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Leads and Placement
This article discusses about leads & their placement and the basics of an ECG grid paper.

ECG Grid Paper

The ECG paper recording speed is typically 25 mm/s.

Fig 1. ECG Grid Paper.

ECG Electrodes

They are 10 in number and placed externally on the skin at various designated positions, as outlined in the following table.


12 lead ECG Electrodes Placement
RA On the Rt arm, avoiding thick muscle
LA In the same location where RA  was placed, but on lt arm
RL On the Rt leg, lateral calf muscle.
LL In the same location where RL was placed, but on the left leg.
V1 In the 4th  ICS to the right of the sternum
V2 In the 4th  ICS to the left of the sternum
V3 Between leads V2 and V4
V4 In the 5th ICS on the Lt midclavicular line
V5 Horizontally even with V4 on the left anterior axillary line
V6 Horizontally even with V4 and V5 on the Lt midaxillary line
Rt-sided Electrodes – To detect Rt Ventricular MI
V1R 4th Lt ICS, sternal edge
V2R 4th Rt ICS, sternal edge
V3R-V6R Positioned as mirror images to the standard precordial leads on the Rt side
Posterior Electrodes – To detect Posterior wall MI
V7 Horizontal plane of V4-V6; posterior axillary line
V8 Horizontal plane of V4-V6; below the scapula
V9 Horizontal plane of V4-V6; at the paravertebral border



The term “lead” in ECG refers to the 12 different vectors along which the heart’s depolarization is measured and recorded. ECG leads are of two types: frontal and horizontal.

Frontal Leads

Frontal leads are also known as “limb leads.” They view the heart in vertical plane.

Frontal leads are of two types:

  • Standard leads (Bipolar limb leads): I, II, & III
  • Augmented leads (Unipolar limb leads): aVR, aVL, & aVF

Einthovens’s triangle

Einthovens’s triangle is an imaginary formation of an inverted equilateral triangle, comprising three limb leads, at two shoulders & pubis with the heart at center.

Einthoven’s law

Einthoven’s law states that when potential difference between the bipolar leads is measured simultaneously at any given moment, the potential in lead II is equal to the sum of potentials in leads I and III.

Fig 2. Einthoven’s Triangle.

Orientation of augmented leads

Unipolar limb leads are of low electrical potential and are instrumentally augmented. They are oriented in the following way:

  • Lead aVR – Augmented unipolar right arm lead, oriented to face the heart from right shoulder.
  • Lead aVL – Augmented unipolar left arm lead, oriented to face the heart from left shoulder.
  • Lead aVF – Augmented unipolar left leg lead.

Horizontal (Precordial) Leads

Horizontal (precordial) leads view the heart in the horizontal plane.

They include leads V1-V6.

Fig 3. Spatial Orientation of the Limb and Precordial Leads.

Fig 4. QRS Depolarization in the Precordial Leads.

Lead Grouping in ECG

Multiple leads that examine the same area are called contiguous leads. They are grouped in the following way:

  • I, aVL, V5, V6 – Lateral leads
  • II, III, aVF – Inferior Leads
  • V1, V2 –  Anteroseptal leads
  • V3, V4 – Anterior leads

ECG Interpretation – Master Basics Concepts of ECG By Dr Najeeb

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Making Sense of the ECG Book

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