ECG Guide

posted in Cardiology on by

The format of this article is atypical for the structure and concept of the website – but it’s always been about learning. Here is a simplified guide to ECG interpretation with a focus on the aspects I find more challenging to understand or recall.

Grid and Leads

The ECG grid
Limb leads
Precordial Leads

Axis

ECG axes

Atrial Enlargement

Atrial enlargement

Normal:
First portion of deflection is RA, second is LA
Right Atrial Enlargement:
P-wave amplitude > 2.5mm in inferior leads
Normal duration P-wave
Left Atrial Enlargement:
P-wave duration increased (terminal negative portion >0.04s)
Amplitude of terminal negative component >1mm below isoelectric line in V1

Ventricular Hypertrophy

Right Ventricular Hypertrophy:
Right axis deviation
Abnormal R-wave progression

  • Increased R-wave amplitude in leads overlying the right ventricle (V1)
  • Increased S-wave amplitude in leads overlying the left ventricle (V6)
Criteria

  • V1: R>S
  • V6: S>R
Left Ventricular Hypertrophy:
Left axis deviation
Increased R-wave amplitude in leads overlying the LV (I, aVL, V5, V6)
Increased S-wave amplitude in leads overlying the RV (V1)
Criteria:

  • Precordial Leads
    • R-wave in V5/V6 + S-wave in V1/V2 > 35mm
    • R-wave in V5 > 26mm
    • R-wave in V6 > 20mm
  • Limb Leads
    • R-wave in aVL > 11mm
    • R-wave in aVF > 20mm
  • Combined
    • R-wave in aVL + S-wave in V3 > 20mm (F), 28mm (M)

Secondary Repolarization Abnormalities

Secondary repolarization abnormality

  • Downsloping ST-segment depression
  • Asymmetric T-wave inversion

Bundle Branch Blocks

Left Bundle Branch Block

Left bundle branch block

  • QRS duration > 0.12s (3 boxes)
  • Broad or notched R-wave with prolonged upstroke in I, aVL, V5, V6
  • Associated ST-segment depression and T-wave inversion
  • Reciprocal changes in V1, V2 (deep S-wave)
  • Possible LAD

Right Bundle Branch Block

Right bundle branch block

  • QRS duration > 0.12s (3 boxes)
  • RSR’ in V1, V2
  • Reciprocal changes in I, aVL, V5, V6 (deep S-wave)

Hemiblocks

His-Purkinje system and hemiblocks (anterior fascicular block, posterior fascicular block)

Other Blocks

  • Non-specific intraventricular conduction delay: QRS >0.10s without BBB
  • Incomplete BBB: LBBB/RBBB pattern with non-prolonged QRS
  • Bifascicular block: RBBB + LAFB/LPFB (by axis deviation)

Ischemia and Infarction

ECG changes associated with ischemia and infarction

  1. Hyperacute T-waves
  2. T-wave inversion: Symmetric, compared to TWI associated with repolarization abnormalities
  3. ST-elevation: Unlike J-point elevation, ST-segment merges with T-wave
  4. Q-waves
    1. Duration > 0.04s
    2. Amplitude > 1/3 R-wave
    3. Normal in aVR

Coronary Artery Territories

Coronary artery territories

Distribution Coronary Artery Leads Reciprocal Changes
1. Inferior RCA, PDA II, III, aVF Anterior, Lateral
2. Lateral LCx I, aVL, V5, V6 Inferior
3. Anterior LAD V1-V6 Inferior
4. Posterior RCA Posterior Anterior (esp. V1)

External Links