Study Notes – Acute Myocardial Infarction & LBBB

Inferior Myocardial Infarction (MI)

ECG Findings

• ST elevation in II, III, aVF

• Elevation in lead III > II is a key clue

• Reciprocal ST depression in I and aVL (especially aVL)

• Rule: ST ↑ in lead III + ST ↓ in aVL → Inferior MI until proven otherwise

Artery Involved

• Usually Right Coronary Artery (RCA) occlusion

• RCA branches:

  • Right ventricular branch

  • Posterior descending artery (PDA) – 80% RCA, 20% circumflex

• Depending on occlusion site:

  1. Proximal RCA → RV + Inferior + Posterior involvement

  2. Distal RCA before PDA → Inferior + Posterior, RV spared

  3. Distal RCA after PDA → Inferior only

Right Ventricular Infarction

• RV is preload dependent → avoid nitrates, beta-blockers, morphine

• Treat with IV fluids

• Diagnosis:

  • V1: ST elevation = RV involvement

  • Right-sided ECG (V4R): ≥1 mm ST elevation with upright T wave

Posterior Infarction

• Look for:

  • Tall R wave in V2

  • ST depression in V2

• Often accompanies RV/Inferior MI if clot is proximal

Clinical Pearls

• Reciprocal changes are very common in inferior MI

• Absence of reciprocal changes in anterior MI is not unusual (20%)

• Always check for RV and posterior involvement when inferior MI is present

Differential: Pericarditis vs Inferior MI

• Pericarditis: diffuse ST elevation (all leads except aVR)

• Inferior MI: look for reciprocal depression in aVL with ST ↑ in III → rules out pericarditis

Left Bundle Branch Block (LBBB) & MI

Normal LBBB Pattern

• Discordance = normal

  • QRS ↑ → ST/T axis ↓

  • QRS ↓ → ST/T axis ↑

• ST elevation ≤ 5 mm

Abnormal LBBB (suggests MI)

• Concordance (abnormal):

  • QRS ↑ + ST ↑

  • QRS ↓ + ST ↓

• ST elevation > 5 mm

• This is essentially what Sgarbossa criteria describe

Simplified Rule

• Don’t memorize Sgarbossa; just learn normal LBBB pattern

• If the ECG doesn’t fit: suspect MI

 Takeaways

1. Inferior MI → ST ↑ in II, III, aVF (III > II), reciprocal ST ↓ in aVL.

2. Always evaluate for RV infarct (V1 or V4R) and posterior infarct (V2 tall R + ST ↓).

3. In RV infarct → NO nitrates, BBs, or morphine; give fluids.

4. Pericarditis vs Inferior MI → reciprocal depression in aVL is the deciding clue.

5. LBBB → normal = discordance, ≤5 mm ST ↑; concordance or >5 mm elevation = abnormal (possible MI).