                  → I, aVL

LCx ────────── Lateral / High lateral / Posterior
               → I, aVL, V5-V6 (or V7-V9)

Left Main ───── Massive anterior + aVR elevation
                → Widespread ST depression + aVR ST↑

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Summary Table: ECG Findings vs. Location

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• Cover page with title and source credits
• Part 1 - Blood Supply - LCA (LAD + LCx), RCA, coronary dominance table, venous drainage, conduction system supply - with two Netter's Atlas anatomical images (sternocostal and diaphragmatic views)
• Part 2 - ECG Leads - bipolar limb leads, augmented unipolar leads, precordial leads with positions and territories, additional leads (V3R-V9), lead grouping by territory, normal intervals table
• Part 3 - MI ECG Changes - mechanism of ECG changes (Ganong's), temporal STEMI sequence, STEMI diagnostic criteria, master localisation table, territory-by-territory detail with clinical pearls, special patterns (Wellens, Sgarbossa/LBBB), plus the real Anterior STEMI ECG from Rosen's
• Quick Reference Summary - artery-to-territory-to-leads table, ECG finding-to-diagnosis cheat sheet