Conventional configuration of bipolar & unipolar ECG leads

I, II, & III are bipolar leads

Formed by connecting one end of the recording galvanometer to one lead and the other end to the other lead

Only three limbs are used (right arm, left arm, left foot) creating three combinations: lead I, lead II, lead III

Lead I: Connects the galvanometer to the right & left arms

Lead II: Connects the galvanometer between the left foot and the right arm

Lead III: connects the left foot to the left arm

The right leg is used as an ‘earth’ to minimise interference

With a bipolar system, one lead has to be connected to the positive terminal of the galvanometer and one to the negative

Einthoven manipulated the connections until the ECG waveform was positive in all three leads (in the test subject he used)

aVR, aVL, aVF are unipolar leads

Eg aVL:

The right arm, left arm and foot are joined together and joined to the negative end of the recorder

The effective deflection at the negative input is therefore zero

The left arm is connected to the positive end of the recorder

Therefore a ‘unipolar’ recording is obtained of the voltage changes occurring at the left arm

The ‘lead’ (ie the combination of connections used) is called aVL

V1 — V6 are unipolar leads

In each precordial lead, the positive (recording) terminal is connected to an electrode at a agreed site on the chest

Since the connection to the negative terminal of the galvanometer is the ‘indifferent’ one formed by joining together leads R, L, and F, the chest leads are called ‘V’ leads and are designated V1 — V6