Recording an ECG - Amplified
The 12-Lead System
Limb Leads (Frontal Plane)
- Lead I: RA to LA (0°)
- Lead II: RA to LL (60°)
- Lead III: LA to LL (120°)
- aVR: augmented right arm (-150°)
- aVL: augmented left arm (-30°)
- aVF: augmented foot (90°)
Chest Leads (Horizontal Plane)
- V1-V2: Septal (right ventricle)
- V3-V4: Anterior (anterior wall)
- V5-V6: Lateral (lateral wall)
Lead Placement Anatomy
Finding the 4th Intercostal Space
- Find the sternal angle (angle of Louis)
- This is at the 2nd rib
- Count down to 4th ICS
Common Errors
- V1/V2 too high: false pathology (poor R-wave progression)
- Limb leads reversed: inverted P and QRS in lead I
- Poor contact: wandering baseline, artefact
ECG Interpretation Framework
Rate
- 300 ÷ (large squares between R-R)
- Or count R waves in 6 seconds × 10
Rhythm
- Regular? Irregular?
- P waves before each QRS?
- QRS after each P wave?
Axis
- Normal: -30° to +90°
- Left axis: negative in II, positive in I
- Right axis: negative in I, positive in aVF
Intervals
- PR: 120-200ms (3-5 small squares)
- QRS: <120ms (3 small squares)
- QT: varies with rate (QTc <440ms)
ST Segments
- Elevation: STEMI
- Depression: ischaemia
- Compare to baseline (PR segment)
Special Leads
Right-Sided ECG (V4R)
- Mirror V4 position on right chest
- Indications: inferior STEMI (assess RV involvement)
Posterior Leads (V7-V9)
- Continue from V6 around left side
- Indications: suspected posterior STEMI