12 channel ECG & Electrode placement

ECG – Electrocardiographs

Did you know that ECG as a concept originated in 1901? They coined a term Electrocardiograph as Electro meant Electrical Activity, Kardio meant Heart and Graph meant to write. Briefly put, a ECG measures the electrical activity of the heart and traces it as a graph helping the physician understand how your heart works. From then (machines with analog electronics), to now(with analog-to-digital converters), Electrocardiographs have evolved and come a long way.

ECG and Cardiac Cycle

Polarization and depolarization of the heart comprises of a cardiac cycle. Starting right from the Sinoatrial node on the top of the atrium, polarization continues to the atria, down to the ventricles through the atrioventricular node. This set of mechanical and electrical events comprise a cardiac cycle.

Leads and Electrodes

A regular 12-lead ECG comprises of:

  1. 3 limb leads
  2. 3 Augmented limb leads
  3. 6 Precordial chest leads

An electrode comes in contact with the body and comprises of a silver/silver chloride conductor and is coupled with a gel. Two leads can share an electrode and hence there are 10 electrodes to capture a complete 12-lead ECG.

Electrodes can either be disposable, bulb or clamp electrodes:

Limb leads are bipolar as they measure differences in two electrodes whereas all the augmented leads and the chest leads are unipolar.

Here are the 12-leads:

  1. Lead 1
  2. Lead 2
  3. Lead 3
  4. Augmented Limb Lead aVR
  5. Augmented Limb Lead aVL
  6. Augmented Limb Lead aVF

The electrodes that are located on the limbs (one on each arm and one on the left leg) forms the Einthoven Triangle.

12- Lead ECG Electrode Positioning:

10 electrodes for a 12-lead ECG are placed in arms, legs and around the chest. They are named RA, LA, RL, LL, V1, V2, V3, V4, V5, V6.

Every lead is derived from voltage differences in respective electrodes.

I = LA -RA



aVR = RA – 1/2 (LA+LL) = 3/2(RA -Vw)

aVL = LA -1/2(RA+LL) =3/2(LA – Vw)

aVF = LL – 1/2(RA+LA) = 3/2(LL-Vw)

The six electrodes V1 to V6 acts as positive poles for all 6 precordial(chest) leads.

ECG wave and complexes:

Every ECG wave is represented as PQRST complexes wherein P represents the atrial depolarization, QRS represents the ventricular depolarization and T represents the ventricular repolarization.
ECG Wave explanation

Source: Queen’s University at Kingston

Types of ECG Machines

Electrocardiographs are classified based on the way they capture and display the leads. They can be:

    Single-channel – records each lead individually and displays one lead at a time
    Multi – channel – records multiple leads simultaneously and are sent to a:
        Three – channel multiplexer in case of three-channel ECG machine
        Six-channel multiplexer in case of six -channel ECG machine
        12-channel multiplexer in case of twelve -channel ECG machine

BPL ECG Machines

BPL introduced their first ECG machine in 1969 – a single-channel ECG and the brand traditionally stands for introducing reliable and sturdy ECG machines. From then on, we have introduced multiple single-channel, three-channel , six-channel and 12-channel ECGs.


Our newest range of ECG machines come with Glasgow Interpretation algorithm that interprets ECG based on race, age and gender.

We also offer portable ECG solutions:

BPL Cardioline Touch ECG – a diagnostic 12-channel ECG with leads that connects to a windows device and comes with Glasgow interpretation.
BPL LifePhone Plus – An no-wire, gel-free portable triaging solution that connects to any android device and displays a 12-lead ECG. It lets you connect with your doctor and stores your health data as well.

Related links

Electrocardiographs range

Consumables and Accessories

Analysis and Interpretation of 12-lead ECG

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