The ECG Leads, Polarity and Einthovens Triangle

Overview of ECG Lead Systems

Types of Leads

• Standard Limb Leads (Bipolar): I, II, III
• Augmented Limb Leads (Unipolar): aVR, aVL, aVF
• Precordial Leads: V1-V6

Standard Limb Leads

• Measure potential difference between two limbs (bipolar)
• Einthovens Triangle: Formed by electrodes I, II, and III at right arm (RA), left arm (LA), and left leg (LL)
• Einthovens Law: Lead II = Lead I + Lead III (e.g., II = 5 + -15 = -10)

Augmented Limb Leads

• Measure electrical forces from one limb at a time using a relative zero potential (unipolar)
• Leads aVR, aVL, and aVF provide additional views on the frontal plane

Right Leg Electrode

• Used to remove artifact, not part of the visible trace

Precordial (Chest) Leads

• Labeled V1-V6, placed on sternum
  • V1: Fourth intercostal space, right sternal border
  • V2: Opposite V1, left sternal border
  • V3: Between V2 and V4
  • V4: Fifth intercostal space, mid-clavicular line
  • V5: Same level as V4, anterior axillary line
  • V6: Same level as V4, mid-axillary line
• Provides transverse plane view of heart's electrical signal

Lead Groupings (Contiguous Leads)

• Inferior: Leads II, III, aVF
• Antero-septal: Leads V1, V2
• Anterior: Leads V3, V4
• Lateral: Leads V5, V6, I, aVL

Polarisation and ECG Trace

• Positive deflection: Wave of depolarisation towards a positive electrode
• Negative deflection: Wave of depolarisation away from a positive electrode
• Biphasic deflection: Wave of depolarisation at right angle to positive electrode

Reference Systems

• Triaxial Reference System: 60° separation between leads, same polarity direction
• Hexaxial Reference System: Combines triaxial system with unipolar leads, determines heart's electrical axis

Conclusion

• The ECG lead systems, including their positioning and polarisation, are crucial for accurately interpreting the electrical activity of the heart.